Monday, November 25, 2013

Project: Mult-Valve: The Universal Pentode Amplifier

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.

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.

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.

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.

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


Lefty McStinky said...

What is a suitable AC rating of a motor run capacitor for a given value of DC?

DividebyTube said...

conservatively speaking:

DC = 1.414 the AC rating.