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.

Dividebytube's Guide to Building an Audiophile System

I've been into this hobby for a number of years and have gone down several dead ends.  This little article is a sort of "lessons learned" piece, which may be of some benefit to a beginner - or even an experienced - audiophile.

Consider the Source:
Like Linn, I believe having a good signal source is the most important part of any good stereo system.  Some others will tell you to spend the majority of the budget on speakers, or even the amplifier.  However, I've found a cheap CD player or turntable will just lead to heartache, making the listener chase their tail trying to find the best speaker or amplifier to match with the inferior sound quality.  Is the speaker too bright or is it the cruddy op-amp in the digital-to-analog converter?  Is the amplifier muddy or does the turntable poorly constructed, picking up vibration?  A fairly neutral source provides the foundation for good system building.

Amplifier Envy:
This idea is closely related to the first concept of source quality - don't go crazy with finding "the right" amplifier until you have nailed down your front end.  Yes, just the right amplifier does make a big difference, especially tubes versus solid-state or Class A versus Class AB, but don't worry about this aspect as much until you have sorted out the source and the speakers.  There are plenty of budget models out there that will do the job until the time is right for an upgrade.

Boom and Sizzle:
Speakers are like luggage - easy to buy and almost impossible to sell.  Before buying any speaker, consider your listening room, environment, and music likes.  For example, a small space will require a small speaker - there is no reason to excite room nodes and get a muddy sound with a monster speaker crammed inside of a tiny space.  If you have little children - a Quad Electrostat or a mini-monitor on a slender stand - may not be the best route.  And small speakers may not reproduce bass all that well, which won't work for head-banging levels, unless a subwoofer is used.

Picking an Amplifier:
I like tube amps, especially single-ended and/or low-powered ones.  Good sounding amplifiers of a higher wattage are few in number, but they do exist.  The Eico HF-60 and (rebuilt correctly) Dynaco Mark III monoblocks come to mind.  However one has to be smart enough to realize that tube amplifiers may not always be the best match for the speaker, especially if low-efficiency, wild impedance curves, or a large listening space is being used.  In this case, perhaps a bigger tube amp or a good solid-state unit is in order.  For example, matching a 20W 6BQ5 amplifier to a pair of Magnepans will lead to nothing but frustration, unless incredibly low playback levels are your thing.

The Good, The Bad, and the Feedback:
Zero Negative Feedback and single-ended was quite the rage for awhile.  However, these types of amplifiers require efficient speakers with higher impedance curves - think Altec-Lansing horns, Klipsch, or (modern) Zu speakers.  Pairing a single-ended 2A3 amplifier to a 4-ohm nominal speaker can result in some strange effects: muddy, underdamped bass or perhaps a rolled-off treble.  Some listeners may like this effect, but it isn't fidelity since the "romantic" tubey sound can dominate the music to the point where the differences between recordings becomes almost negligible.  That is to say, if every record or CD sounds the same, then you're listening to a pleasant distortion machine, not a high-fidelity amplifier.  In this case feedback - if judiciously applied - can actually help, providing speaker damping and extending frequency response.

Short Circuit:
Another strange audiophile fancy is the love for boutique parts.  Now don't get me wrong, I've personally heard differences between coupling capacitors, resistors, and wire.  But I also know that the circuit itself is the most important part, not the parts supporting it.  For example, a poorly design SRPP tube linestage isn't going to be greatly improved by slapping in a Teflon capacitor.  The circuit itself needs to be corrected, or an alternative and improved preamplifier needs to be purchased.

Flash in the Pan:
The high-end audio scene is filled with hyped products and broken dreams.  I could name company after company that have failed since the 1980s, and some of them even produced well-reviewed winners that should have seen these start ups to the road to success.  However, the audiophile market is a niche one and it's apparently quite easy to fail.  Unless you are handy with a soldering iron, my advice when purchasing equipment is to buy from established brands that have been around for awhile.  This will lead to easier service, possible product updates, and better resale value.  Some of the classic brands: Audio Research, Conrad-Johnson, Mark Levinson, KEF, Quad, etc have a long track record and the quality of their gear, plus the resale value shows this.

Vintage Madness:
I love old tube gear and vintage speakers.   However, in the world of amplifiers, only a few of the old designs really stand the test of time.  These were usually the expensive pieces back then  - Marantz, Fishers, the top-level Eicos, Fishers, Dynacos, Grommes, Fairchilds, etc.  On the other hand, the more budget integrated and console amplifiers suffer from poorer circuits, cheaper parts, and output iron with less bandwidth and smaller cores.  The same holds true with speakers - only some have really held up to the test of time, while the majority of the vintage units sound rolled-off and not very detailed.  Sure, these old-timey pieces can sound enjoyable, but they ain't hi-fi.

Separated at Birth:
If you're a simple kind of audiophile who wants the minimal moving parts, then by all means go the integrated or receiver route.  Just keep in mind that this path will minimize - unless in/out RCA jacks are included - your upgrade possibilities.  Personally I prefer separates - preamplifier and amplifiers - which allows different combinations of gear to be tried.  Difficult speakers to drive?  A tube preamplifier and a hefty solid-state amplifier might just be the perfect combination.

Budget Talks:
Even if you are silly rich, every stereo system is built around a budget.  Things to consider: how much are you willing to pay and how will it match with the rest of your system?  I could, with enough bad decisions, put together a fantastically expensive system of Stereophile Class A components that would make any listener run out of the room screaming,  For example, couple a low powered singled-ended amplifier with an inefficient speaker with a low 1-ohm impedance curve.  Though painful, it's best to start with a good front-end, and then buy and sell up the chain, keeping in mind that a manufacturer's lowest priced product is not always the best place to start.  For example it will be better to save to buy a Rega P3 than the entry-level P1.

David and Goliath:
There are giant-killers out there - budget gear that punches above its low price and gives a great listening experience.  KEF and Wharfedale speakers easily come to mind, along with some of the cheaper Chinese tube amplifiers.  However, no matter what you tell yourself, they still are budget pieces that can be surpassed with something more expensive.  It's just a matter of how much you are willing to spend and if you ears really demand that much perfection.  I think it's important not to lie to yourself and accept the limitation of your system, which brings me to my final point.

Times are A-Changing:
When buying gear and bring in a new component into the system, it's all too easy to get swept away thinking that the latest change is always for the better.  It's also far too easy to think an expensive unit will always surpass a cheaper piece.  It would also be wrong to think that newer is always better than older.  Again, system matching and careful listening is required here.  It's also good to have a few audiophile friends over who don't have an emotional investment in your stereo.  If they have experienced ears - and are of an honest nature - they can also tell you what faults they are hearing, instead of just concentrating on the best parts of upgrade.

As always, comments and suggestions are welcome.

Review: Denon DL-110 high-output moving coil

(picture taken from Google Images)

My previous phono cartridge was a Denon DL-103R, the famed 'upgraded' version of the DL-103.  Mine had been modified with an aluminum body.  Partnered with Cinemag step-up transformers, the DL-103R was a great performer, offering plenty of hi-fi goodies - soundstaging, detail, etc - at a budget price.  It really is an excellent low output moving-coil alternative to the more expensive units out there.

Sadly, the cantilever of my DL-103R was accidentally broken when I moved to my new house - don't ask!  Since this new place is just temporary and has the misfortune of a smaller listening space where I cannot use the UREI 813A speakers, I instead opted to buy the budget DL-110.  Regarding type and specifications, the Denon DL-110 is a high output (1.6mV) moving coil cartridge that can work well into a standard moving magnet phono input.  At 4.8g of weight, it's also very light and with some tonearms may require the use of the included metal plate.

Using the VPI HW19 Mark III table with a Rega RB300 arm, I was immediately taken with this cartridge.  Though it lacks some of the finer points of the modified DL-103R, it still in no slouch.  No immediate shortcoming come to mind, though the midrange is perhaps slightly forward.  Bass detail is also good and treble extension was spot on with minimal aggression or roll-off.  Detail retrieval - hall ambience, trailing edges - aren't exactly on par with the best I've heard, but once again, any sins were of omission.  So all in all, a very good cartridge for the $139 asking price.

I recently purchased a new turntable: a VPI Aries I with a JMW 10.5" tonearm.  Since I only had the Denon DL-110 on hand, I was forced to use this cartridge which had a cost some 20X less than the turntable.  Due to the light weight of the cartridge body, I as forced to add the included metal plate under the headshell.  This was a tricky operation that required much fiddling and curse words.

Even with the upgraded turntable and arm - and much to my surprise - the DL-110 did not fall on its face.  For example, Neil Young's "Live at Massey Hall" never sounded so real.  Soundstaging was deep and wide and the music - much like the real, live thing - came at you instead of hanging out behind the plane of the speakers.  There was a nice shimmer to the slightly forward midrange, while bass was low but still tuneful, giving great response with the small KEF speakers.  Treble was very extended with more detail than the old HW19 table.  I'm sure I would find more faults with the UREI speakers installed, but at least with this system, the sound was much better than anticipated.  Highly recommended for the budget conscious audiophile.!

System
VPI HW19 Mark III with Rega RB300 or VPI Aries with JMW tonearm - both with SDS Power Supply
Denon DL-110
Cardas Quadlink 5C 1M interconnects
Quicksilver preamplifier with Gold Lion 12AX7 re-issues, RCA 12FQ7s, Raytheon black-plate 5814
Cardas Cross 1M interconnects
EICO HF-60 monoblocks with Mullard XF2 EL34s, GE 6SN7GTAs, Genalex CV4085s
Kimber 8PR/4PR 2M bi-wire cable
bi-wired KEF iQ30 with VTI UF29 stands
VTI BL503 equipment rack

New turntable: VPI Aries I

This was an unexpected purchase since recent events have cut back on my audio acquisitions.  But this was one deal that I couldn't pass up.  Featuring a thick MDF plinth, the heavy-duty metal platter from a TNT table, and a JMW 10.5 tonearm, the Aries is one heck of a piece of engineering.

I've always been impressed with the sound of my old HW19 Mark III turntable and though the Aries would only be a minor step up in quality.  But even with the only cartridge I currently have - the Denon DL-110, I heard a major improvement.  With the Aries I the background is quieter and blacker with less of a washy sound.  Bass goes lower and with more impact, while detail and treble quality has also improved.  I obviously need to upgrade the cartridge, so this isn't a real review, but more of a revelation.  I've always distrusted solid plinth turntables, intellectualizing the superiority of suspended designs.  But I guess with enough mass loading, a great tonearm, and a heavy platter, the more "primitive" solid plinth designs can compete.

More later when I've had time to process the changes and have a better cartridge installed.

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.