New Project: a SE 6CB5A amplifier

After the completion of my 1625 "Command" amplifier, I began to cast my eye around for another project.  This time I wanted to yet again tackle a triode amplifier, perhaps doing a push-pull 2A3/6B4G type.  However, the pesky cheapskate that lives inside my head just couldn't spend that sort of coin on a set of output tubes.  While searching around, I read a thread on diyaudio that lead me to this blog post on Vinylsavor.  Check it out for the signal schematic.

The 6CB5A is a 1950s television sweep tube that wasn't designed specifically for audio, but the curve traces in triode mode are quite good.  This cheap tube can also produce 6-8Ws in single-ended, which is pretty close to the mojo of a 300B.  I also liked the idea of using the 6N7 driver, thought I opted to not implement the expensive interstage transformer, and instead went with RC-coupling.  Hey, this is a budget build after all.

Output and power iron will be from Edcor - I really liked the quality of sound I got from them in the 1625 amplifier.  Chokes will be from Triad and the rest of the parts will be standard bog stuff you can get from Mouser, Allied, and Digikey.  I've also opted for some current meters to monitor the health of the output tubes.

Since a single 6CB5A takes a mighty 2.5A @ 6.3V to light up, I had to purchase a separate filament transformer to power the 6N7 tubes.  Other than that, this is a pretty standard power supply with a fake dual-mono separation for the two channels.

Output iron has been ordered.  Everything but the top plate metalwork has already arrived, so I will provide another post when all the parts have come in and everything has been wired together.  I'm quite curious to hear what this amplifier sounds like since the build/design approach is much different than the 1625 amplifier.  The 6CB5A amplifier will have no negative feedback, a pure triode amplification chain, and minimal parts.  In my experience this should lead to a big sound with a slightly underdamped bass.  Stay tuned to find out!

New Project: Heathkit W4-AM rebuild

I didn't even get a chance to buy another Heathkit W-2, when I saw this golden beauty on Ebay.  A few bids later and I won.  Now the Heathkit W4-AM isn't exactly their best product.  There have been several complaints about the quality of the output transformer, stability issues, and the "lower rent" build quality.  Personally I think it's a lovely little amplifier and worth the time for a good restore.

The first thing to do is to replace the power supply and coupling capacitors.  After that, I plan on doing the bricktop mod to increase stability and improve the quality of the square waves.  And of course I have to score another one so I can listen to stereo.  But even with one amplifier running, I like what I'm hearing out of a completely stock unit.  I also dig the use of all octal tubes and wonder how some Gold Lion KT66 reissues would sound in there - the 4A filament supply should be able to handle the extra load.

The Most Important Preamplifier In the World: The Dynaco PAS

Introduction:
The venerable Dynaco PAS is perhaps the most famous stereo preamplifier of all time,  Much like the Dynaco ST-70, it introduced the magical sound of tubes to thousands of audiophiles throughout the past decades.  The PAS is still a popular model, both in stock form or as a platform to be tweaked and modified.  In this article, I will touch on a general outline of the circuit, the sonics strengths and weaknesses, some of the more popular modifications, and an idea of what to look for when rolling tubes. 

The Circuit:
So what is a Dynaco PAS?  There are three iterations.  The PAS-2, the earliest stereo version, with circuitry taken from the PAM-1 mono unit and a gold faceplate with brown knobs.  The PAS-3, with a more modern silver faceplate with matching knobs.  And finally, the PAS-3X with improved tone controls.  This is a full-function preamplifier with tone controls and inputs for phono, reel-to-reel, tuner, and others.  Gain, of course, is higher than modern preamplifiers since this was the days before high output CD players.

All PAS preamplifiers share the same circuit: For the phono stage, one 12AX7 per channel, each section in series, using loop negative feedback connected to a capactior and resistor network to provide the RIAA equalization.  For the linestage, a 12AX7 per channel, each section in series, utilizing loop feedback to lower distortion and provide a low enough output impedance to drive cables into a high impedance input of an amplifier.  Power supply rectification is handled by a 12X4 tube.  Filaments for the 12AX7s come from a voltage-doubler circuit, which - as the namesake suggests - doubles the supplied AC voltage from the transformer and smooths it out to DC.  Having DC on the filaments also helps to reduce hum.

Stock Sound:
My own experience with this unit started back in 1990, after the electrical failure of my first preamplifier, a solid-state SAE Mark XXX that I was using with my first ever amplifier, a Dynaco 70.  Compared to the SAE, the Dynaco PAS-3X had a more organic sound that was imbued with a mellowness that is often attributed to the term "tubiness."  Such a strong sonic fingerprint can mask detail and transparency, but the PAS never suffered from being unmusical.  Since that initial PAS, I've owned many other preamplifiers, each with different strengths and weaknesses, but none having quite the same magic as old vintage tubes.  Some of that is due to nostalgia, and the urge for musical enjoyment over technical perfection.  No, the Dynaco PAS is not a perfect preamp, but it sure is a fun one.

In the bass department, a stock PAS suffers from a little sloppiness and lack of control - usually a sign of aged power supply capacitors - but with a pleasant warmth reminiscent of a good tubed radio or an underdamped woofer.  Rhythm, which carries the beat and timing of a song, suffers compared to some of the better tubed and solid-state units.

The midrange is the Dynaco PAS strongest point.  There is a golden coloration throughout the music, which, though definitenly not neutral, is a pleasant addition for digital sources or hotly mastered albums.  This same effect shortens soundstage depth and width, diminishes transparency, and hides inner detail.  However, the listener may not care since the sheer musicality outweighs the apparent weaknesses.

Of course the treble follows the same character of the midrange: smooth and pleasantly colored.  Ultimate extension is limited, giving a rolled-off effect that may work to the benefit of the user, depending on source and material being played.  However, with some vintage speakers, the end result may lead to a rather dead sounding system.

The stock Telefunken smooth-plate 12AX7s are perhaps some of the better tubes for a stock Dynaco PAS.  The top end 'hotness' compliments the circuit limitations better than say, a Mullard long-plate, but still, the Dynaco PAS is remarkably tolerant of many different types of tubes from the lowliest Chinese 12AX7B to an expensive Amperex.

Modifications:
The number of changes that can be done to a Dynaco PAS are practically innumberable.  There are simple improvemnts on the stock circuit to full-bore modifications that use nothing but the chassis and power transformer.  Personally I prefer to keep the general character of the PAS intact, but improve the weaknesses to the point that the preamplifier can be used as part of a high-end system.  Some of these mods are simple to perform but many will require experience working with high voltage electronics.  Be warned!  If you don't know how to solder or understand electronic safety, please have the work done by someone who does.

Bypassing the tone controls [link] is an easy modification with a great improvement on transparency and detail.  Since the signal path is shortened and the effect of various capacitors are removed from the circuit, the overall quality take a jump for the better.  Bypassing the loudness switch is also a good idea since these older slider switches can be troublesome.

Signal capacitor replacement is another simple change that benefits detail. treble extension, and transparency.  The stock green paper capacitors are hardly high-end fodder.  As to what capacitor to use, that is up to the individual user.  However, space on the PCB is limited, so gigantic oil caps in metal cases may lead to troublesome short circuits.  For the budget-minded, any metallized polyproplene - Orange Drops, Panasonics, Russian military - will be an improvement.  Auricap, Obligatto, and Jantzen are well-priced improvements over the cheap units.  After that, the sky is the limit with some of the better Teflon, film and foil, and oil capacitors beckoning the well-heeled audiophile.  All have a slightly different flavor that will work better to varying degrees depending on other system considerations: type of amplifier, speakers, room, and signal source.

The next improvement can be a little more problematic.  Since the power supply capacitors - for both the main and the filament - are aged beyond any margin of safety, this should be a no-brainer.  However it will be noted that this step does heavily change the character of the Dynaco PAS.  Bass extension especially improves, but some of the golden glow is lost since the replacement electrolytics discharge faster and provide better regulation than the vintage pieces.  Replacing the stock selenium rectifiers with moderm diodes is a necessity.  If one doesn't want to source the can cap and two filament electrolytics, there are PCBs available - notably SDS and Curcio - that can be used. These boards also give the option of using solid-state diodes instead of tube rectification.  Personally I prefer the 12X4 rectifier, but some may prefer the solid-state replacement.  The end result is a more modern sounding preamplifier that can run with some of the entry-level pieces from Conrad-Johnson, Audio Research, etc.

The next modification isn't particularly difficult but requires some mechanical work to do it.  Replacing the stock carbon volume control with a Noble, Alps, or even a Goldpoint will lead to better channel tracking and improved transparency.

The final modification is one that won't necessarily change the sonics, but will stop the user from pulling out their hair in frustration.  You see the stock RCA jacks are cruddy little ceramic units that don't work particularly well with modern cables.  Various vendors offer replacements that bolt into the stock location.  This will require much wire tagging and soldering to accomplish, but the end result will be worth it.

End Result:
A fully modified Dynaco PAS loses some of the coloration, much of the excessive bass warmth, and the rolled-off treble.  Transparency and detail is much improved and is now a good match with many more amplifiers and speakers.  The phono and line sections, though still warm and full-bodied, now reveal more subtle differences between phono cartridges and DACs.  Though not quite top tier material, such a PAS can easily serve as the heart of a moderately priced or even a lower-priced high-end stereo.

Simple modifications after this are of a minor nature: better signal wire, source selector switch, tube rolling, and various damping and vibration control products.

Of course major changes are still possible - whole new PCBs and circuits - but be warned that these updates will drastically change the character of the preamplifier, making it sound much more modern but losing that vintage magic.  However, the amount of money one could throw into such a project may be better spent on buying a used but newer preamplifier from Quicksilver, Conrad-Johnson, Audible Illusions, or whatever your budget can afford.  After all, the Dynaco PAS is a fifty-year old unit that can now have several mechanical issues: switches, AC cord, and RCA and AC jacks that will need to be addressed if long-term service is of importance.  As is, the Dynaco PAS is much like a old car, requiring some work to run its best.

Review: Rotel RA-840BX3 integrated amplifier

When it comes to solid-state gear, I seem to be cursed.  Whatever used piece I buy somehow ends up having a problem.  Now I love my Audio Analogue Puccini amplifier for its smooth and warm sound, but the selector switch eventually became dodgy, often dropping the right channel of the phono, which is the one I use the most.  With that in mind, I decided to have it repaired but needed a short-term replacement since this stereo sees almost constant use.  I hunted around until I found a Rotel RA-840BX3 (what a mouthful of gobbedlygook) on Ebay. 

This is a budget amplifier from the early 90s that still features a phono preamplifier, and the normal CD, Tuner, and even video inputs.  Power output is an unremarkable 50Ws, plenty for your average speaker and small to midsized room.  Build quality is pretty good with an actual metal case and a mix of metal and plastic knobs.  It runs a tad warm and is a popular spot for my cat to hang out on the colder nights.

Sound quality:  I'm not going to wax overly poetic on this little budget integrated, but in comparison to the Puccini, it did not fail on it's face.  Bass seemed a tad deeper with the Rotel and on one of my favorite albums, Mirrors - Lights and Offerings, I would swear I heard more detail than before - a bit of hidden synth that changed the overall mood of the songs.  The 840BX3 is exceedingly polite in that British way but still not dark.  It certainly is not an overly "fast" sound unit either, but with the KEF iQ30s speakers the sound is very easy on the ears.  For a budget piece, I couldn't be much happier.  Highly recommended - within the confines of a low priced system.

The Most Important Amplifier In the World: The Dynaco 70

 

The Dynaco 70 is the most important tube amplifier ever made.  Sure, some may quibble at that characterization, but I can’t think of any other amplifier – at least within North America – that has turned on more audiophiles to the possibilities of tube amplification.  Like countless others, it was the amplifier that turned me on to an alternative way of listening to music. 

Back in the late 1950s, the stereo wars were heating up.  Many audiophiles had invested in the expensive monoblocks or stereo amplifiers from Marantz, McIntosh, and Fisher.  The smaller players – Heath, Dynaco, EICO, and Pilot were nipping on the heels of their more upscale brethren.   David Hafler made the right move at the right time by introducing a $99 stereo wonder – the Dynaco 70.

Sure, this little budget amplifier couldn’t compete with the big boys at a performance level, but it was such a bargain that no one seemed to mind.  The sales numbers certainly back this up.  Yes, the power transformer was woefully underrated for supplying the current to four EL34s, and yes, the 7199 tube is not an ideal driver/phase-splitter, but whatever failings the Dynaco 70 has is ameliorated by the warm forgiving sound that throws a big wide soundstage.  This is an amplifier made for playing music, and if paired with good tubes, the performance is damn well good enough to have made me a tube convert for life.

I bought my first Dynaco 70 at the tender age of nineteen.  I was heading off to college and needed a stereo system.  At the time I didn’t know anything about tube amplifiers but came enamored with them after hearing a McIntosh 240 over at a friend’s house.  I loved the black transformer and chrome chassis look and lusted after the glowing tubes.  When he asked if I wanted a tube amp, I readily agreed.  However, I will admit that I had to hide my disappointment when he brought out the brown caged Dynaco 70.  It certainly couldn’t compare to the McIntosh in the looks department.  But little did I know how this little tube amplifier would change my life.

The amplifier started stock with Siemens EL34s, a Mullard 5AR4, and RCA 7199s.  After a few weeks of that, the same friend suggested running an outboard tubed regulator to supply the front end.  With the shortage of 7199 tubes – this was before the Internet, mind you – I converted the PCB, through the use of a RCA manual and my first soldering iron, to use 6GH8As.  After that, it was modified to run the EL34s in triode.  After that, it was sadly sold to another friend who began his own audio journey with tubes.  At that point I was running modified Scott or EICO integrated amps, a small step before my DIY journey.

A few years ago, I exchanged emails with the fellow who bought the Dynaco 70.  He still had it!  We did a swap and I had my very first amplifier back in my possession.  The original circuit board was long gone, the tube sockets were shot, and the can cap had been replaced by a giant oil power supply capacitor.  This old amplifier was due for a rebuild.

In order to stay true with the spirit of the Dynaco house sound, my restoration was fairly limited in scope.  The input circuitry was replaced with a PCB from Triode Electronics that utilizes a pair of EF86 tubes and a single 12AU7.  The power supply can-cap was replaced with a SDS circuit board that fitted underneath one of the output transformers.  A bit of wire and solder, some new output tube sockets, and the amplifier was ready to sing again.  

For tube selection, I went with some budget Russian valves – EH EL34s, Sovtek EF86s and a 5AR4.  The sole American tube was a late production Phillips 12AU7.  The sound, as to be expected, was a little on the harsh side.  This was due to the “solid-state in a bottle sound” that afflicts many of the lower-end Russian tubes.  So it was time for a spate of rolling.  In the end, the amplifier finished with Matsushita EL34s, Dutch EF86s, a Hitachi 5AR4, and a 1950s-era RCA blackplate 12AU7.  This change made for a much smoother and cohesive amplifier that was quite enjoyable to listen to.

But, I’m not the sort of chap who wears rose-colored glasses.  The Dynaco, even with an improved power supply and new input circuit, suffers from several flaws that stop it from being a truly great amplifier.  

First of all, the power transformer is helpless underrated in the current department which will cause the voltage to sag at higher output powers.  Another issue is the use of a single 5AR4 rectifier to handle all four output tubes.  Sure, it’s within the specification of that tube, but still it is hitting the top of the envelope.  Of course these two problems can be rectified (ha! – editor) by an aftermarket power transformer and a different rectifier or even the use of solid-state diodes (which have their own issues).

However, even with those changes, I believe the fatal flaw of the Dynaco 70 rests with the A-470 output transformers.  Like many things Dynaco – especially on their budget gear – this output transformer was built to a price point.  At higher output levels, the amplifier sounds like the core is saturating, giving that pleasant “tube-like” compression.  It sounds like the images are on springs, causing the soundstage to shrink in size as large dynamic swings occur.  Why do I pin the blame on the output transformers?  I’ve heard the same effect on the Dynaco Mark IV, the monoblock version of the Dynaco 70 that have a much beefier supply that isn’t shared between two channels.

So yes, the Dynaco 70 – and all the new kit versions now available – is still a great introduction to the world of tubes, but it isn’t the end game.  There are much better - and admittedly more expensive – amplifiers that really lift the veil over the music and provide the sort of output that an audiophile wants to hear.