Tuesday, December 23, 2008

Pictures from the Fish Ladder

We've been hammered with snow here in the great wilds of Grand Rapids, MI. I thought it would be a good time to take some pix from the fish ladder located on the Grand River. This is a favorite locale for my family to visit, but it is usually a little more pleasant in the spring and summer!


Wednesday, December 17, 2008

Threshold S/500 amplifier review

I've been a tube guy ever since 1989 when I first heard a pair of Quad ESL-63 speakers driven by a heavily customized McIntosh 240 amplifier. The glow of the vintage Genalex KT66s were magical as was the sound coming out of the electrostatic speakers. It was that day that I became an audiophile for life - a man obsessed with wringing the last bit of detail and realism from my favorite recordings.

So far it has been an interesting journey - filled with tubed gear from Dynaco, Harman Kardon, Eico and my own DIY creations. Over the years I've also dabbled with solid-state, owning at various times a SAE preamplifier, a McIntosh 250 amplifier, an Adcom 565 preamplifier and even building a kit or two. But in my heart of hearts I still remained a tube guy. To my ears tubes always came out on top for sheer musicality. But I've always kept an interest in the industrial art solid-state 'super' amps - brands like Threshold, Krell and Mark Levinson still hold some magical awe for me. These were the amplifiers that only the rich and well-heeled reviewers used with their uber-expensive exotic loudspeakers. However I've never actually known anyone who has owned any of these 'super' amps so I never had the opportunity to hear one. Surely they couldn't compare to a good tube amp!

Recently I've started to have an interest in purchasing a pair of Magnepan 1.6QR speakers. The problem was the low efficiency. I didn't trust my refurbished and upgraded Dynaco Mark IIIs to provide enough power, so I started thinking about getting a more powerful McIntosh solid state amplifier. I love the sound of my vintage MC250 and if I could capture that sound in a higher wattage model then I would be rather pleased. Well a chance conversation led me to buying a used Threshold linestage and amplifier instead.

The Nelson Pass designed Threshold S/500 was built from 1983-1988 and boasts 250WPC into 8 ohms. The power doubles into 4 ohms and the amplifier still has the grunt to even drive a 1 ohm load. This kind of power was needed for driving Apogee and other high end speakers of yore. Even today, some manufacturers build speakers with difficult and punishing loads that require arc-welding amplification.

For a sense of history the original Stereophile review of the S/500 can be read here.

The S/500 amplifier is huge and weights in at a portly 92 pounds. Removing the top panel reveals a massive toroidal power transformer, four huge power supply capacitors and ten pairs of output transistors per channel. The input/driver board is gold plated but to my jaded eyes some of the passive parts certainly look a little pedestrian. But you have to realize that this amplifier was made before the days of ultra-tweak parts.

It is rather funny how bias works - and I'm not talking about Class A versus Class B operation here - but human expectations. My mind was biased against this amplifier for several reasons - it doesn't have tubes, it is some 25 years old and it is high powered. I told myself that solid state design has surely improved over the years and low-powered amplifiers always sound better than their big brothers. And take a look at the insides there - it is just a bunch of silicon parts - where is the mystery and romance of the vacuum tube? A nice black-plate Tungsol 6550 or a Western Electric 300B output tube must be the only way to sonic nirvana. At best I was hoping a tube preamplifier could tame some of the solid state nastiness.

With low expectations I hooked the S/500 up to the Threshold linestage and as I switched it on, I was greeted with a small thunk through my KEF C-75 speakers. I turned up the volume, sat back in my sofa and gave it a listen.

The S/500 simply dominates any speaker it drives. The sheer power, low impedance and high damping factor controlled my KEF speakers more so than any of my tube amplifiers. But yet the sound had a refinement that was totally unexpected from such a monster. Treble was light and airy with incredible extension. Not only was it clean, but it was also free of etch. Based on my past experiences with silicon amplification this is the total opposite of what I had predicted. The midrange was glare and grit free, reminding me of some of the best tube amplifiers I have heard. Not exactly like a tube amp mind you, but very similar with warm musicality and excellent body. And the bass - the bass! - I cannot accurately describe the bass except to say it is the best I've ever heard. Not only is it low and controlled, but it is extremely musical. I can hear different bass shades and musical interactions that were just hints when coming through a tube amplifier output transformer. I never knew a KEF C-75 8" woofer could go so low!

This amplifier is really a high definition piece, besting my old McIntosh 250 by leaps and bounds when retrieving detail. I also feel it is an improvement over my Dynaco Mark IIIs, offering more bass, detail and drive. In some way the clarity reminds me of my SE EL156 amplifiers. But my EL156 amplifiers certainly can't drive difficult low impedance loads like the S/500 can.

Is this the best amplifier in the world? I don't know about that, but I'm certainly interested in seeing how it sounds with my UREI 813A and future Magnepan speakers. The S/500 has forever changed my mind about solid-state amplifiers. They can sound very good indeed and money permitting, I'm curious enough to try out some more upscale solid-state pieces in the future.

If you're interested in trying out a Threshold S/500 for yourself (or any vintage amplifier for that matter), you may want to consider a replacement of the aging power supply capacitors done by an experienced technician. That and some passive parts upgrade will not only increase the longevity but it could also improve the sonics. Right now my S/500 is running completely stock but I will be upgrading once I can.

Source: Phillips DVD player with RAKK DAC
Preamplifier: Threshold FET-10HL
Amplifier: refurbished Dynaco Mark IIIs or Jenison Audio Ultra SE-1 monoblocks
Speakers: KEF C-75 or PSB Stratus Bronze
Wire: Cardas 300-B Microtwin interconnects and Canare Starquad speaker cable


Tuesday, December 16, 2008

Speaker Review Updates

I gave the PSB Stratus Bronze and KEF C-75 speakers another listen using some solid-state amplification. I added my findings to the bottom of each review.


Monday, December 15, 2008

Threshold FET-10/HL linestage review

The past few days I've been listening to the Threshold FET-10/HL linestage. This is an 'old' stereo unit built back in the late 1980s by the great Nelson Pass. It uses discrete FETs throughout with no op-amps in sight. It has tape outputs, selector switch for five inputs, a stepped Nobles balance and volume control. Be warned that there is no phono stage - that was available by purchasing the stand alone FET-10/Pc.

Opening the case of the 10/HL reveals an amazing layout - all discrete components, gold plated circuit boards and an attention to detail that would make a mil-spec electrical engineer gawk in admiration. Output capacitors are basic ERO polypropelenes and the resistors appear to be very nicely made metal-films. There is also an outboard power supply that keeps hum down to near zero.

In all honesty I wasn't expecting much from this linestage. I had faith that my trusty 6N6P based unit could trounce anything commercially made. After all I had MIT RTX coupling caps, all wirewound resistors, a SMD Goldpoint volume control and a choke-based tube rectified power supply. Surely some great parts combined with a simple topology would beat out an 80s produced solid-state piece!

Well, I'm prepared to eat a healthy helping of crow now. The FET-10HL is a wonderful performer and makes me question the need for ultra trick parts. I never knew that solid-state could sound this good.

First of all, it has the ability to resolve the smallest details in the recording. The brush of the cymbal, the breath against the microphone, the way the stick hits the drum and a myriad of other audiophile tricks that I crave in a high performance preamplifier. This is one high resolution piece of gear and on everyone of my recordings I heard details that I never knew existed before. But this resolution does not lose out in musicality. There is a natural sweetness to the treble and the midrange is remarkably grain free. The bass is deep and well defined, adding even more control to my EL156 single ended vacuum tube amplifiers. Instruments have body - maybe a bit less than the best vacuum tube preamplifiers - but more than enough to satisfy me. I wish the gain could be just a little higher for the use with a non-OS DAC, but it hasn't been a problem with my transformer-coupled RAKK DAC.

Considering the performance, a used FET-10/HL is an audio bargain. Prices vary since this was never a mass produced unit. However consider a power supply capacitor upgrade as part of the overall price. This will help for long-term reliability and if you're not handy with a soldering iron, have this work done by an experienced technician. Also a coupling capacitor upgrade may improve the sonics even more, but for now I'm quite happy with the stock caps.

System used:
Source: Philips DVD player and K&K RAKK DAC with passive output
Speakers: KEF C-75 or PSB Stratus Bronze
Amplifier: Threshold S/500 or Jenison Audio Ultra-SE1 EL156 monoblocks
Wire: Cardas 300-B Microtwin interconnects, Canare Quad speaker cable


Tuesday, December 9, 2008

U.S. source for the EL156 tube

For anyone interested in making my Singled Ended Ultralinear amplifier, Antique Electronic Supply is now carrying the EL156 output tube. Price is a bit higher than a Chinese supplier, but at least you can try to return the tube if you have a problem with it.

I've been buying from Antique since 1990 and continue to do so because of their great customer service. Plus this year for Christmas they sent me a small tin of mints shaped like a 2A3! Of course Front Panel Express sent me some chocolate shaped like a laser cut panel. I guess being a 'big spender' has its perks. Heh.


Tuesday, December 2, 2008

The C3m pentode

The C3m is a loctal socket metal-shielded pentode use by the German Post for audio repeater amplifiers in long distance telephone lines. They are extremely linear and were rated for 10,000 hours of use.

The use of this tube was popularized by Thorsten Loesch in his 'Legacy' 300B amplifier. There are also a few commercial pieces of gear (eg Ayon Audio has a full function preamp) that use this tube. I have experimented with these connected as a triode and my experiences reflect this use.

Breaking a defunct one open and removing the mesh shield, I found a black plate inside with beefy construction. There are no side supports on the mica, which leads to one of many issues with these tubes - microphonics. They are extremely microphonic - reminding me of an old 71A globe linestage I built many moons ago. The switch bounce from my Goldpoint stepped volume control was easily picked up and amplified by the C3m. I tried many brands of C3m tubes (Valvo, Siemens, Lorenz, Philips) and got varying results but some microphonics always remained. So if you do use this tube, I strongly suggest using an extremely heavy case.

Another issue is the loctal socket and 20V/125mA filament requirements. This puts them in the realm of DIY use. I used a 24V transformer with an regulator board using a 7820. The more rare (read expensive) C3o uses 6.3V filaments but is an equivalent.

Another concern is fragility. When making my linestage, I once fired it up for 2-3 minutes to take some voltage readings. I pulled the C3ms out, made some changes and put in the C3ms back in. The movement of the heated tube was enough to make one stop working - ie, it would no longer draw current. I don't suggest ever 'hot-switching' tubes, but it is something to consider if doing some work on a C3m equipped unit. Let them cool down for a few minutes before moving.

Ok - how do they sound? Connected as a triode and used in a linestage, I found them to be very neutral. No glare or grain, they are even-handed from top to bottom. They reminded me more of a really good 6SN7 'type of sound' than (let's say) a 6DJ8.


Friday, November 28, 2008

Starving Student Headphone Amplifier update

I added a 5uF/100V ERO Polycarbonate capacitor bypass to each of the 300uF output capacitors. That and a little break in seems to have 'opened up' the treble a bit. I brought the headphone amplifier over to my friends house - he is currently running an all tube OTL headphone amplifier built by me using a EF86 triode connected as the gain tube and a 6922 as a White Cathode Follower.

In comparison, the Starving Student has better bass control and sounds more neutral top to bottom. To my ears the all tube unit sounds a bit more musical, but that could be from the higher output impedance having a little less control over his 150ohm Sennheisers. The 'Starving Student' sounded better and better the longer it was kept on and really had great impact and detail. I could live with either circuits - but the EF86 unit cost $500 to make versus $80 for the Starving Student! Of course the EF86 linestage has multiple input switching, RCA outs, tube rectification and premium parts.


Wednesday, November 26, 2008

Starving Student Headphone Amplifier

Today I finished a Starving Student headphone amplifier as designed by Pete Millet. It is a hybrid design using 19J6 tubes and a low-z mosfet output. I did double the output coupling capacitor to 300uF (three 100uF capacitors in parallel) and increase the power supply capacitance a bit to 220uF. I mostly stuck to pedestrian parts - the only specialty capacitors I used were .1uF/63V ERO Polycarbonate coupling capacitors, a few 220k KOA resistors and AB carbon composition for the gate of the output mosfet.

I won't bother to show off the insides since it is nothing but quick n' dirty spaghetti wiring!

How does it sound? Very good. Using Sennheiser HD-570 headphones I hear no obvious distortion or coloration. Nice bass control, great mids and tons of details I don't hear using my speakers. For an overall budget of under $80 it is a good project - for electronic beginner and experienced alike.


Tuesday, November 25, 2008

EF40 - the rimlock version of the EF86 pentode

For anyone using the EF86 pentode, NOS tube prices have been rising! What to do? There are new production tubes out there - the discontinued Svetlana EF86 which has a harsher top-end than an old Euro tube. The EH EF86 is pretty good, though it seems to lack some music 'color', reminding me of solid-state. I have yet to hear the latest JJ version - prices are still high on this model.

Back in 1948, Philips introduced the rimlock EF40 tube which was the precursor to the EF86. This tube was used in several pieces of Siemens theater gear and in Philips radios. What makes the EF40 different than the EF86? In this case it is just the base - the EF40 (along with several other excellent tubes - notably the ECC40) uses a B8A socket. The B8A or rimlock socket uses a nub on the side of the tube which centers the tube in the socket. Kind of neat bit of engineering and also forces good contact with the pins. The B8A rimlock socket was eventually discontinued as the majority of small tubes went to U.S. standards. Therefore B8A tube sockets are available mostly through European dealers and can also be found on Ebay. Avoid the Chinese sockets as they do not have the metal outside base to center the tubes.

Who manufactured the EF40? Sorry - no GEC or Telefunken here - just Philips (branded Mullard, Valvo, etc) from Holland and (seems to be mostly) France. Earlier versions were made in the Netherlands while perhaps production was quickly be moved to France. Later 60s production was made by Tungsram. There are also Siemens versions out there but I have yet to hear a pair and I've suspected they are actually Philips since they have the same construction. I do have a single Siemens supposedly made in Germany - I would like some further confirmation on this. Check out a nice gallery here of EF40s.

Best sounding are the Holland versions followed by the Tungsram. In general the French versions seems to lack the last bit of finesse of the Holland ones - and I've found this true even for EF86s - but the French versions are still very good. Tungsrams have good slam/dynamics. All versions seem to be very low in microphonics - perhaps it is the rimlock socket holding the tube tightly. The rarest (and earliest) versions feature a metal base for the nub and a solid metal screen. Within a year or two the the centering nub was included as part of the glass.

Going prices of the EF40 make them bargains! I've paid anywhere between $2 to $8 each. Provided the socket to be replaced is hardwired, it would only take a little work to convert to use the EF40. Not many of these show up in the U.S, but Canadian and European sellers have them and would be happy if you took them off their hands.


Friday, November 21, 2008

The Tube Detective: The GEC E3375

One day I was shopping for EL38s on Ebay. The EL38 is a Mullard designed pentode used for television line output stages. It features a 25W plate along with a beefy 400V 8W screen. The downside for many users is the top plate connection. With plate (anode) caps high voltage is outside the chassis and for safety reasons this tube may not appeal to everyone. However this may not be such a concern if you're making a DIY tube amplifier. Specifications for the EL38 can be found here.

I came across a very strange looking EL38 that I had never seen before. It looked exactly like a KT77 with a top cap. There was only one, but I was curious enough to bid and ultimately win.

Upon receiving the vacuum tube, I confirmed that the plate, getter rings and mica conformed exactly to the GEC KT77. I knew there were two versions of the KT88 with top caps - the TT21 and TT22. Was this a top cap version of the KT77? If so, I may have discovered a cheap way to get the famed KT77 sound at reasonable prices. So I started to use Google and find out everything I could about this version of the tube.

On this site I found references to the GEC EL38/CV450 and saw that the GEC version was called the E3375 - the 'E' standing for Experimental. I was curious enough to buy some, so I found a supplier in England called Langrex. At the current exchange rate, I was able to order a quad of E3375s for roughly $40ea. A little steep for an unknown tube, but perhaps I was letting my emotions get the better of me.

While I waited for the tubes to arrive, I bought a set of 1/4" top cap connectors on Ebay. Langrex shipped quickly and I popped open the first box to find this:

The only push-pull tube amplifier I had on hand was my pair of trusty Dynaco Mark IIIs. I added the plate cap leads to pin 3 of the tube socket, plugged in the tubes and set the bias adjustment at the lowest point. And got no sound. Hmm... this is strange. Fearing something was wrong, I shut the power off and suddenly, as the amplifier powered down, music started to come out of the speaker. Here was a clue. The bias voltage dropped as the amplifier power supply was shut down, allowing the tube to conduct. So the E3375 is in fact the GEC version of the EL38. Like the EL38, it requires much lower bias than the stock 6550/KT88 used in the Mark IIIs. See the specifications here for the operating points for the EL38.

I quickly modified the bias circuit, replacing the 10K pots with 25k units. The larger range of adjustment allowed me to finally bias these tubes in. In the end the sound was pretty good, but not at all what I was expecting from Genalex. Perhaps the Dynaco Mark III output transformer was not the optimal load. From my further experimentation with a single ended amplifier, the entire EL38 family of tube seems to 'work' better biased hard into class B.

As to why the E3375 was even made, I can only surmise that GEC wanted to compete with the Mullard EL38 for TV tube sales. Or perhaps the British military required a batch of new EL38s for some older equipment and GEC took on the contract. They already had the KT77 on hand and just modified the screens to conform to the EL38 specifications.

So if you're into rolling EL38s and don't mind spending a little dough, the E3375 is worth checking out. But be aware that using them in a stock amplifier may require some modification since they will require a lower bias point.


Wednesday, November 19, 2008

Pictures of the 6th Street Bridge

Michigan's oldest iron truss bridge, the 6th Street Bridge was made in 1886 by the Massillon Bridge Company. It has a 535 foot span and is made of wrought iron. One of the trusses on the west bank has been shortened and you can see the additional support that was added when the modification was done. Under the bridge you can see the original brick mortared pilings which have been buttressed by modern concrete. The original ornate guardrails are still there for the wooden sidewalk. If you get a chance take a walk across the spans and check out the construction details.

more information here.

Click on the pic for the larger view. Pics were taken by my lovely wife:

I enjoy walking across the bridge, examining the details like the 'Carnegie' stamped on the steel and other little touches that only an artist would have added. It reminds me of simpler times and I wonder how many times my great-grandfather drove over this bridge. He had one of the first cars in Grand Rapids and must have been over this span several times. I still wonder if two late 70s Cadillacs could actually squeeze past each other. Well I don't think I'll be doing that experiment any time soon.


Tuesday, November 18, 2008

Upgrading a Dynaco PAS tube preamplifier

I was searching for a cheap preamplifier to use with my recently bought McIntosh 250 amplifier when I stumbled upon a nice Dynaco PAS-3X on Ebay. I've owned a few of these back in the day and even though they aren't the ultimate in definition, they certainly are pleasant to listen to. With Tube rectification, simple circuitry and decent reliability the PAS has been used throughout countless systems. In fact the very first tube preamp I've ever owned was a PAS. Since this is for a second system that is used by the rest of my family, I decided to go for it based on the ease of use and AC switching for the McIntosh 250. The upgrade I did was simple and I did not go the route of the complete rebuild (ala Curcio).

First thing to do is to replace the filament diodes and power supply electrolytics. Triode Electronics carries an SDS board that does both.
SDS Board

I then bypassed the tone controls using instructions on the Curcio website
Tone Control Bypass

While I was in there I also removed the loudness switch. Basically removing the connection from the tap on the volume control to the loudness switch. The volume control seemed to track well between channels so I did not replace it (yet).

After that it was time for a capacitor upgrade. In the line section I replace the .02uF caps with some .022uF Russian K40s I had on hand. Just about everything else left in the signal path I replaced with Obbligatos from DIY Hifi Supply. I've found the Obbligato to be the best 'bang-for-the-buck' budget capacitor out there. Not the best of breed, but its sins are few compared to most metalized capacitors.

I plugged everything in and everything worked without any smoke or sparks. Always a good sign. This particular unit came stock with three Telefunken and one GE 12AX7s. Since I mostly listen to CD, I opted to keep the Telefunkens in the linestage while using a pair of Shuguang 12AX7C9s for the phono. Sound in my second system is not ultra-fi by any stretch of the imagination, but the Dynaco PAS has proven to be a good combination with the McIntosh 250 amplifier.

The modifications proved to be a success - especially the tone control bypass and capacitor upgrade. The unit now sounds cleaner, less grainey and with a subjective wider frequency response. The 'magic' of the tubes is still there providing a natural midrange and vocals.


Friday, November 14, 2008

A visit with the UREI 813A

If you can find a pair, the Urei 813 studio monitor is an excellent bargain. Deep bass, a time-aligned crossover, and the foam UREI horn takes care of many concerns with the original Altec/JBL drivers. UREI also QCed and blueprinted every driver used in their speakers. UREI speakers were used throughout American studios in the 1980s and can still be found in use even after all these years. There are three different iterations - the original rare Alnico magnet Altec based 813, the ceramic magnet 813A with the foamed horn, and the JBL driver based 813B.

From Mix Magazine:
In the mid-’70s, UREI founder Bill Putnam—unhappy with the sound of the Altec 604 monitors in his United Western Studios—worked with UREI’s Dean Austin and Dennis Fink on ways to improve the 604. They replaced Altec’s multicell horn with a wider dispersion design and added a 15-inch Eminence woofer to boost LF output. Ed Long applied his Time-Align™ crossover techniques to achieve time- coherent, true point-source performance. Engineers and producers mixing on the system were so enthusiastic about its sound that UREI started producing the monitors as a commercial product, with the first UREI 813 debuting in 1977. Typically soffit-mounted, these large, double-15 monitors were ideal for the larger, higher-SPL control rooms of the time.

I was lucky enough to pick up a pair of 813As from a local speaker manufacturer. Mine were particularly well-cared for with intact foam on the horns. They are extremely heavy and required two (relatively) fit adults to drag them down the stairs. If you ever have the misfortune of having to move these monsters, make sure to remove the drivers beforehand.

So how do they sound? In a word explosively dynamic, clean and powerful. The only speaker that I've ever owned that truly captures what impact a drum kit can have. They also capture guitars and bass just right. Vocals are amazingly real with the ability to pick up the different types of microphones used.

Weaknesses? Maybe because of the large baffle area, these are not soundstaging champions. Most mixes never go beyond the sides of the speakers. Another downside is that the sound never truly opens up until these are playing very loud. This makes them unsuitable for apartment and condo dwellers. Because of this, they don't quite work with baroque music either. They are also a touch dry and unrelenting - a bad recording is well, a bad recording and they will certainly show you the weaknesses of your favorite not-so-well-mixed music.

If you enjoy hard rock, run them with a solid-state amplifier - bass was deep and controlled even with my McIntosh 250. However for the best overall sound I preferred to use my refurbished Dynaco Mark IIIs off of the 4 ohm tap.

Sadly the UREIs are sitting under the stairs in my new house with the drivers out and safely tucked away. For my current living conditions they were permeating my entire house
with music - even at normal listening levels - annoying my other family members. So they will stay retired until I have a larger area to use them in.

Manuals from JBL:


Thursday, November 13, 2008

McIntosh MC-250 update

I decided to give my McIntosh MC-250 a much needed update - here is the input board:

Here are the parts (circled in red) that I am going to be updating: the main input board power power supply capacitors will be increased with 1000uF Philips units. The input capacitors are replaced with Ero Metallized Polypropolenes and the output capacitors should be replaced with bipolar electrolytics.

Once the two bolts on top have been removed you can see the bottom of the PCB board. I applied my soldering iron and removed the old parts. If you do this yourself, be sure to work carefully since these vintage PCB boards are fragile. Also be sure not to bend the attached wires too much as these can also break. It certainly would help to have a 3rd hand - one hand to hold the board still, the other hand to pull on the leads of the old component and another to handle the solder iron.

Here you can see the new capacitors are mounted. The Ero .47uF capacitors were tight fit and needed to be raised off the board a bit in order not to interfere with nearby components.

So how does it sound after the changes? It definitely makes the MC-250 sound 'less vintage' with a more modern representation. The original capacitors are over 30 years old and must have drifted in value since then. There appears to be more detail too - but I'm doing comparisons by memory so you can take this observation with a grain of salt. Some people may prefer the vintage sound - so keep this in mind if you are thinking of upgrading any old amplifier. Of course old electrolytics should be replaced anyways as a safety measure.

Next time I'll bypass the main reservoir capacitors and replace the other aging electrolytics mounted on the PCB boards.

For schematics and part values the McIntosh MC-250 service manual is available here.

Tuesday, November 11, 2008

Soviet Madness : the 6N6P dual triode

The 6N6P is a large 9A base dual triode - under the cyrillic alphabet it is called the 6H6. The 6N6P is a wide band power amplifier and has nothing to do with the American designated 6N6 which is an octal power tube from the days of yore. The Russian 6N6P is a modern built tough tube obviously built for military specifications with the ability to handle high-g loads.

The 6N6P is starting to get used by manufacturers (K&K and Lamm comes to mind). It is also one of the last cheap tube values available on Ebay - going rate is $4 to $6 per tube. It looks quite like the 6H30, though the 6N6P has different characteristics (higher mu and higher rp). Though the 6N6P has the same pinout of the 6922/6CG7 it is a much different beast - be warned since it uses more than double the filament current of the 6922 and an extra .15A over the 6CG7. It is actually in operation closer to the 5687 family.

I bought a pair of these from Ebay and they look extremely well made with a tall bottle and massive getter area. I plugged them into a White Cathode Follower circuit based around the 6922 tube. Not an ideal circuit, but I was quite surprised on how good they sounded. My curiosity got the better of me so I took my existing 5687 linestage and re-wired the filament leads to accept the 6N6P.

Initial impressions it has quite a bit of the 5687 character, but it has a certain spaciousness and speed the 5687 (or at least the Tungsol) can lack. Back to front depth is very good, which really helps with the sense of 'being there'. The 6N6P is very neutral top-to-bottom and appears to be a good match with the rest of my system. This is a surprisingly refined tube and in the end I preferred it over my vintage Tungsol 5687s.

An excellent option for building headphone amplifiers, linestages, and for driving output tubes.


Monday, November 10, 2008

KEF C-75 review

Introduction: From my review of the PSB Stratus Bronze:
After the creation of my single ended EL156 amplifiers, I needed to buy a pair of speakers that would work well with a minimal of wattage. Sure I own a pair of UREI 813A loudspeakers that would do the job, but the 813As are massive and fragile with their exposed drivers. They were meant to be mounted high in a mixing room, not to be used in a residential situation with cats, children and klutzy owners like myself.

So I searched around and found a pair of KEF C-75s on Ebay. The KEF C-75 appeared to have everything I needed - high efficiency, a time aligned (or near enough) coaxial drive unit (Uni-Q) and a low profile. I put in a bid and won them from a seller with a 100% feedback rating. Well the speaker gods have never looked kindly down on me - when I received the speakers and plugged them in I found that one of the Uni-Q drivers was already blown with a fried voice coil. This C series of yore was never exactly a popular speaker, so finding a replacement was going to be hard to do. So with great disappointment the C-75s went under the stairs until I could find a driver
So there the C-75s sat as I searched through Ebay and Audiogon looking for a suitable Uni-Q driver replacement. A few weeks later, I finally found a pair of C-35s that appear to be a mini-monitor version of the C-75 - only using just a single Uni-Q driver in a smaller box. Information on these early versions of the Uni-Q is sketchy at best so I had to go on looks alone. I won the auction and waited impatiently for the C-35s to arrive.

Well nothing ever seems to go as planned - though the C-35 driver has exactly the same 8" diameter as the C-75 driver, upon closer inspection the C-35 has a slightly smaller tweeter. Pulling out the driver, the C-35 version also has a smaller magnet. I'm not sure why KEF bothered to make two versions of the same driver, but I decided to go ahead and make the swap. I had to desolder the tweeter connections to the crossover boards and swap everything on over to the new drivers. Let the Franken-C75 awake! With some trepidation I plugged everything in and decided to give 'em a listen.

System: RAKK DAC and/or Sony SCD-CD595 SACD player, DIY 6N6P linestage and DIY EL156 monoblocks. Various Cardas and Canare cabling.

Review: I setup the KEF C-75s in the same location st the PSBs once sat. I did not bi-wire as the KEFs only have a single set of banana jacks. I wired them up with an old pair of Canare 4s11 speaker cables that I used back in the day with my Adire HE10.1s.

Initial impressions were very good with surprisingly deep bass, a very nice laid-back midrange and a treble that was non-fatiguing. Because the crossovers use electrolytic capacitors, I let the speakers play a bit more before trying to draw any conclusions. However, I was immediately struck in the differences between this and the PSB. The PSBs try to be neutral while the KEFs decided to err on the side of warmth.

Richard Hawley's "Darlin' Wait For Me" on Cole's Corner is a extremely well record track. The KEFs made a big wide soundstage that really seemed to fill the room. While the PSBs kept the sound behind the speakers, the KEFs seemed to be more forward and captivating. Treble was warm and well-rounded as the recording intended. The drum kit however seemed a touch smaller than the PSBs, but still had a great sense of depth - sounding as if it was ten feet behind Hawley's dark vocals.

Nickel Creek - s/t is well recorded and highlights some great post-modern bluegrass. The recording captures Chris Thile's mandolin's attack and percussive sounds just right. The violin (or viola) never sounded etched, highlighting the natural beauty these instruments can have if the recording engineer does their job right. The KEFs captured the recording rather well, not hiding any of the blemishes or multi-tracking done with some of the vocals.

Earth Wind & Fire - "That's the Way of the World" (Mobile Fidelity) started to show some weakness of the speakers. Imaging sounded a little strange at higher output levels and the speakers definitely began to strain as the volume went up. An 8" woofer in a small cabinet can only move so much air and the dynamics became constricted at even decent output levels. Bass on some tracks was a little sloppy and uncontrolled.

Conclusion: Imaging is very good with the speakers nearly 'disappearing' into the room. The sound never seems to emanate from them, but the KEFs just create a pleasant soundfield. Driver integration is excellent with the three drivers combining into one seemingly single-source. Treble is clean with a touch of a roll-off, but you can still hear the differences between recordings. The midrange errs on the side of warmth which is a good thing in these hotly recorded digital days. Listening fatigue is very low and this leads to extended music listening sessions.

Weaknesses? Bass is a little uncontrolled, leading to a fatter warm sound than neutral. The overall sound can get a little grainy if pushed too hard. So don't expect clean sound if you're cranking up AC/DC to ear-shattering levels. They are also not the ultimate in definition but they make up for it with sheer musicality.

I also ran through plenty of different music listening to these speakers and like anything else, they have their limitations. For the money on the used market they are a bargain - pure and simple. Efficient enough to use with lower wattage amplifiers, the 4-ohm impedance doesn't appear to be a hard load even for a tube amplifier to drive. In the future I will be trying the speakers out with some higher powered tube and solid-state amplification.

12/15/08 Update: Using a 250WPC Threshold S/500 (500WPC into 4 ohms!) definitely made a change for the better. Bass is deeper and soundstaging opened up even more. The speakers really shined and to my ears outclassed the PSBs on several levels. There was a real sense of the musicians being there and playing together. Recording after recording revealed how musical the KEFs are. Ultra low bass on some recordings could be a bit uncontrolled, but I'm not expecting perfection for this kind of price.


Wednesday, November 5, 2008

PSB Stratus Bronze review

Well another election has come and gone. Looks like we're in for some more 'big government' programs coming down the pipeline. Needless to say I'm not a fan - bureaucracy invites corruption, sloth and becomes a permanent part of 'the system'. Maybe I'm just old fashioned but I believe in self-reliance, hard work and privacy.

Well enough of that - here is my review of the PSB Stratus Bronze speakers:

After the creation of my single ended EL156 amplifiers, I needed to buy a pair of speakers that would work well with a minimal of wattage. Sure I own a pair of UREI 813A loudspeakers that would do the job, but the 813As are massive and fragile with their exposed drivers. They were meant to be mounted high in a mixing room, not to be used in a residential situation with cats, children and klutzy owners like myself.

So I searched around and found a pair of KEF C-75s on Ebay. I liked the sound of my diminutive Q30s and thought a larger KEF model would be an improvement. The KEF C-75 appeared to have everything I needed - high efficiency, a time aligned (or near enough) drive unit (Uni-Q) and a low profile. I put in a bid and won them from a seller with a 100% feedback rating. Well the speaker gods have never looked kindly down on me - when I received the speakers and plugged them in I found that one of the Uni-Q drivers was already blown with a fried voice coil. The C series of yore was never exactly a popular speaker on this side of the pond, so finding a replacement was going to be hard to do. So with great disappointment the C-75s went under the stairs until I could find a suitable Uni-Q driver.

That very next day I stopped in at the Corner Record Shop and sauntered into their used stereo equipment room to take a look at their current speaker offerings. Right away I spotted these tall black tower speakers made by PSB priced at $225. I looked a the back tag and saw they were called 'Stratus Bronze'. After buying a Tyrone Davis LP, I went home and did some googling on the PSBs. Reviews were favorable and they seemed sensitive enough for my needs. These were originally priced at ~$1100, so the asking price of $225 was a screaming bargain. So back I went with my friend Chris and I gave them a quick listen on a vintage receiver to be sure there was nothing obviously wrong with them. No apparent problems so into the Volvo they went for their journey back home.

An interesting thing about PSB, they are a Canadian company and the government has actually funded research so their speakers could be internationally competitive. So lets find out how well a pair of 'big government' speakers do. ;)

The PSB Stratus Bronze is known as a 2 1/2 way system. The tweeter (of course) handles frequencies from 2kHz on up, one of the woofers handles the bass under 500Hz, while the second one just handles the midrange. If you bi-amp, both of the woofers will be handled by one amplifier while the other drives just the tweeter. I would have preferred a bass woofer and midrange woofer/tweeter bi-amp setup.

The physical design is the popular tall but not wide tower. The popularity comes from the small footprint and the wife friendly (make them disappear!) looks. Of course this makes it easy to squeeze in near your TV which is where the majority of speakers end up these days.

I did my listening with a K&K RAKK DAC, 6N6P linestage (or Bride of Zen linestage),my pair of EL156 amplifiers, and various Cardas and Canare cabling.

Setup was easy enough - move my little KEF Q30 speakers out of the way and screw down the speaker cables. I played a little bit with placement and found that I preferred them two feet away from the back wall. Putting them closer to the wall would have increased bass response to the detriment of imaging. Perhaps it is some psychological trick, but speakers put close to the wall seem to image flatly (that is to say with little depth). It may have been (re-)breaking in of the capacitors in the crossovers, but the sound seemed to improve with bi-wiring. Don't ask me why this would make a difference technically, I'm just reporting what I heard.

First impressions were of an extremely clean and extensive treble, a very neutral midrange and a somewhat dry bass response. Overall soundstaging was a little flat with a small loss of body. The speakers never sounded forward and the sound always hung behind them without projecting into the room.

I listened to various audiophile and non-audiophile releases. Richard Hawley's "Coles Corner" has some well record tracks - notably "Darlin' Wait For Me" with its simple instrumentation. With the PSBs the drum kit actually seems to be sitting far behind Richard Hawley's voice. A neat stereo trick that not all speakers can pull off. Two guitars sit left/right in the stereo spread and there seemed to be some loss of definition - like the speakers did not fully resolve the actual presence of the instruments. But still you got a feeling that a whole band was right in front of you, just playing through a bit of a fog. Hawley's distinctive voice however was very clean and powerful as ever.

Earth Wind & Fire - "That's the Way of the World" (Mobile Fidelity) is full of 70s goodness and deep funky bass, but the PSBs 6.5" woofer could only reproduce the top of the bass spectrum. The low bass was rolled off considerably compared to the mighty 15" woofer on the UREI 813s. Of course the 813A takes up 4X the space and weight 5X as much. Well you still can't break the laws of physics - a small woofer can only move so much air. Some of the multi-vocal harmonies also broke down with the PSBs making it harder to hear in this particular mix. I know, I'm expecting a lot out of the PSBs if I'm comparing them to the UREIs which were a studio standard for years.

Bela Fleck - Drive (Mobile Fidelity) is a well recorded bluegrass bit of music. It is multi-miked, giving a rather artificial listening experience. Of course everything about audio reproduction is artificial, but if I was the recording engineer I would have done this a little differently. Anyways the acoustic instruments sound quick with plenty of fast picking going on. A lesser speaker falls apart, making the music become just a blob of noise. Heck, 99% of the speakers on the planet render music like this! Well the PSBs do a competent job of keeping it all together, but with a slight loss of definition. However I can pick out the different instruments and follow their separate line in the music.

I briefly tried some bi-amping, using four monoblock tube amplifiers to drive the speakers. My Dynaco Mark IIIs drove the woofers while the SE EL156s amplifiers just drove the tweeters. This seemed to open up the dynamics a touch but I wasn't too keen running all that vacuum tube power on a daily basis just to listen to music. That's a lot of power and power tube usage.

As you can tell, my praise isn't exactly resounding with the PSB Stratus Bronze speakers. However at this price range, every speaker is going to have faults that are easy to pick up on. Though I mentioned 'definition' problems, I'm comparing the sound to the UREI which was used in mixing rooms across the world. Every engineering choice is about trade-offs, and the designers at PSB did what they could to minimize colorations. And that is perhaps my biggest fault with these speakers - they tend to sound too polite and just a little too reserved for my tastes. This reserved sound seems to be very popular with many audiophiles but I'm not a fan of it myself. Dynamics were never thrilling or expanded like some other (though more expensive) speakers I have heard. Music naturally breaths with changing dynamic peaks and valleys - the PSBs however seemed to minimize these changes. Perhaps this was the fault of my amplifiers - maybe the PSBs require some larger solid state power for me to fully appreciate them. A 4 ohm load for a SE amplifier (even with negative feedback) can be a bit of a difficult load. Perhaps I should have pulled down my McIntosh MC-250 from upstairs and see if that improved anything. I guess this opens me up for a future update.

So if you come across a pair of PSB Stratus Bronze speakers, I would suggest some good low impedance amplification that can handle the 4 ohm load. If you have an extra amplifier to spare, check out bi-amping too. Listen carefully and see if you can live with the aural vision that PSB is offering. I should also note they worked flawlessly at all times and sounded very good for home theater use.

12/15/08 Update: I recently got a Threshold S/500 amplifier that boasts 250WPC into 8 ohms. It also boasts a staggering 500WPC into a 4 ohms load like the PSBs. I hooked up the PSBs and gave them a listen. The extra power and current driving ability helped flesh out the bass a bit more, but I was still disappointed by the lack of low bass. Impact/dynamics were helped out but still seemed a little muted even compared to the KEF C-75 with its 8" woofer. These speakers definitely need some subwoofer help if you plan to listen to rock or action movie soundtracks at decent listening levels.

Midrange and treble remained the same with lower definition than my other speakers. The overall sound was polite, reserved and smooth. If this is the type of sound you are looking for, then go ahead and give the PSB Stratus Bronze speakers a try.

Frequency Range : 40Hz-21kHz +- 3dB, 45Hz-20kHz +/- 1.5dB LF Cutoff -10dB 29Hz,
Impedance: 4 ohms
Power required: 15-200Watts
Tweeter: 1" Aluminum Dome with Ferrofluid tweeter
Woofers: two 6.5" Felt Cone Rubber Surround 1" VC/16oz magnet,
Crossover: 24dB/octave Linkwitz Riley, 500Hz Butterworth
Internal volume: 1.56cu ft
Design Type: Bass Reflex
Size: 9" X 36.75" X 12.75"
Weight: 39lb
Misc: Dual 5-way gold plated binding posts and bi-ampable

from here.


Tuesday, November 4, 2008

Triode Connected EF86 linestage and headphone amplifier

This is an old post from my now-defunct Jenison Audio website.

EF86 Linestage: I've been messing around with the EF86 linestage for a number of years. This was based on the old Svetlana schematic available here. I've build several generations of this circuit, each better than the other. I initially started with a simple cathode follower using a 6SN7. Then I built the 'ultimate' version using tube regulated power supplies and a white cathode follower - various experiments with this made me eventually pull out the regulation and I preferred a simple choke and capacitor combination.

The final version of the schematic I built is here:

A note on the headphone outputs - this will work best with a pair of high-z headphones - a 32ohm pair will be just too difficult for the white cathode follower circuit to drive well. If you need to drive a pair of low impedance headphones I would recommend some loop feedback from the headphones out. This would reduce the output impedance and distortion. Something worth experimenting with at the very least. In any case I would also add a good bypass capacitor on the 470uF output capacitor - something like a 1 to 4.7uf film capacitor will help.

Power supply was a 6X5GT rectifier, cap/choke/cap/resistor/cap. I've kept this part vague since everyone has their favorite ways of building a PS and it turns out I never made a schematic for this portion of the circuit!

and the finished 'product' looks like this:

I ended up selling this linstage to a friend of mine who gets daily use out of it. He has rolled in several different types of EF86s and the best ones have been the GEC CV4085s and the Blackburn Mullards. However just about any EF86 sounds good. Gain is a touch high, which is why I built a switch so the cathode capacitors on the EF86 can be bypassed - this adds local feedback and reduces the gain. Not an optimal solution, but something nice to have if your amplifier is too sensitive.


Monday, November 3, 2008

Local Record Store Reviews

(pic from Corner Record Shop's site)

Vertigo Music in Grand Rapids, MI - owner is a nice chap who was originally one of the co-owners of my once favorite but now departed Vinyl Solution. Vertigo is located on Division Ave, famous for drug dealers and prostitutes - but the block that Vertigo is on is relatively safe most hours of the day. A nearby coffee house provides all-age punk rock shows. Vertigo is in an older building with creaky floors, high painted tin ceilings and a large collection of music. Selection is geared towards newer indie - but there is a fine representation of older music and 'dinosaur rock'. Used vinyl selection has usually been picked through but I still manage to find a gem every once in awhile. Used CD is alphabetized only by letter, not by artist name. This makes it more difficult to find particular items on your list. But nonetheless I rarely go there without buying something.

Corner Record Shop in Grandville, MI - owners are friendly and there is a massive collection of vinyl here. Most of their large floorspace is dedicated to records though I've been surprised on what I've found in their CD bins. They also have a used vintage stereo equipment room where I've bought a few pieces - prices here are very fair. Most of their selection is geared towards mainstream/dinosaur rock with a smattering of punk. The CRS carries new vinyl, used audiophile releases and plenty of old records. Again, most of the selection has been thoroughly picked through but I've found some rare stuff by taking my time and checking out the latest arrival bins. They've also given me 'first crack' at some just arrived goods. Highly recommended.

Flat, Black and Circular in Lansing, MI - an old college record store geared towards indie and new releases. Plenty of older stuff too. CD selection is very good and I can usually find something to buy. Record collection is big, though it seems very crammed together almost as an afterthought. Honestly I have a hard time mustering the energy to dig through the vinyl - for example the R&B section shares the same space as their Rap/Hip-Hop 12"ers and its hard going even for a zealot like myself. Prices on used CDs are very good, but new releases can be a tad high.

Encore Recordings in Ann Arbor, MI - this is exactly the old school type of record store you would expect to find in such a big college town. Records and CDs are piled everywhere and the floorspace is squeezed tight by the sheer amount of vinyl. Half the time you are saying 'excuse me' to dodge past some other yob looking for their music fix. An extensive collection of every genre, you may be disappointed on the actual slim pickings. Like every popular record store 'destination', the selection has usually been picked through by the local regulars. But still I've been able to find some obscure stuff and I always enjoy the hunt there. Encore keeps the actual CDs behind the counter and you bring up the cases to purchase. I've had several occasions whereI brought up the case and they couldn't actually locate the CD - a little disappointing but I still visit the store when I'm on that side of the state.

Sunday, November 2, 2008

First Generation KEF Uni-Q Driver

A few months ago I bought a pair of KEF C-75 speakers - these utilize KEF's earliest version of the 8" Uni-Q driver which is essentially an advanced coaxial speaker. With the C-75s an additional 8" bottom woofer provides the bottom bass. The MB200 Uni-Q driver is an 8" woofer with a neodymium tweeter which is small enough to be placed right inside of the bass voice coil. This makes the speaker act as a true source point. Oddly enough for the all the hype that surrounds the Uni-Q drivers, KEF's historical section fails to mention the C-series of speakers from the late 80s. Go figure.

One of my KEF MB200 drivers was bad - turns out the seller fried the voice coil. Probably a bit overzealous with the power. I've had to search and search, but I managed to find another driver. Here is the broken down one:

I pulled off the attached crossover and had to desolder the wires leading to the enclosed tweeter.

And removing the magnet assembled I was able to get to the tweeter inside.

I failed to remove the bad voice coil in the woofer magnet - it was actually fused due to the voice coil varnish being heated up (clipping solid-state amp for sure!) and the varnish bubbling off.

I have a pair of KEF C-35s being shipped my way - I will have to switch over one of the drivers, attach the C-75 crossover and I should be back in business. Stay tuned for an update coming soon.

Friday, October 31, 2008

A visit with the McIntosh MC-250

Introduction: I've always had an interest in the McIntosh MC-250 ever since I learned it used a transformer to couple the output transistors to the speakers. This seems to be one of the traits that make tube amplification great, so when I had a chance to buy a 250, I went for it. My McIntosh MC-250 was bought at a local record store for a bargain price of $325. The chrome and overall condition is excellent with only a few scratches on the chrome and autoformers. Looking inside mine, it appears that all components are original except for the power supply diodes. It has a big muscular look which works with my three tenets in life - sex, power and chrome (ha!)

There certainly is a tube mindset, not only in the schematic, but in the physical execution. Rumor is this amplifier was voiced to sound just the venerable MC-240. The 250 looks much like a MC-240 chassis with solid-state guts and the power devices mounted on top.

After my purchase, I initially hooked this little beast up to my UREI 813A speakers which are brutally honest in their presentation. To tell you the truth, I didn't expect much from this amp except for the classic grainy solid-state sound from the days of yore, butI was suprised by the high quality sound coming out. Well, the autoformers seem to make a difference - compared to my tube amplifiers, the treble seems slightly rolled off but the midrange is very warm and forgiving. Bass control is excellent and this certainly sounds more powerful that its rated 50Ws. This amp is heavily biased into Class B and can produce big peak power when needed. Soundstaging and depth is definitely a little flatter compared to my tube amps, but the McIntosh 250 excels in rock music with big dynamics.

left side:




History: The McIntosh MC-250 (metered version is the 2505) was McIntosh's first foray into the field of solid-state amplification. They certainly held off for awhile since the amp did not come into production until 1967. Most of the hi-fi companies about that time had left vacuum tubes forever. It could be argued that this sudden shift of technology led to the downfall of many of these companies. Early solid-state technology was tenuous at best and the first generation amplifiers were not known for their reliability. McIntosh wanted to get it right the first time. Of course McIntosh is a company that continues to follow the beat of their own drum. The MC-250 has proven to be a reliable beast, having survived the tortures of time with its own unique circuitry - notably the Autoformer and the Sentry Monitor which continue to be used in their top-of-line models.

The autoformer is controversial, here is how Roger Russell explains it -

from http://www.roger-russell.com/mcintosh1.htm
Transistor power output circuits can match 8-ohm loads directly. This eliminates the need for the output transformer for most manufacturers. However, output stages that are designed to operate into an optimum load of 8 ohms can double or quadruple heat dissipation when operating into 4 or 2 ohm loads. At some frequencies, speakers rated at 8 ohms can dip as low as 4 ohms. Some 4-ohm systems can dip even lower. This mismatch can cause the amplifier to exceed its thermal dissipation limits.

On the other hand, if an amplifier is designed for an optimum load of one or two ohms, a low impedance load would be no problem. However, less power would be available for a speaker having 4 or 8 ohms impedance.

The unique McIntosh output autoformer was the answer. Since McIntosh output stages were connected in a single ended push-pull circuit, one side of the output was always connected to ground. They were typically designed to work into an optimum load of 2.1 ohms. The matching autoformer was connected directly to the output. In the MC2505 amplifier, the matching output was for 4, 8 and 16 ohms. Other impedances became available in later amplifiers. Full continuous amplifier power could be delivered to each of these loads. There was no danger of exceeding safe limits or overheating.

The autoformer also protected the speakers from damage in the event of amplifier failure. Should a direct current component appear at the amplifier output, it was shunted by the low DC resistance of the autoformer, instead of passing through the speaker voice coil, which could damage the speaker or even cause a fire.

McIntosh autoformers continued to be used in the "top-of-the-line" amplifiers. They were all designed and manufactured by McIntosh. Although the autoformers added extra cost, weight and took up extra space, they assured a safe, optimum match to a variety of speakers and speaker hook-ups. They were constructed and performed in the McIntosh tradition of excellence.

Although the autoformer provided an efficient match between the power transistor output and a variety of load impedances, a short circuit at the amplifier output or a load that was much lower than the selected autoformer tap could cause excessive current to flow in the output transistors. To complement the new transistor amplifiers, the McIntosh Sentry Monitor circuit was developed which prevented destructive current levels from occurring under any conditions. This circuit sensed the dynamic operating time, voltage and current of each amplifier output stage and controlled the current flow, confining it to non-destructive limits. The arrangement assured complete circuit reliability for all load conditions. The Sentry Monitor did not limit the rated power output available from the amplifier in any way. McIntosh power amplifiers continue to use the Sentry Monitor circuitry.

Conclusion: So if you're looking for some cheap and reliable amplification, check out the McIntosh 250 or its big brother the 2100. The metered versions (2505 and 2105) look prettier, but my manly (ha!) instincts prefer the industrial look. A perfect piece for any mancave or swinging bachelor pad. This amp now sits on my living room bookcase providing hours of daily musical enjoyment.

McIntosh is an American icon and they continue to make their amplifiers at their Binghamton, New York plant. The new ones are bloody expensive, but you can own one of their older amplifiers for a decent price. Do your ears a favor and check 'em out.

Tuesday, October 28, 2008

Building a Single Ended EL156 Tube amplifier

This post is from my original Jenison Audio website which is now sadly extinct. If you're interested in making your own amplifiers and have never worked with high voltage before, be sure to follow correct safety procedures. I am not liable for your death or dismemberment.

Introduction: The articles you read in the stereo rags usually start with some clever comparison between the author's Porsche, gardening, or the different flavor of wines from Tuscany. Well, I'll skip all that and jump right to the point - I've built a few low powered SE amplifiers in my time using the 2A3 and 300B, but I've always found their real world performance disappointing. Even with fairly efficient speakers, I could hear the output clip when listening to some solo acoustic guitar! I like listening at levels that attempt to give some sense of the actual event. For example, throw on some AC/DC and the 300B amplifier would quickly fall apart. I'm not ready to invest in some mojo-efficient horn speakers yet and since my music tastes vary, I needed a single-ended amplifier that could at least drive some fairly efficient speakers, pump out at least 20Ws (which seems to be the bare minimum for real world amplification) and have some bass extension. This limited me to three choices - parallel 6550s/EL34s/KT88s/6L6GCs, a single 845/211s, or a bigger pentode.

Paralleling tubes was out since the extra cost, matching and fiddly bias adjustments would be a headache. The 211/845 was out because I have a morbid fear of working on anything over 600Vs. High voltage has a way of playing tricks and jumping if you're not careful - there's a reason to use ceramic sockets. So I decided on the 50W plate dissipation Shuguang EL156. I picked up a quad of these before the value of the dollar dropped like a rock and was impressed by their construction and massive plate. I also really like Ultralinear operation since it strikes a decent balance between the power of pentode and the delicacy of triode.

A quick word on the EL156 - this was originally a 10-pin European socketed pentode designed by Telefunken to be used in cutter amplifiers and has a big 50W plate dissipation with a 450V/8W screen. The new Shuguang version uses an octal socket and uses the same design/layout as the original Telefunken. Pin out is the same as an EL34 - meaning it is a true pentode and you will have to tie pins 1&8 together. Rumor is that Telefunken even had this tube made in China towards the last days of major tube production and even provided the cathode emulsion. The EL156 is one mighty tube and takes 6.3V/1.9A to light up, however it doesn't need big plate voltage like the 211/845 tubes. I grew interested in this tube as a replacement for the 6550/KT88s in my Dynaco Mark IIIs - but had problems getting it to bias without modifying the stock circuit. I was however convinced by my experiment that the EL156 deserved some further attention. Specs are available here. These tubes are available on Ebay or from Super TNT.

Note - I built these as monoblocks, so two will be required for stereo. eg - each schematic is for one channel. Total cost was just over $1000.


Power Supply is very basic - started off with a choke input power supply that was 'tuned' with a 1.5uF capacitor to get the voltage I needed. Even with the slow-warm rectification of the 5AR4, I would suggest using 600V (or two 350V in series) capacitors as the voltage does initially rise above 500V before settling down to ~475V as the EL156 filament takes some time to come on line. In my case I used two 350V/100uF capacitors in series where each capacitor had a 470k/2W resistor in parallel.

I got away with AC on the filaments of the EF86 too - careful lead dressing plus the negative feedback helped in that regard. Note with the AC from my wall, filament voltage was a touch high a 6.7VAC - I added two .10 ohm dropping resistors to get this more in line with the standard 6.3VAC. Hammond transformers are known to run a little high.

Amplifier circuit is simple. You may have seen this same type of input stage used on several other amplifiers - a basic EF86 pentode RC coupled to the next stage. You could also try a 6SJ7/6J7/EF37/etc in this position. Pentode drivers are not favored by many people, but I like them for their detail and snap - plus it only requires one stage to get the desirable gain. Of course the minus is the higher output impedance which can be an issue when driving some tubes. Luckily the EL156 is an easy tube to drive and doesn't require the extra complexity of a cathode follower or mosfet to drive it. I used high quality but not crazy expensive parts - KOA carbon film resistors, Russian Teflon FT-3 coupling capacitors, old Allen Bradley carbon-composition grid-stoppers, PEC mono volume controls and teflon-coated wiring. Cathode bypasses are just Nichicon electrolytics. The output stage cathode has a potentiometer with one leg tied to the wiper. Using the current meter allows me to dial in bias for the output tube. You can play with different bias points to find the best mix of current/voltage. It should be noted that the voltages are approximate as the power supply will slightly increase the B+ if the current is dropped. The zener diode on the screen tap provides a bit of regulation and also drops the g2 voltage down to a safer region. I got this idea from Bill Kleronomos in his Sound Practices #4 article "The Classic Williamson 1993 Style" where he noted a decrease in THD when using zeners on the screens.

If you wish to use other output tubes than the EL156, for example the KT88 or 6550, you will have to increase the 150 ohm output tube cathode resistor or use a larger potentiometer in the cathode circuit. This is to get the right bias range and you may have to change the B+ via the first PS capacitor to your favored operating point. An optional 25K linear potentiometer with one leg tied to the wiper could be added before the 8.2k feedback resistor - this will allow you to adjust the amount of feedback, though amplifier sensitivity will change as the amount of feedback is varied.

Initial Listening Impressions: Listening was on a pair of budget KEF Q30 speakers, a 'Bride of Zen' preamplifier, and a RAKK DAC. The amps are extremely quick with a fast rise and settle time. I'm not sure if it is the Teflon capacitor I'm hearing, but I'm mighty impressed when listening to the Mobile Fidelity release of Bela Fleck. Fast acoustic music never sounded smeared or has the notes run together. It's like a really good solid-state amplifier but without any of that slight grit/glare you hear in the upper-mids. Instruments are well fleshed out and voice is very natural. Listening to a needledrop of Willy Nelson's Stardust, I heard lots of little details (reverb effect, drum brush, etc) that were just a touch more apparent. Maybe one of the better amps I've heard, I'm curious to see if it holds out on something more hi-def like my UREI loudspeakers. In a short word (and I hate using this word mind you), the amps appear to be very neutral. They don't sound like 'tube amps' in the classic sense, but they don't sound like solid-state either. Amazing considering the pedestrian Hammond iron, electrolytic power supply and overall budget.

Further Impressions: My UREI 813A speakers are still in storage so I gave these a listen with a pair of 90dB efficient 4 ohm PSB Stratus Bronze speakers. The amps are breaking in nicely - there is enough power to drive these speakers and there is very good bass definition and control too. If anything the massive Hammond iron can pass some bass. Very nice midrange that has a tight grip on the music, but yet I don't feel a lack of detail. The sound has some of the smoothness of a SE triode amplifier, but to my ears they have more speed and control. These amplifiers are now a permanent part of my system and will be useful for future bi-amping.

A small video of a single amp being tested with some R.L. Burnside.