Review: VTL Stereo Deluxe 90/90 power amplifier

 

A year or two into my audio journey, I was in college when an friend replaced his heavily modified McIntosh 240 amplifier with a set of VTL Compact 100 Monoblocks.  Our little audio community were a little disappointed since this change also meant the removal of his Quad ESL-63s.  These electrostatics were replaced by a big ol' pair of UREI 813 dynamic speakers, using the Alnico version of the Altec 604.  But, to be fair, the Quads - without a subwoofer, weren't the best for rocking out.  Understandable to us 20-somethings.

Along with the amplifiers came the VTL book, something that he loaned to me.  At the time I didn't know too much about circuits so I read David Manley's words with much interest.  I was a poor college student at the time and was firmly stuck in the (then) more affordable world of vintage gear.  Dynaco was more my speed with the PAS-3X and the Mark IVs being in my system at the time.  Owning new tube gear was just out of my means.  

I'll hit the fast forward button on my life.  So many years later my interest in modifying or building my own tube or solid-state gear has declined.  Some of it is cost, why make a tube amp when there are so many reasonably priced units out there?  And a lot of it is patience and my close up vision isn't what it used to be.  The idea of staring at a circuit board or a snake pit of wires hour after hour no longer holds the same interest.  It is so much easier to buy something, and at such a reasonable price, this used VTL Stereo Deluxe 90/90 power amplifier was a must buy when I saw it on Audiogon,

 

Okay, what is the VTL 90/90?  It is a 90WPC - their claim, not mine - tube amplifier using a pair of 6550s (or KT88s or KT90s) and a pair of 12AT7s per channel.  No tube rectification, no B+ regulation, and no power supply choke.  Instead they use a large amount of capacitance to get the B+ ripple down.  The output tubes are also connected in Ultralinear, which means the screen grids of the output tubes are connected to taps on the output transformer primary.  Bias is relatively low, only 35mA per tube, meaning heavy class B operation.   The output iron is of note - being made by Mercury Magnetics - which are known for high quality.

This is not really the way I would build an amplifier.  I prefer, at least philosophically, pentode output stage and some type of regulation for the screens and the input stage.

Physically speaking the VTL is not as imposing as my Sonic Frontiers SFS-50.  The SFS just looks better built with the meter, larger chassis, and bigger output transformers.  It also has a trick cascoded differential 6922 input stage, and well, lots of regulation going on.  The VTL, on the other hand, has a Mullard 5-20 type circuit with a parallel 12AT7 input tube and a long-tail phase splitter (with no current sink) afterwards.

So, you are already asking after that long introduction, how does the VTL sound?  I'll try to keep this short. I will be comparing directly against the Sonic Frontiers SFS-50.

A quick note on tubes:  I used vintage three-hole Tung Sol 6550s and GE labeled (made by Lansdale) 12AT7s for the VTL.  Not the pictured tubes which came with the amplifier.  The SFS-50  amplifier used the same output tubes and Tungsram ECC88s. 

The VTL has a big bold and dynamic sound.  This is a fun amp to listen to!  Bass, something the company was known for, has a deftness that a lot of solid-state amplifiers could only dream about.  How is this possible?  I don't know given the lower output impedance of silicon based amplification.  The Sonic Frontiers, however, seems to best the VTL in the very lowest bass notes.  I suppose that makes sense with the bigger output iron and all that regulation. 

 

What surprised me the most about the VTL 90/90 is how natural it sounds.  The character of the amplifier is the same from top-to-bottom.  It's just an easy amplifier to listen to.  The SFS-50, on the other hand, is more neutral.  When it comes to detail the Sonic Frontiers is the clear winner, but that, as I will mention later, this comes at a cost.

Soundstaging?  The VTL is w-i-d-e and deep.  Vocals, like in The Ubiquitous Mr. Lovegrove by The Dead Can Dance has real presence and body.  This is a real treat, as was my American copy of Pink Floyd's Dark Side of the Moon.

So where does this amplifier fail?  This is a thorny question and one that drives into the pros and cons of this hobby.. It is a matter of detail.  I love detail but I can also be actively annoyed by excessive amounts of it because lesser recordings sound, well, bad.

The Sonic Frontiers, with its 6922 front end, has so more detail.  Like the sound of the hands striking the aforementioned Dead Can Dance song.  It really breaks down the recording into more component parts that the VTL amplifier.  The VTL blurs this detail - only by a bit - but is also a touch more engaging.

What amplifier do I prefer?  It's a matter of taste.  The VTL is just enjoyable, sounding more like a powerful Dynaco amplifier.  The Sonic Frontiers is better for those days I want to eek out more detail and hear the best that my system can do in a purely audio geek sense.

  

System:

Rega P8 turntable with Rega Ania Pro MC cartridge 

to a Denon AU-S1 step-up transformer

to Cardas Neutral Reference interconnects

to an Audio Research SP-8 (with a mix of vintage Mullurd and Amperex tubes)

to Cardas Iridium interconnects

to the amplifier(s) being tested here

to Cardas Iridium speaker cables

to KEF R500 loudspeakers 

 

Review: a Frankenstein Eico ST-70 tube amplifier

 I bought this strange modified amplifier on Ebay. It looks as if someone took a stock integrated and modified it to be a basic power amp. Gone is the normal set of controls on the front plate, replaced with a generic black panel and power indicator light.

 

A peek on top of the grille - what is inside?

Of course you know - you read the title of this post! Someone took an Eico ST-70, removed the preamplifier portion and converted it to use 5881s instead of the stock 7591s.

Since I didn't have any 5881s on hand, I modified this amplifier to use the military 6AR6. Since the 6AR6 has a completely different pinout (and bias points!) it took an entire rewire of the output stage, along with more negative bias than the stock circuit.

I also changed the power supply with new electrolytic capacitors for the main bucket, 6SN7 phase-splitters, and 12AX7 input tube. I also added a little 1.5H Dynaco choke to reduce the B+ ripple. After a little troubleshooting, and bias modifications, I finally got this amp to work; with 40mA per output tube. 

So how does this pentode output Frankenstein weird output tube sound? Even with old Russian 6N8S tubes, surprisingly good. Bass extension - with the large output transformers - is really deep. And there is the old tube midrange and smooth treble thing happening. Maybe it's been too long with an SS (Aleph J clone) amplifier, but this little push-pull tube amp is quite magical.

Chet Baker Sings, for example, really is holographic - for a mono recording - with his voice and trumpet sounding natural and forward. Compared to the Aleph J solid-state amplifier, music is more relaxed but, at the same time, more dynamic. The tube amplifier sounds way more powerful than its approximate 30 or 35Ws per channel.

I'm surprised that the KEF R500s work so well with this vintage Frankenstein; you never really know what you are going to get with modern speakers.

Once I get over the excitement of having a new amplifier, I plug in some old Zenith 6SN7s that I have on hand. Maybe things will get even better-better.

 

 

Short Review: Lepai LP-2020TI amplifier

I'm in the midst of building a small home studio for recording synth and guitar.  My plan for monitoring was to use a pair of KEF Q350 speakers with a small amplifier.  With my past love of the Charlize and other Class T amplifiers, and a low budget, I decided to give the Lepai LP-2020TI, available from Parts Express, a try.  For $25 it is hard to go wrong.  If I didn't like it, no big hit to the pocket book.

This thing is tiny - like the size of two packs of cards.  It is also extremely light; helped by the 12V outboard power supply.  Build quality is just okay.  I'm not a big fan of the spring clip speaker wire connectors or the tone controls; which can thankfully be bypassed.  The light-up volume control is gimmicky too but no real major complaints given the silly low price.

In my garage system - replacing the venerable Adcom 545 - I was surprised how good CDs from the old Pioneer DVD player sounded.  There was a toe-tappin' excitement to the music with plenty of kick.  The Wharfedale Denton speakers are a good match with this amplifier, making more volume than I expected.  There was a nice speed to the sound too.  Over all I would give the slight nod of the Lepai over the Adcom, but we are talking a garage system so let's not get too carried away!

I originally wasn't thinking of plonking this little amp into my main system, but given the amount of free time I have due to being furloughed, I decided to give it a try.  Initial impressions were rather eye opening.  This is one very nice sounding amplifier.  But a few minutes of listening, even with my Dual CS5000 and Schiit Mani phono, I could hear a loss of detail and blurring compared to the Aleph J.  Bass of the Lepai was also a little rolled off and there was some solid-state grit and forwardness on top.  But to be fair, the Aleph cost in the neighborhood of $900 to build; a far cry from the dinner-for-one price of the Lepai.

Conclusions?  It's a good amp for cottages, garages small apartments, dorms, or where critical listening is not of upmost importance.  I could easily imagine a system with a Rega RP1, Schiit Mani, the Lepai, and your favorite budget mini-monitors.  Such a stereo would best a lot of ones that I've heard (or even owned!). 

Garage System:
Pioneer DV-V7400 DVD/CD Player
Schiit SYS passive linestage
Adcom 545
Wharfedale Denton 80th Anniversary
Belden 9497 speaker cable
generic interconnects

Main System (second turntable)
Dual CS5000
Schiit Mani
Classe Five preamplifier
DIY First Watt Aleph J amplifier
KEF R500 speakers
Cardas Twinlink speaker cables
Cardas Neutral Reference and Iridium interconnects

Review: Classe Five preamplifier

 
Introduction:
Given my heavy valve past going with an upscale solid-state preamplifier was a difficult choice.  Though certainly not a heart-wrenching one as I do have more recent experiences with Threshold, Adcom, and even some homebrew gear.  And since I was considering going all separates, different preamps for both phono and line, my choice to buy a full-function Classe 5 may also be a puzzling one.

Classe, however, is one of those companies that have been on my radar ever since I was a twenty-something reading Absolute Sound and Stereophile.  Back then Classe was considered very "tube-like" and warm, completely opposite to the common complaint of much solid-state gear: harsh or strident, especially in the treble region.  Such descriptions, like the warm sound of early Mac SS gear, piqued my interest.  But since I was a poor college student, living on a diet of vintage tube amps, Mac 'n' Cheese, and $2.99 records, anything in the price range of a Classe DR-5 or DR-6 preamplifier was unthinkable. So instead I soldiered on with DIY gear, plenty of Dynaco PAS preamps, and an Eico HF-85.

Well doesn't time fly?  Perusing through the Audiogon ads, I saw a recently listed Classe Five preamplifier.  It had a phono stage, switchable from MM or MC cartridges, and even a remote control.  Their isn't much information out there on the Five but some search engine sleuthing and I found it is essentially an upgraded 1990s version of the original DR-5.  It also features balanced (XLR) or single-ended (RCA) outputs; a single set of balanced inputs too if I ever needed them.  The XLR outputs will come in very handy for my planned bridged ACA amplifiers that are being built sometime very soon.

With the price being right, I put an offer in and it was accepted.  Given the proximity of the seller, I received the preamplifier the next day.  Unpacking the Classe Five and I found myself with a very heavy duty unit that was nearly flawless.  The switches, which have a wonderful tactile feel, worked perfectly, as did the remote control and the various knobs.  The wireless remote has buttons that causes little motors on the volume and balance to physically turn the knobs.  Very neat (I'm still an adolescent at heart!).  This is also great for adjusting the gain to get that perfect playback level from the comfort of my favorite listening chair.

Initial Impressions:
It is always exciting to listen to something new - but the Five wasn't exactly earth shattering.  The sound was dark, a bit muffled, and seemingly missing some bite on the top end.  Bass, however, was controlled and the overall sound was smooth.  With only an hour or so of time, I went to bed, leaving my new preamplifier in standby mode since there is no power off button.

The next day, after work, I heard an improvement in sound.  The smoothness was still there but everything sounded a lot more cohesive.  I fought the urge to do any serious listening, waiting for a quiet Sunday afternoon where I could experience the Classe Five without any familial distractions.



In my doddering middle-age I've become a bit of a Chet Baker fanatic, even exploring his musical output beyond his 1950s heyday.  Nonetheless I still have an appreciation for his signature album, Chet.  It's a brooding masterpiece that has the fingerprints of Bill Evans all over it.  Listening to this album - 1980s Riverside pressing - reveals a lot more detail than I was hearing through the Adcom GFP-555.  The Classe Five does a better job of revealing the light cymbal work, which sounds deeper in the soundstage but with the attack and decay being more apparent as is the metallic nature.  Too many systems turn cymbals into a hard SHHHH noise, instead of the shimmering, complex sound it should be.

Instrument - piano, the brass instruments, and drum - have a more natural body than my old preamplifier.  The overall sound is very smooth but doesn't inhibit the more aggressive tones of the music.  It is a different type of smoothness than I ever heard; not imbued with a golden hue like a vintage tube preamplifier but more as if the sonics were cut from a single, grain-less cloth. 

Willie Nelson has a rich and immediately familiar voice.  On Stardust, the vocals come across as completely natural with just a hint of studio reverb added.  The instruments remain firmly behind the singer, reproducing the very intimate nature of this album.  Tonality is some of the best I heard, exceeding my expectations given the budget nature of the speakers and my turntable and cartridge. Cohesiveness is another word that came to mind, and one that kept popping up as made notes.  No single instrument seemed crowded out while another took the center of attention.



For something more dynamic, I turned to one of the greatest live blues albums ever recorded: Drinkin' TNT 'n' Smokin' Dynamite which has an all star backing band behind Buddy Guy and Junior Wells.  The Classe Five captured the audience background noises allowing me to place them in location and distance from the stage.  The dynamic shifts, given the limitations of the Wharfedale Denton speakers, were impressive; never sounding bloated or blurred.  An example of this is the stuttering starts and stops of the lead guitar, along with the deep bass foundation.  Each instrument had its own space but jelled together as one whole, making for a very toe-tappin' experience.


After that good time, I needed a melancholic change of pace.  Richard and Linda Thompson deliver this in spades with their album Shoot Out the Lights.  Linda's voice is sublime here, all barely hidden pain while the barbed wire guitar of Richard grinds away as if stripping the heart out of his troubled marriage.  His backing vocals sit separate from hers, as if he is giving his ghostly agreement.  This album, needless to say, a heart-wrenching series of songs.  The Classe Five does an excellent job of baring these emotions.





I listened to several other albums of varying quality - like The Fixx, Wardruna, The Police, and even some old 1930s-era Billie Holiday.  Poor recordings sounded like, well, poor recordings.  And mediocre ones weren't given any extra magic either.  The Classe Five appears to be rather neutral, low in coloration, and even-handed.  This one is a keeper.


Takeaways:

The Classe Five has the best treble I have ever heard from a solid-state preamplifier.  At first I thought it was rolled-off; lacking in air and excitement.  However further listen revealed that it is actually a lack of glare and edginess that is often common in lesser solid-state gear.  If a recording is bright, you will hear that.  If it is dull, or poorly recorded, that will be revealed.  There is no heavy coloration going on here.

Along with this natural treble comes the feeling, from top to bottom, that all frequency points are sonically similar; that is to say no part of the sound, be it treble or bass, sticks outs or draws attention to itself.  As I mentioned above, everything is cut from the same sonic cloth.  It's something I never quite heard to this degree with any preamplifier that I've ever owned and took some time to understand this new experience.  A similar situation occurred when I first heard the VPI Aries 1, only then did I hear the warble and background rush of lesser turntables.

Bass control is excellent - tight and dynamic without any bloat or smear.  This is essential for providing the foundation of the song.

Detail is very good; though some listeners may prefer a more hyper detailed presentation.  Perhaps this is the limitation of my front-end or the nature of the Wharfedale speakers.  Some future upgrades will see how well this holds true.

The quality of the Classe Five makes me question, to some degree, my allegiance to valves.  At least with valve gear you can roll in different tubes to find the best match with your system.  But, if you're tired of buying high priced NOS valves or even new stock, then give the Classe Five a try. You may be surprised.


System:
Adcom GFA-545 amplifier
Dual CS5000 turntable with Ortofon OM20 phono cartridge
Pioneer DVD-V7400 DVD player
Kimber 8PR/4PR bi-wire speaker cables
various budget interconnect cables

Short Take: the Adcom 545

(picture from Google Images)

My old McIntosh MC2100 is in desperate need of a rebuild.  This poor amplifier, after all, is knocking on forty or more years of usage, which means capacitors, resistors, and other bits are due for replacement.  This age has shown up as "vintage" warmth, but also a rolled-off top, plummy bass, and some signal degradation.  To put it bluntly, this amp sounds old and tired.

With that in mind I decided to replace the Mac with a partner to the Adcom GFP-555, the venerable GFA-545.  Of course the 545 is no youngster either, but it is inexpensive and can also be stacked on top of the preamplifier.  Given my tight listening space this was the biggest reason to go this route.

The Adcom GFP-545 is a conservatively rated 2-channel power amplifier making 100WPC.  5-way binding posts, RCA inputs, captive power cord, etc.  Very basic and no frills with only the mentioned connections, an on-off switch and indicators if either channel is clipping.  Designed by Nelson Pass, it sits between two other models, the 60WPC GFA-535 and the 200WPC GFA-545.

In the 80s and even up to the 90s, Adcom amplifiers were mainstays in many, many systems,  My father still has an original 555 running in his downstairs home theater system, so that speaks of some good reliability.  But, like my McIntosh 2100, they are getting to the point where some maintenance is due.

How does it sound?  At least in my budget system, very good.  With the fresh power supply and direct coupling, the Adcom had much better and tighter bass than the McIntosh.  It does lack some of the warmth, but the clarity and detail is higher along with a less grainy presentation.  Top end is extended, perhaps a little etchy compared to the best tube amps, but nothing that will drive the listener out of the room.  So a good basic power amp that can be used in a variety of systems.

Ultimate fidelity?  In a word: no.  But my current system is a whole bunch of compromise, and with the soft presentation of the Shure M97xe / Dual CS5000 combination and the nature of the Adcom GFP-555 preamplifier, it somehow all works, even blunting the forward character of the B&W 805s,

So for now the Adcom 545 delivers music and does it in a way that I find inoffensive.  But with tube amps coming soon it's time in my system is short.

New Project: The Command 1625 Deuce Tube Amplifier

It's been a few years since I've built a DIY amplifier, so I approached this project with some trepidation. With my new house with no dedicated listening room, I went from a multi-kilobuck system to something considerably more down market. But as always, the itch to get back into tubes was too much for me.

So what to build? I'm an audio cheapskate at heart - not because I don't like spending money - but it just irks me to throw down too much money for output tubes. Even the price of an old EH 300B has risen a lot the past few years. So with that in mind I decided to use the 1625, which is a 12V filament version of the venerable 807. Most available 1625s were made during WW2 and used for fighter plane radio sets. There are still scads of them out there and the prices are cheap. I've also used this output tube with another amp and liked the results. So why not use it again?

And for those keeping track, this is the second time I've built an amplifier using the 1625, so I added Deuce to the Command 1625 name.

I know I wanted some power which gives me greater flexibility when it comes time to replace my B&W Matrix 805 speakers. So I decided to do something different and parallel the two output tubes. Guesstimated power output would be 16-20Ws if I run the 1625 in pentode (ok Tetrode!). So I scoured the web for ideas, checked out different schematics and asked a few questions. And I came up with the following design (no schematic yet):

12HG7 pentode driver, parallel 1625 output tubes into a 2500 ohm Hammond 1627SEA, a real beast of an output transformer. No global feedback around the transformer, instead I decided to experiment with plate-to-plate feedback.

Screens for the input and output tubes are regulated via gas tubes. Raw power supply is a simple CLC filter using a 5AR4, large polypropolene capacitors, and a 6H choke.

Off on switch, gain controls and banana jacks for 4-8-16 ohm speakers. Also current meters to see the health of the output tubes since in the past I've had gassy 1625s that went into runaway.

Once I was done with the design, it was time to start gathering parts and figure out top plate dimensions and where to drill the holes. For a clean, non-DIY look I went with Front Panel Express.

Soldering everything together was a matter of patience - and my usual sloppy spaghetti point-to-point wiring. I would only devote an hour or two a day to minimize mistakes. I also ran into some fitment issues that required some uh, judicious modifications.

The first time I turned the amplifier on, I was a nervous wreck. As I said it had been a few years since I built anything. Was I going to get a ton of hum? Distortion? Some problem I couldn't track down? Or worse, a whole bunch of smoke?

Well there was no smoke but I did get some hum and a lot of distortion. And so began some very frustrating troubleshooting. Long story short, plate cap tubes made for radio transmitting like to make a lot of RF. Two tubes in parallel only compounded the issue. I'm no expert HAM radio guy, but the differences between the two output tubes may have caused a tank circuit, or something weird like that. Luckily there was a solution - add carbon composition anode stoppers to the output tubes. Since these are plate cap tubes the resistor and small hand-wound coil will have to be added to the lead. Once I did this the distortion and hum dropped to the point where they were no longer audible.

Now I have to build another monoblock and make some cleaner plate cap leads since I'm no fan of exposed voltage.

Sound? Since with this amp I can only listen through one speaker I can't say much about soundstaging depth. But using an extra DVD player and a single B&W 805, I heard a very clean 'n' clear presentation with a good top-end and good bass control from the 4-ohm tap. Not very "tubey" warm sounding but more neutral than that

Stay tuned for Part 2. where I build the second monoblock, give a schematic, and do some listening tests.

Review: McIntosh MC2100 power amplifier

It was vintage McIntosh tube gear that launched my interest in high-end audio which is a bit funny since - and I'll be sure to get some flack for this - these older preamplifiers and amps aren't necessarily the greatest in the world.  But I will give them credit for making a piece of gear that is certainly memorable; the lit glass fronts and layouts are visually striking while the sound coming out of the speakers is warm and robust.

As I already mentioned in the previous post, I've sold off my previous system, retaining only the UREI 813A loudspeakers - for now.  Part of this was due to the location of the system - the dark basement isn't exactly my favorite place to hang out - and the realization that I was spending thousands of dollars on something that was only being used a few hours a month.  So with that in mind, I decided to simplify; concentrating on the musical aspects of reproduction instead of the ultimate fidelity.  Since the B&W 805 speakers like power I thought it best to replace the Nakimichi receiver in my upstairs system with something with a little more drive.

I remembered how much I liked my departed McIntosh MC-250.  I also noticed that prices are rising on these vintage amplifiers.  But my luck held out and I managed to find the 250's big brother, the heavy-duty MC2100 for only six hundred dollars.  It has twice the power and the same utilitarian (some might say industrial) look of its smaller sibling.  When funds permit, I may go for a matching McIntosh preamplifier - though of a later vintage - and tuner, but for now I've gone the budget route with a first-generation Adcom GFP-555.

The McIntosh MC2100 is rated - very conservatively - at 105WPC and features autoformers on the output.  It can, at least according to various sources, easily exceed that value.  And after hooking it up to my B&W Matrix 805 speakers, the first thing that I noticed was more bass oomph and dynamic drive.  The little 5" woofer of 805s don't exactly move a lot of air, so this was a pleasant surprise over the Nak receiver.  Perhaps it is the nature of having an autoformer on the transistor output stage, but this bass had a slight (and I mean very, very small) loosey-goosey nature.  It wasn't sloppy, but was just a touch less tight and less sterile than the Japanese receiver.  In my book that's a good thing.

With these early SS pieced of McIntosh there have been many comparisons to tubes.  The midrange, with well-recorded pieces like Steve Forbert's Jackrabbit Slim, or Gary Numan's Splinter had a warm character, but lacked that ultimate finesse and clarity that the best tube amplifiers have.  Instead I'm vaguely reminded of the venerable B&K ST-140, which has a (non-linear?) dc-blocking electrolytic capacitor on the input stage.  It adds some "FM Radio" character that is pleasant and conducive to long-term listening but should not be mistaken for uber-fidelity.  Of course that's not what I'm striving for here so there are no disappointments in this area, but just something to be noted for any readers looking for the "ultimate" amplifier.

It is the upper-mids and treble where most solid-state amps fall on their face.  I would give the McIntosh MC2100 a high passing grade here.  It suffers from the sins of omission, clouding some detail and rolling off the treble.  Again we are talking about musicality over the ultimate extension and pin-point imaging.  A rough analog: perhaps the MC2100 is closer to a good moving magnet instead of a hyper-detailed moving-coil cartridge.  As a side note: when I had a MC250 around, I noticed some grain in the texture, but, at least with this current setup, I'm not hearing that same kind of effect

Imaging and depth are good but not superlative.  Of course some of this is the fault of my speaker setup, the limitations of the Dual CS5000 turntable and Nagaoka MP-110, and perhaps the use of an Adcom preamplifier, which isn't exactly the best in the world.  Future upgrades will give a clearer picture of the limitations of this amplifier.

At least to my ears, the McIntosh MC2100 is a fine solid-state piece of gear if - and this is an important point - one is not reaching for the ultimate fidelity, but instead prefers emotion.  It is here where this amplifier shines, being a sort of "poor man's tube amp."   No, it doesn't sound like a Dynaco 70 or even come close to my departed Eico HF-60s, but it does capture the essence of the musical performance and minimizes the worst aspects of a budget system.  So in that regard, the McIntosh MC2100 is a clear winner.

System:

Dual CS5000 turntable with Nagaoka MP-110

Pioneer DVD-V7400

Adcom GFP-555

B&W Matrix 805s on stands

Wire: Various brands

Review: Nakamichi SR-3A receiver

For some strange reason I decided to simplify the second system, replacing the odd couple Rotel integrated and Sansui tuner with a receiver.  Now I've never been a big fan of the "all in one box" approach to audio components, but having a single device handle the amplification chain and radio certainly takes up less space than all separates.  After cruising through several hundred items on Ebay, I finally zeroed in on a Nakamichi SR-3A receiver.  Why?  It uses a "Stasis" output stage, the very same topology that my now departed Threshold S/500 had.  This circuit was designed by the great Nelson Pass who continues to make some of the most interesting amplifiers around.  Though obviously built to a price point and lacking the muscular grunt of the Threshold, I thought the Nakimichi would provide good enough service for less intensive listening.

When I received the package, I opened the box and found a rather Plain-Jane receiver in that most boring of colors: black.  Hooking it up was a bit of a nightmare - the receiver depth is larger than expected, making speaker and RCA cables with large jacks almost impossible to fit because of the limited Ikea Expedit shelf space.  Also the banana jacks in the back really don't have any metal contact and are too small for the banana plug(!).  So in the end I ended up using insulated wire twisted together.  Other than that, build quality is pretty good - though not exactly McIntosh.

Once I finally had everything in place, I could finally turn the Nakamichi on.  There is a fairly dim display for the radio station, tuner signal strength, and that's about it for the light display.  There is an input for turntable (with a switch for MM and MC), CD, Video, and Radio.  The tuner portion is switchable between AM and FM (no surprise there) and stations can be programmed into memory.  Bass and Treble controls are defeatable.  There is a loudness contour button and two speaker selections: A or B.  The right side is dominated by a volume and balance combination - the inner ring controlling the former while the outer ring does the latter.

Listening to a record, the sound coming out of my B&W Matrix 805 speakers took on a different character.  The mids were cleaner and leaner, but, strangely enough, the lowest bass seemed to have more depth and control.  The top end had real "presence" - ie, slightly forward in it's presentation.  The phono section itself seemed to be very good with excellent overload characteristics and lacking that op-amp etch that many receivers have.  Though only rated at 45WPC, the Nakamichi SR-3A seemed to punch much higher than one would expect.  There was more than enough volume for my not-quite efficient B&W speakers and mid-sized room.   Dynamic contrasts were also more evident, making this receiver a good match for these classic monitors.

The tuner - which is popular for listening to classical music - was excellent.  I'm normally not a fan of digital tuning, but the Nakamichi really locked into the signal, easily bettering my analog Sansui AU-777.  Music from the radio also seemed more lively compared to my old integrated/tuner combination.

In conclusion, the Nakamichi SR-3A is one of the better "vintage" receivers I've heard.  It certainly doesn't sound overly warm or suffer from solid-state harness.  Instead, it's quite even-handed, even with my finicky B&W tweeters.  The Stasis output circuit really does remind me of a mini-Threshold amplifier; giving the user a taste of the real high-end.  Highly recommended.

Second System:

Analog: Dual CS5000 turntable

Digital :Pioneer DVD-V7400

Speakers: B&W Matrix 805s on stands

Wire: Various brands

Project: Mult-Valve: The Universal Pentode Amplifier

Introduction:
It seems like a long time ago, but I remember when the first Sound Practices magazine came out.  For many American audio nuts, the idea of a using an ancient triode like the 300B in single-ended mode was a new revelation.  Of course this was merely revisiting an old technology, spruced up with some modern twists and a hearty dollop of audio hipsterism.  I easily fell into this new movement, making my first ever piece of DIY gear, a 71A linestage using dual-mono mil-spec potted transformers, gas tube regulation, and some vintage oil caps on the output.  After that, it was a series of amplifiers, including PP 6B4Gs and single-ended 2A3s.  All this on a college student budget.  Once I had a real job, I built was my first 300B amplifier using Thorsten Loech's SV83 pentode driver.

But after a while, my interest in this triode movement began to wane.  It was a problem of speakers:  Low power amplifiers require efficient drivers and my experience with the lower priced offerings, including vintage coaxes and more modern designs, left me unfulfilled.  My first experience with audiophile gear included the wonderful Quad ESL-63 electrostat speakers, and nothing I bought seemed to meet that detail, speed, and snap that I remembered so fondly.  I did, however, found myself willing to compromise, thoroughly enjoying the UREI 813A loudspeaker, which ideally requires more power than I could get out of a common single-ended triode running at sane voltages.

With the UREIs, the best all-around amplifier I've heard to date are the Eico HF-60s.  It's a classic 1950s ultralinear design, but runs the EL34s in heavy Class A.  There is a dynamic ease and sense of unlimited power with these classic 60W monoblocks.  Of course some modern components, the addition of a choke in the power supply, and a "battery bias" mod on the EF86 really brought these ancient wonders up to a new level.  But a close runner up was the 1625 amplifier I built, another Class A wunderkind that sounds much better than one would expect considering the budget build cost.  I began to wonder if I could make an amplifier that would make enough power to drive the UREIs, provide class A output, and have the ability to roll in multiple types of output tubes.  The concept of Multi-Valve was born!

The Design:
Most single-ended amplifiers use Cathode or self-bias.  This is an easy way for the circuit determine the idling current of an output tube.  But since I wanted to roll in different sorts of valves, I decided to go with fixed bias.  This decision complicated the power supply since an adjustable negative voltage is need to put across the grid of the output tube.  This also changes the requirements of the driver tube since output tubes using fixed bias need a smaller grid leak resistor.  That meant my favorite pentode driver was out of the question.  Instead I went with the 5687 dual triode which is a low mu tube that would work well cascaded.



For the power supply, I was limited by the small size of the chassis I had in mind.  So I went with a a simple CLC filter using a 5AR4 rectifier, a Solen, a Triad Choke, and then a larger motor run polypropolene capacitor.  RC filtering with some nice Panasonic capacitors and wirewound resistors provide the lower voltage for the driver stage.  Filament is all AC, but I referenced it to ground using a pair of 100 ohm resistors.  The bias supply is a standard Dynaco-type half-wave solid-state rectifier and some CRC filtering.  A pair of linear potentiometers provided the necessary adjustment, while current meters let me dial in the current of the output tube.  With Pentode amplifiers, a reduction in distortion can be found by the use of a regulated screen power supply.  To meet this requirement, I went with a pair of VR tubes connected in series.

Parts:
Instead of Edcor output transformers with their single output tap, I went with potted James 6123HS, which have the outputs for 4, 8, and 16 ohm speakers.  They also look really nice.  However, for the power transformer, I went with Edcor, but bought plain steel bell covers to match the rest of the metallic look I was going for.  Except for the octal sockets, all the small tube sockets are NOS.  A motor run cap suitable for 500VDC was selected, along with some small current meters, miniature pots for the bias control, and some Tocos volume controls.

The most difficult part of building a tube amplifier is the metalwork and chassis.  Instead of the common wood sides, I wanted something different.  Searching through Ebay, I managed to find a nice Chinese unit that included a bottom and top plate, holes drilled in the back for speaker binding posts, an IEC connector, and a pair of RCA jacks.  It's about the size of a Dynaco 70, but with the size of the output and power transformers, I knew it was going to be a tight fit.  To make the top panel, I did a lot of measuring and made several different versions using the Front Panel Express software.


Testing:
Once I had everything wired up - and it was a tight fit getting everyting inside that small chassis - it was time for some testing.  I first tested the bias controls, making sure that the maximum voltage was set.  I then plugged in all the tubes - GE 5687s, Sovtek 5881WXTs, a GE OA2, and a GE OB2 - except for the Shuguang 5AR4.  Leaving the rectifier tube out allowed me to check that I hadn't messed up on the filament wiring.  Once that checked out okay, it was time for the smoke test.  After I plugged in a pair of cheap test speakers and a running CD player, I had the DMM ready to go with the ground lead already attached to a ground point.

Is there anything more exciting than turning on a brand new tube amplifier?  Okay, don't bother to answer that question - of course there is!  But I still get a little rush even though I've been at this still hobby for 24 years now.  To my surprise there was no smoke or flames shooting out the top.  Instead there was a nice purple glow of VR tubes and that warm orange filament color.  After some quick voltage measurements, I adjusted the bias upward and music started flowing out of the little 4" woofer of the Pioneer test speakers.  Ah, it sounds like magic.


Listening:
After some further measurements and some very minor fixes, it was time for some listening.  The Eico HF-60s were unplugged and disconnected from the system.  Up went the Multi-Valve amp and on went the Cardas wiring.  I replaced the Sovtek 5881s with a pair of vintage Tungsol 6550s which required a bias change.  I started with a Schiller CD on the Sony player, adjusting the gain knobs until I found a suitable match with the Quicksilver preamplifier.  After I was happy with that, it was time to spin some vinyl.  I started with some Tom Waits, checked out some Willie Nelson, and ended up with some Police.  Initial impressions was an amplifier that sounded a little lean, but had plenty of detail, very good bass control, and a suprising amount of dynamics for such a relatively low-powered amplifier.  There was a real 3-D sound with the vocals projected in front of the speakers.  A good beginning...

Of course this amplifier was still incredibly fresh.  Except for the tubes, all the parts were brand new.  So over the course of the next few nights, I used this amplifier to provide background music as I worked out, played video games, or started to brew another batch of beer.  As the hours went on, the lean sound began to recede and was instead replaced by something quite neutral.  I also tried different pair of output tubes that I had lying around - Mullard and Shuguang EL34s.  Both provided very pleasing results, but the vintage Tungsol 6550s seemed to sound the best.


No More Games!
Now it was time for some serious listening.  For this, I invited my fellow local audiophile friend over.  He has a great stash of quality records and also had some different vintage output tubes to try out.  I also replaced the rectifier with a 5AR4 made by Mullard.  Here are some notes from this session:

The first pair of output tubes was a pair of vintage Mullard XF2 EL34s.  Listening to Neil Young - Live at Massey Hall, the darkness and classic warmth of this venerable pentode was obvious.  There was a slight loss of detail, but the overall effect was quite musical.  Some of the macro dynamics seemed a touch restrained.

Next up was the Tungsol 6550.  This pair was from the early 1960s with the grey plates with three holes.  Dynamics really improved here with tight and well-defined bass.  There was also a sense of ease to the music, making the amplifier sound much more powerful than the estimated 15Ws.  Treble detail and extension wasn't the best I've ever heard, but it was still very good, but perhaps lacking that last bit of shimmer and liveliness.  Robert Ludwig's mastering of The Band - s/t album was the best I've ever heard it, managing to find clarity in this darkly recorded album.  On the other hand, Donald Fagan - The Nightfly still had speed and the latent fingerprint of a digital recording.

For the next record, a pair of 1950s Dutch Philips EL34s was selected.  These metal base tubes are quite expensive these days, but there was some real magic here.  Listening to Cat Power - Jukebox was an amazing experience, pulling me into the music in a way that made it seem real and alive; as if I were sitting thirty or forty feet away from the band in a small hall.  There was an evenness to the music from the top to the bottom without any  part of the frequency spectrum sticking out in an unnatural way.  Dimensionality and soundstaging was some of the best I've ever heard.  Even more surprising was the controlled but taut bass through the 15" Eminence woofer.  Very impressive.  These tubes appear to be made for this amplifier.

The following experiment was not much of a success.  A fresh pair of early 1950s Tungsol 6AR6s was selected, along with an octal socket adapter.  The sound on Steely Dan - Greatest Hits and The Who - Tommy, became very uninvolving and flat.  Perhaps some more break-in time was needed or a different bias point.  It was time to move on and try something different.

1970s Gold Lion KT77s (with black bases):  These sounded much like the Tungsol 6550s with excellent bass control and incredible dynamic range.  It was, however, a little richer, hewing somewhere between the dynamic Tungsol 6550 sound and the classic richness of the EL34.  Neil Young - Tonight's The Night delivered all the tortured darkness that I expected with a sense of realism that was inviting.

For a more budget choice, I also tried a pair of SED 6L6GCs, one of my favorite modern production tubes.  Though there was a slight loss of the last bit of detail, the overall sound was very pleasant with only some slight overlaid grain over the music.  The explosive dynamics on Blue Nile - A Walk Across the Rooftops were really a surprise, while the Police - Synchronicity kept the rhythm and pace of the music together in a bouncy and engaging way.


The Scorecard:
Bass: Normally a weakness of tube amps, and even more so for single-ended, the Multi-Valve was a real surprise here.  The lowest notes were deep, well-damped, and had excellent control without the slight tubbiness I normally associate with tube amplifiers.  This is a testament to the James output transformer and the feedback loop controlling the woofer.  Not only was there depth, but detail, whether it was the smack of a tom-tom, or the playfulness of a skilled bass player.

Midrange: Clear, defined, and transparent are the first words that come to mind.  The music is served in an even-handed fashion without tube bloat or an overly analytical presentation.  The quality here is quite dependent on the output tube used, ranging on the scale from amazing (Tungsol 6550s and Philips EL34s) to a more middling sound (Mullard EL34s or SED 6L6GCs).  But at no time, except for the 6AR6s, was I actively disappointed by the music coming out of the speakers.

Treble: This again was dependent on the output tube selected for use, with varying levels of quality.  I preferred the Philips EL34s and the Gold Lion KT77s here, while some listeners, depending on the speakers used, may prefer the warmth of the Mullard XF2 or the qualities of the Tungsol 6550s.  At its best, the Multi-Valve amplifier treble is perfectly integrated into the midrange, not calling attention to itself, but not lacking in detail or extension.

Soundstaging: Whoah!  Images are way beyond the edges of the speaker, making my listening space sound larger than it is.  There is also an incredible amount of depth.  Vocals are also projected forward in a nice fashion, keeping separate from the rest of the music.  This clarity comes from the lack of sonic mud, allowing the listener to "see" deep into the musical performance.

Intangibles: Considering the limited power that single-ended can deliver, one of the biggest revelations was the handling of macrodynamics.  Even at lively listening levels the amplifier never failed to deliver the wattage when needed.  This, along with the various microdetails and instrument shadings, gave a unique presentation that makes so many other tube and solid-state amplifiers sound cloudy or unfocused.  The Multi-Valve isn't stubbornly analytical either, bleaching out warmth or lacking humanity.  Instead the presentation is incredibly balanced.  An amplifier like this would be a great tool for mastering or component evaluation.


Conclusion:
There really is something special about single-ended designs, whether they are triode, ultralinear, pentode, or even solid-state.  It's the Class A power that really matters in the end, along with the uncomplicated driver circuits and simplified power-supply requirements.  A pentode, at least with regulated screens and negative feedback, gives a different presentation than, for example, the classic WE91 300B design.  Of course listening preferences, speaker selection, and source components will greatly effect the best amplifier to use in a given system, but with mine, the Multi-Valve is perhaps the best I've ever heard.  Really.  It's also great fun to sample different output tubes and the unexpected differences between them.  Highly recommended.

Parts list and Front Panel Express layout available upon request.


Review System:
VPI Aries with JMW 10.5i tonearm and SDS Power Supply
Dynavector 10X5
Cardas Cross 1M interconnects
Quicksilver preamplifier with Mullard short-plate 12AX7s, RCA 12FQ7s, Amperex 12AU7
Cardas Quadlink 5C 1M interconnects
EICO HF-60 monoblocks with Mullard XF2 EL34s & 5AR4s, GEC CV4085s, and GE 6SN7GTAs.
Cardas Hexlink speaker cable
UREI 813A monitor speakers
VTI BL503 equipment rack

Refurbishing a vintage Sony TA-3120A solid-state amplifier

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The original Sony TA-3120 is quite the ancient (solid-state-wise) piece of gear.  As far as I can tell, the original TA-3120 amplifier was released in 1966, the start of the decline of vacuum tube technology.   It was this model, along with many others, that was also the beginning of the end for American stereo manufacturers.  Thanks to truly portable radios, the Japanese had a leg up on making solid-state gear that didn't blow up and could provide countless hours of trouble-free operation.

(right click to see larger image - or check out the service manual)
Note: this is the 3120 schematic, the 3120A appears to be slightly different.

The TA-3120 and the latter TA-3120A features very simple circuitry, looking quite valve-like in operation.  A quick look at the schematic reveals a very simple single voltage power supply, a single transistor gain / and single transistor phase-splitter, two pairs of output transistors per channel, and a large capacitor coupling the transistors to the speaker.  My own interest in these types of amplifiers are purely historical.  Though I believe a good tube or a modern circuit will lay waste to these early units, I also have an interest on hearing just how good these things could potentially sound.  Those vintage electrolytic capacitors, especially back then, weren't particularly well-made, not compared to the tightly wound models of today.

These stand-alone "one third" amplifiers seemed to have been rarely used with a preamplifier - instead they were supposed to be mated with integrateds, powering the back channel in early Quad / primitive surround sound systems.  Making a decent - for the time - 50 watts per channel this amplifier will have enough moxy to drive most speakers, provided the they don't dip below 4-ohms impedance.  The capacitor coupling will also save woofers if the output transistors decided to go south.  I also like adjustable gain knobs in back, perfect for matching up with any number of different preamplifiers.  With that in mind, along with the handsome chrome shoebox looks and potted power transformer, I decided to buy one for a paltry $129 and see how it sounded.

  

Initial listening results through the UREI 813A speakers were unfortunately short-lived.  However, before the problem came to light, I heard an amplifier that sounded pretty good but a tad "confused" with not very good control over imaging and soundstage placement.  There was a "blur" to the sound, along with a recessed midrange and a slightly tizzy treble.  Bass response was half-decent though definitely not approaching the uber-depth and control of my old Threshold S/500 or even my Eico HF-60 tube amplifiers.  But still, I would rate these higher than stock Dynaco ST-80/120s or even many vintage receivers I've heard.  I could tell the fine fellows at Sony had put some serious thought into these amplifiers but were hampered by the technology of the time.

A few minutes of listening and the sound suddenly cut off.  And then mysteriously it came back.  Only to be cut off again.  At first I thought some speaker protection relay - as seen in the TA-3120 schematic - was clicking off and on.  However, an examination of the 3120A innards showed no such circuit.  A quick check of the multimeter showed that the power supply was undulating from 62 to 82Vs roughly every two seconds.  As the voltage dropped, so would the sound.  This pointed to a potential capacitor problem - a device that stores and releases voltage.  To the Digikey site I went - it was time to replace every capacitor on the circuit board.  Like the original amplifier, I went with Elna (mostly Silmic) capacitors where possible.  I also discovered that the TA-1320A is quite different than the original 3120, with different values and some there not at all.  So if you need to do this job, record the capacitors needed from the components on the circuit board, not the schematic.

Yikes!  This was a nightmare.  Even though Sony had thoughtfully made it easy to pop off the three circuit boards in question, I still opted to keep the multiple leads intact.  This left me a few inches between the back of the PCB and the heatsink.  The solder traces are also extremely fragile and appear to be made of very thin copper - use caution!  When I was done, I had a nice pile of used capacitors and a much more modern looking board.

 

When I finally finished, I hooked up my Pioneer test speakers and took the output directly from the SACD player.   I turned the amplifier on ... and hurrah!... there was music!  I let it play and tested this power supply voltage.  This time it was a rock steady 86 volts, higher than the initial value.  Since I hadn't replaced the main bucket capacitor which I thought was the source of the original problem (and the most expensive fix) I was happy to hear undistorted sound.  The Pioneer speakers, however, aren't the highest in fidelity, but still good enough for some initial thoughts and component break-in time. 

Hmm... good bass response and a bouncy engaging rhythm.  A very pleasant background sound as I help my fellow marines hammer through the defense of Iwo Jima - courtesy of the virtual world of Rising Storm, a Red Orchestra 2 upgrade pack.  However, the old saw "all amplifiers sound the same" really does hold true, but only if the speakers in question can't resolve the differences.  It was time for a true sonic torture test, the venerable UREI 813A monitors with their compression horn tweeters, a speaker that has broken many an amplifier.

Hooking up the Sony amplifier was easy as pie - the back connectors work great with the mid-sized spades of the Cardas Hexlink speaker wire.  Large spades and banana plugs please do not apply.

The first cut was my original U.S. pressing of Tom Waits - Closing Time.  This recording is a tad dark and it showed.  The upper midrange / lower treble grit I normally associate with solid-state gear was pretty suppressed - you had to really listen for it to hear it.  Only a few non-tubed amplifiers truly get rid of it - at least according to my experience.  The next recording was the 180g re-issue of Ray Charles - Genius + Sould = Jazz.  I was taken aback by the sheer musicality of the sound.  I began to feel a tad suspicious, wondering if I was hearing one of the better amplifiers of my life.  Seriously.  The dynamic swings worked well and the spaciousness of the recording was evident; this sort of depth is something I normally associate with tube amps.  Hmmm....

Next up was a first pressing of Donald Fagan - The Nightfly.  Ah, now I could hear the limitations of this early digital recording.  The treble was noticeably splashy and forward.  The very deepest bass was not exactly missing, but not as low as I remember with other, more powerful amplifiers.  The separation of the vocals were not as defined as other amplifiers caused by a slight blurring and loss of detail.

Now it was time for something really dynamic - The Classic Records re-issue of The Who's - Tommy and my original pressing of Frank Sinatra - Sinatra At the Sands.  With these two recordings, everything was there and the music was still emotional as ever, but the lowest bass registers were just slightly underdamped and lacking - perhaps an effect of the music going through such a large electrolytic output capacitor.  The very top could get hard and uncontrolled when listening too loudly, the sign of an amplifier running out of steam - obviously 50Ws can only do so much.

Conclusions?  This is a very nice amplifier, especially with a bevy of new Elna capacitor replacements.  Perhaps it is the simple circuitry or the effects of the (fairly) non-linear output capacitor, but the Sony TA-3120A lacks much of the grit and harshness I associated with solid-state gear.  Sure I've heard better - notably a large number of tube amplifiers, but if I didn't know any better, I could really live with this piece of vintage amplifier.  What it lacks is the ultimate finesse of the best gear I've heard - detail retrieval, bass control, and soundstage depth.  However, it does - much like the B&K ST-140 but not quite as warm - convey the music in an enjoyable way.  Of course much of this may come from my VPI table and Quicksilver preamplifier; the flaws of the Sony diminished by the strength of the other audio gear.  Nonetheless, recommended for vintage junkies who want to try something different.

Review System:

VPI Aries with JMW 10.5i tonearm and SDS Power Supply
Dynavector 10X5
Cardas Cross 1M interconnects
Quicksilver preamplifier with Mullard short-plate 12AX7s, RCA 12FQ7s, Amperex 12AU7s

Cardas Quadlink 5C 1M interconnects

EICO HF-60 monoblocks with Mullard XF2 EL34s & 5AR4s, GEC CV4085s, and GE 6SN7GTAs.
Cardas Hexlink speaker cable
UREI 813A monitor speakers
VTI BL503 equipment rack

Building a single-ended 6CB5A amplifier

Introduction:
After the success and sale of my single-ended 1625 amplifier, for my next amplifier build I decided to return to the venerable triode.  However, I wanted a bit more power than your average single-ended 2A3 amplifier, but still didn't want to use the pricey 300B.  With that in mind, I looked at several push-pull designs using the 6B4G or 2A3.  Once again I ran into expensive tubes, even for Russian or Chinese versions, and pricey interstage iron.  However, after reading several threads on diyaudio.com, I came across Thomas Mayer's blog describing the use of the 6CB5A - a TV vertical deflector tube - in single-ended mode to make 7Ws.  He touted it as a budget alternative to the 300B, and at ~$5 to $6 a pop on Ebay, it certainly would cut down on the output tube cost.

Circuit:
Once I had the signal schematic in hand, I went and designed my own power supply circuit around a Edcor power transformer.  Note that the 6CB5A uses a mighty 2.5As of current at 6.3V.  This required an additional filament transformer for the 6N7 driver tubes, and, in order to reduce costs, I stuck with a 5V rectifier tube since the filament tap was available instead of the recommended TV damper diode which would require an additional transformer..

Parts:
Once again, this is a budget build, so Mouser and Allied Electronics were used heavily.  This means electrolytics used in series to get a high enough voltage rating, Cornell-Dublier metallized polypropolene coupling capacitors, plastic speaker binding posts, and plenty of KOA resistors.  Output transformers are 3.5K/6-ohm units from Edcor.  The power transformer was also sourced from them.  For the top plate I used Front Panel Express.  Wood chassis is from Valab, an Asian Ebay seller.

Construction:
This was an exercise in point-to-point wiring, which can be frustrating if one is not experienced.  A single star ground is used near the RCA input jacks.  Wiring is all plated solid-core.  Due to the simplified power supply and lack of regulation, this unit was much easier to finish than the 1625 amplifier, my last project.

Listening:
After a quick voltage and current check, I hooked this amplifier up to a pair of Pioneer BS-21 speaker that I use for test purposes.  At 84dB efficiency, these aren't exactly a great match for single-ended amplifiers, but at least I can tell if music is being made.  The sound, via my VPI table and Quicksilver preamplifier, was very smooth without any noticeable hum or other noise.  With my medium-output moving coil cartridge, I had to turn the volume control way up to get any decent playback level.  Of course such a low gain amplifier will prove to be useful for much more efficient speakers than the test Pioneers units.

After everything checked out, it was time to listen to the 6CB5A amplifier with the KEF iQ30s.  Once again my big UREI 813As, which would be a much better match, are in storage so I had to make due with what is on hand.

This amplifier is smooth, coherent and almost touches the better 300B amplifiers I have built.  The soundstaging in uncluttered with a deep and immersive soundfield - all traits of a triode amplifier.  The bass was also surprisingly well-controlled, lacking that fat underdamped sound that plagues some tube amplifiers.  Treble extension was smooth with a nice shimmer and swirl to high hats.  Detail was also very good, providing plenty of definition - eg, recordings sounded different and weren't congealed into a "it all sounds the same" blob.

Switching to the less efficient B&W 805s and I noticed the amplifier would run out of steam on big peaks.  Obviously not a good match, but at lower listening levels it was quite the pleasurable listening experience.

Conclusion:
Single-ended amplifiers - even using triodes - require careful speaker matching.  Many speakers are too inefficient and have wild impedance curves that require more power and low output impedances. However, the 6CB5A coupled to the Edcor iron did a decent job driving any of the speakers I have on hand.  Those into rock or heavy orchestra would benefit with more power or horn speakers, so keep that in mind if you're interested in building something like these amplifiers.

Rebuilding a pair of Heathkit W-2 tube amplifiers

 The Heathkit W-2M amplifier is a fairly rare beast compared to the more later versions of the Williamson line of amplifiers.  It is also perhaps one of the earliest incarnations of a commercial ultralinear circuit.  The earlier predecessor, the W-1, used 807s connected as triodes, with pretty much the same power supply and driver circuitry.  Of note, it's would be an easy project to convert the W-2 to match the W-1, since the layout and even the output transformers are so similar.

Anyway, for my W-2 rebuild I decided to stick with a retro-modern ethos: keep the same circuit but improve the parts and biasing method.  Since I consider myself a bit of an audio historian, I prefer to hear the amplifier as the designer intended.  If someone wants to tackle an improved Williamson design, I would strongly suggest the Chimera Labs modification which will also reduce distortion, provide cleaner power, and improve square wave performance. 

(right click and open to see more detail)

Power Supply Chassis: The stock power supply is rather anemic with barely enough capacitance to reproduce transients and extended output levels.  In my case, I replaced all of the electrolytics, also increasing the values except for the first bucket since it is attached to the 5V4 rectifier.  I also added bypass resistors to normalize the voltage between the two series capacitors.  Parts required per unit: (2) 22uF/350V capacitors, (2) 150uF/450V capacitors, (4) 470k/2W resistors, capacitor mounting hardware, and nuts & bolts.

(right click and open to see more detail)

Signal Chassis: Once again, new electrolytics, also increasing the values to improve power supply stability.  Potentially leaky paper coupling capacitors were replaced by new film units.  The input capacitor was removed and replaced by a small value (1K) carbon composition grid stopper resistor.  Aged resistors should also be checked to see if they are within specification.  This is especially important for the ones requiring matched pairs.  Parts required per unit: (2) .047uF/400V capacitors, (2) .22uF/600V capacitors, (2) 1K carbon composition resistors, (1) 47uF/450V capacitor with clamp and nuts & bolts, and (1) 22uF / 450V capacitor with two lug terminal strip for mounting.

 (right click and open to see more detail)

Bias Supply: The 1/4" phono test jacks were replaced by easier to use banana jacks and 10 ohm resistors which now allow the output tubes to be balanced by using a multimeter: set the meter to the lowest voltage range and adjust the 100-ohm potentiometer until a reading of zero is found.  This biasing method is pretty much straight from the Heathkit W-5 schematic and is much easier than trying to juggle current meters with the clunky phono jacks.  Parts required per unit: (2) 22uF / 150V capacitor, (2) 10 ohm, .5W resistors, (2) banana female jacks with washers and nut, and one terminal strip.

Miscellaneous: The stock RCA jack can easily be replaced with a modern gold-plated unit.  On the other hand, the single speaker screw terminal strip is a slightly more difficult proposition.  At this time I don't have an easy solution that doesn't require a drill press and new hardware.  For now I suggest using speaker wire with small "vintage" spades, or bare solid-core wire.

Listening results: The stock pair of Heathkit W-2s, even with ancient electrolytic capacitors, were still quite pleasant.  They sounded a little muddled and lacking in power, but still possessing a musical charm that many modern units have a hard time matching.  The excellent Peerless output iron certainly helps.  However, this simple upgrade really helped, making the bass firmer and the dynamics better; all while keeping the golden midrange and easy going treble detail.  With new parts, the W-2s really excel at musicality and now rate among my favorite vintage amplifiers.  At least in my listening situation, I would still give the nod to the Eico HF-60s, but that's due to the higher power and the overall sound of the EL34, which is a tube I prefer over the 6L6GC family.  But for a different listener and a different setup with more efficient speakers, it really would be a matter of personal preference.

Tubes: With the rated 4A filament supply, it is possible for the power transformer to handle an output tube with a little more heft than the 6L6GC.  Since I'm not one to trust vintage iron too far, I probably wouldn't go beyond a KT66 type.  I would be curious to see how a set of Gold Lion re-issue KT66s would stack up against the vintage Tungsol 5881s.  Another option for the cheapskate would be to rewire the output sockets to use the 6AR6 which is still cheap and plentiful.

With two 6SN7s handling gain and phase-splitting duties, there is a vast world of tube-rolling opportunities here.  For now, I'm sticking with GE 6SN7GTAs since they are easy to get and sound great for the dollar. 

The 5V4 rectifier can be replaced with the 5AR4 which will slightly bump up the voltage and offer some of that classic Mullard sound.  However, with the high price of good vintage versions, one may want to stick with the more common 5V4 with all the various manufacturers and versions.

Conclusion: Bringing an old tube amplifier back to life and ready for another twenty (or more) years of service is always a feeling of accomplishment.  The Heathkits aren't the easiest to rebuild, not with some of the strange capacitor can sizes and small interior chassis depth, but the time and work was worth the effort.  These are wonderful amplifiers with that vintage smoothness but still modern enough for electronic music and pop.  Highly recommended.