I'll admit right off the bat that I've never been a huge fan of small signal Russian tubes. Overall they seem to have a "solid-state in a bottle" effect and certainly don't come close to some of the better NOS offerings from Mullard, Amperex, RCA, and company. I would even prefer to spend my money on GE, Ei, Raytheons, or CBS tubes.
The other night I was given a chance to borrow a pair of re-issue Genalex B759 / 12AX7A tubes. The boxes are certainly a nice copy of the original, as is the logo imprinted on the glass. However, if you compare the "re-issue" to the original, they are in fact nothing alike. Of course the machinery that made the original British tube is long gone. It's a small point, but I never liked the use of the term "re-issue" which means, at least to me, that the product actually has some connection to the past.
Instead, the Genalex B759 looks like a Sovtek 12AX7LPS - the plate, mica, and layout are the same. Perhaps there is some magic going on with the screens and cathode chemistry, but that's difficult to tell without breaking them and taking a look inside while doing a chemical analysis. So one theory - these could be just highly selected and matched "super" Sovtek tubes. Perhaps a softer vacuum could come into play.
Okay, enough intellectual yammering. How does the tube sound?
My Quicksilver full-function uses a 12AX7 as the first tube in the phono section. The linestage also has one. I pulled out my 1960s short-plate Mullards and replaced them with the Genalex B759. I had a fellow audiophile over to listen. As we talk tubes and single-ended amps, we let the whole system warm up for a while before listening. Hmm... this actually sounds better than I expected. Yes, the Genalex tubes do sound different than the Mullards, but within the confines of this system, the B759s are completely acceptable.
The first word that comes to mind is fast - perhaps due to a bit of additional upper-midrange/lower-treble excitement. This extra bit of zing makes music fun and engaging, all without sounding overly bright or with the type of "solid-state" grain I normally associate with many small signal Russian tubes. Overall depth seemed a tad flatter than the Mullards, but the effect lessened the longer the preamplifier was on. Coherency from the lowest to the highest octave was even handed, except for the already noted touch of forwardness. "I could live with these" was one comment I made as Fleetwood Mac was spinning away.
So yes, the Genalex B759 / 12AX7A does deliver the goods, though I fear a more revealing speaker - like the UREI 813As - would show more limitations. I suspect this tube may not be a good match for forward systems - especially ones using horn loudspeakers. However, they would be a great match for some darker sounding gear like the Dynaco PAS or Eico HF-85 preamplifiers. With the ever rising cost and increasing scarcity of decent NOS tubes, it is good to know that there are real alternatives out there.
I'll admit that I'm not a tuner kind of guy. The source material coming from the radio stations is usually grossly compressed and lacking any real fidelity. The classical stations usually offer the best sound, but that kind of music isn't normally my cup of tea. Having said that, why exactly did I buy a tuner? Well I do any number of hobbies that often require my undivided attention. Getting up to change the record or CD can break my concentration. Having a radio for the background takes care of this problem and also exposes me to some different music other than my MP3 collection.
So why did I pick this tuner in particular? It all started when I bought a Sansui integrated to fix up. I was searching on Ebay for its radio mate when I saw this particualar model: the TU-777. I was taken by the round tuning dial which reminded me of something from the era of classic 1930s radios.
The Sansui TU-777 is a solid-state tuner that saw its introduction to the market way back in 1968. There are no preset buttons here! But the dial action and signal strength meter both still work, giving a pleasant green glow to boot. Sound quality is surprising good, much more warm and natural than I expected for something with silicon bits and aged capacitors. It also pulls in stations reasonably well, though obviously not with the same strength as more exotic pieces. But really, what do you want for $75?
So highly recommended - just on looks alone - if you need a simple tuner.
Things are never quiet here at the 6th Street Bridge. Having just finished up the 1625 amplifier, I decided yet another project was worth pursuing. This time it's yet another rebuild of a vintage piece of gear - the Heathkit W-2. This was a fairly popular "20W" mono amplifier made in the early to mid-1950s. It uses the classic Williamson circuit with 6SN7 driver tubes, 5881 output tubes, and Peerless output iron - running in Ultralinear. Another special thing to note is the separate chassis for power supply and the signal which is a great way to reduce hum.
The W-2 I received was in pretty good condition. Popping the hood and the resistors and capacitors look to be in mint condition. But sadly, the amplifier didn't work. Not a single hum, burp, or anything out of the test speaker. A quick diagnoses with the multimeter and it appears that the original builder had neglected to ground the two chassis together via the umbilical cord! A short length of wire, two solder joints, and music was coming out the speakers. It makes me wonder if I was the first to ever hear this particular amplifier. Very weird to say the least.
Anyway, the sound - with almost 60 year old passive parts - was rather lifeless: undynamic with some treble roll-off.. Here's hoping that some coupling and power supply capacitor upgrades will bring this old amp up a couple of notches. Of course I also need to score another unit so I can listen to stereo.
Introduction:
In the world of single-ended amplifiers, the venerable triode still gets preference. It's easy to see why since the 300B, 2A3, 845, and 211 - among many others - has a certain catchet and a sonic signature that is honestly very hard to beat. The triode is quite the linear device with the downside of needing a bunch of voltage and current to make relatively low power.
The other option when running single-ended, minus the odd solid-state design, is to use a pentode/beam-tetrode in ultralinear or native operation. This not only gives plenty more power, but these tubes also have the benefit of being inexpensive to buy. Based on my experience with various SE amplifiers, most of the magic of single-ended is the circuit simplicity - no phase-splitter - and the Class A output stage. Now this design I'm about to introduce was not meant to be an assault on the very best amplifiers, but instead offer a budget alternative to the sometimes stale and now very mundane 300B amplifier. It's also an attempt to replicate 1930/1940s technology' but with modern components. Much of the 'sonic footprint' of those pre-war years was not from inferior amplification, but the bandwidth limited source material.
Of course running in pentode mode requires some negative feedback - a new fangled concept back then - in order to reduce the output impedance to something that modern speaker can use. An alternative would be to experiment with Schade feedback - taking voltage from the plate of the output tube and tying it to the plate of the input tube.
Tubes!
The first decision was tube choice. If I wanted to be just a cheapskate, there are plenty of TV tubes that could be used. Instead I went with a vintage classic that is still widely available - the 1625 output and 12J7 driver tube. (Note that the 807 and 6J7 can be used - just don't use the voltage doubler for the filament supply.) Expect to pay $5-$10 per tube, which is a bargain considering the quality of these vintage beam tetrodes. Also note that the 1625 has a low screen voltage maximum, so running ultralinear isn't an option unless one can live with reduced voltage and the resulting lower power output.
The UX7 base top-plate cap 1625 is part of the famed 807/6L6GC family and was used in WW2 to provide radio communications - the AN/ARC-5 Command transmitter - for the majority of the planes in the United States Army and Navy. This tube was made to military specifications with wonderful assembly and a high quality control - when it came to radio pilots lives were on the line. Since there was a war on, scads of them were made and every example I've seen is an impressive piece of engineering. In tribute, I have given this project the name 'The Command 1625'.
The grid-cap 12J7 pentode is the 12V filament version of the 6J7 tube which is the octal version of RCA's earlier 6C6 tube, which in turn was their answer to Western Electric's famed 310A. These tubes take the signal voltage and bring it through the cap on top of the tube - a nice way to keep the fragile signal away from noisy power supplies. The 6J7 eventually begat the 6SJ7 which eventually became the smaller EF40/6AU6/EF86 types. The 12J7 was selected for the fact that NOS versions are still cheap and plentiful in metal or glass versions.
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A quick look at the signal schematic shows nothing too exciting, though using a pentode on the front end is considered a heresy these days of current sourced triodes and LED biasing schemes. My idea was to keep this design as simple as possible - in the spirit of the vintage tubes selected - with minimal parts count and matching required. Eagle-eyed reader will note that this same type of front end was used on my EL156 amplifier. It's a good sounding front end so it was easy decision to use it again.
The feedback resistor for a 4-ohm output transformer is 6.8K. For an 8-ohm secondary, increase the value to 8.2K.
(right click to open and see larger version)
For the power supply I stuck with tube rectification which has less switching noise than solid-state and better visual aesthetics. The 5Z3 - a precursor to the 5U4G - was selected on looks and the decent prices, plus the ability to easily handle the current requirements for two output tubes. Plenty of other options are available here. Two Triad chokes and a smattering of generic capacitors provide the raw filtering for the output tube plate. Of interest is the regulation for the screens of the 1625 and the input stage. This stiff front end gives more control and separates it from the wranglings of the output stage. For easy implementation, shunt regulation is accomplished with simple VR tubes: the OD3 and OC3 are put in series for 255 volts. The small bypass cap here is just to clean up noise - going for a larger value will cause instability with these types of regulators.
The voltage doubler circuit is used to take the 6.3VAC filament tap and boost it up to ~12VDC required for the 1625 and 12J7 tubes. Note, do not ground the center-tap. If one wishes to use 807s, 6L6GCs, 5881s, or KT66s along with a a 6J7/6SJ7/EF86 input tube, the voltage doubler can be skipped. In this case, do ground or lift the center tap of the 6.3VCT winding.
Parts:
Some of the passive-parts decisions were based on what I already had in stock. For example, the PEC volume control, Wima coupling caps, and power supply electrolytics were chosen because I just happened to have them in my stash. Builders can select their own goodies, provided they meet the electrical requirements of the circuit. Other than that, I went with Kiwame and carbon-composition resistors, Nichicon cathode bypass capacitors, and a few Solens.
For the the output transformers I went with the Edcor CXSE model. Alternatives are James, One Electron, or even Hammond. For those on a tight budget, there are cheaper alternatives, but these will limit bass and treble response, not to mention that the core will be taxed more heavily. I've found that overkill output transformers lead to a more relaxed, transparent sound.
Top aluminum panel is from the steadfast Front Panel Express. Size was selected since I already had a leftover wooden chassis from an older project. Layout was done to minimize power and signal circuitry interaction, while also giving a vintage "juke box" aesthetic.
Potential modifications:
Different Output tubes: With socket changes and 6.3VAC from the transformer instead of the voltage-doubler filament circuit, the 807 or any of the 6L6GC family can be used - 5881, KT66, 350B, etc.
Different input tubes: With wiring changes: the 12SJ7. With filament and/or socket and wiring changes: 6C6, WE310A, 6J7, EF86, EF80. 6AU6, EF37, etc
Different rectifiers: 5U4/G/GB is the Octal version of the 5Z3.
Listening Results:
Turning on a new, freshly built amp is always an exciting moment. With the test speakers - Pioneer SB-21s - place on the ground, I was immediately taken with the fast quality of this amplifier. After checking over the power supply voltage and bias readings, I moved on to my main system consisting of Spica TC-50 speakers, a Quicksilver full-function preamplifier, and a VPI HW19 Mark III turntable with a Denon DL-110 cartridge. This is a smaller, more budget orientated system than the setup at my old digs, so I'm still feeling my around the various changes.
First impressions - revealed an amplifier with a nice, neutral midrange. This is not a romantic sounding unit but I would never mistake it for solid-state either. Bass quality was very good: controlled and with no tubbiness. This is hardly a surprise due to the heft of the Edcor output transformers. The already mentioned speed was evident in the treble too - giving a slightly lean presentation which may be more a character of the Spica TC-50s than anything else. Soundstaging, also strong point of these speakers, was wide and deep. Texture and inner-detail was also top-notch. With more dynamic music, the amplifier had no problem delivering the power while keeping everything in line. This amplifier sounds much more powerful than its expected 10Ws, indicating good clipping characteristics common with Class A output stages.
Overall, I would still give the overall nod to the EICO HF-60s, but those monoblocks usually cost three or four times as much, and also sport much more exotic output iron plus increased power output. Having said that - and it's always difficult to fairly judge your own creations - the Command 1625 amplifier offers great sonics for such a low price point. It is also a great reminder that pentode outputs can compete in the world of hi-fi.
Parts list and Front Panel Express file are available upon request. Shoot me an email.
The job of updating the Acrosound 20 monoblock amplifier became much easier once I saw a PCB for sale on Ebay. It's a modern iteration of the same circuit, but with all new resistors, capacitors, and tube sockets. Replacement was an easy job and with all the leads soldered into position, the amplifier fired right up on the first try. Now that's always good news.
Some new power supply capacitors are still needed, but I'll wait until I have another matching amplifier before I make that move.
Listening in mono through a fairly efficient test speaker, it's a pretty good sound amplifier. The Acro has some of that general "Class A" sonic footprint - an ease and naturalness to the music that makes tubes worthwhile. It does run quite hot, and appears to be exceeding the maximum plate dissipation of the 6BQ5/EL84, but hey, this is a cheap little tube, so no worries there. Buy 'em cheap and burn 'em up!
