This project started during some early spring cleaning, when I un-covered a very nice power transformer with only 12v filament supplies.
It wasn't long before I had a pair of JAN 1626/VT-137s and a very cool H&K 'Gammatron' 12SL7GT on the bench as well. It took some more looking, but I came up with a pair of 12X4 rectifiers as well. Before long, the amp you see here was finished.
All AC power, switching and fusing is confined to the extreme left rear of the chassis and isolated from the actual amplifier circuit.
Tube rectification is accomplished with a paralled pair of 12X4s feeding a CLC filter network. The big 4H choke (any choke labeled as a ‘Mojo Triple Seven’ has to be good!) kills the hash very effectively. The choke provided plenty of stiffness such that filter capacitance could be kept on the low side to maximize rectifier life.
DC filaments were used to minimize noise, and are set at a conservative 12.2 volts to maximize tube life.
All new circuit components inside are top-shelf and over-built to the extreme. I used Solen coupling caps and EPCOS cathode bypass caps on the 1626s. I had gain to spare on the 12SL7GT, so I did not bypass the cathodes to allow some local feedback at that stage only. Everything is wired point to point, and the big copper buss bar ground lines maintain an equipotential ground for the entire circuit.
This darling runs the 1626s a bit hotter than the textbook values, as shown on the load line below. It does slightly exceed the maximum operating conditions listed in the RCA spec. sheet, but remember; those values were generated for CCS (Continuous Commercial Service) conditions. All operating points and temperatures are below the Western Electric maximum ratings for IMS (Intermittent Mobile Service). With the controlled filament voltage, you should easily get 5000+ hours of life out of the output tubes, and even more out of the 12SL7GT.
Output transformers are vintage Hammond units that were originally used in the VM 730 model reel to reels. They are air-gapped and interleaved with 5K primaries and 8 ohm secondary/speaker outputs. Although they are physically small, you can see from the frequency response plots below that you don’t need a huge output transformer for good bass response when you are only kicking out only a watt or so per channel.
I have to say that this project has completely exceeded my expectations for sound quality and range.
At idle with open inputs the amp is completely silent; I measure 0.8 mV AC across an 8 ohm resistive load. Against the rated output of 1.0 watts, this gives a SNR of 70 dB – almost unheard of for an under 1 watt amplifier. Credit here goes to careful wire routing, AC isolation, DC heater filaments, and good grounding.
Input sensitivity is sufficient that standard line level inputs (Ipod, CD player, Tuner, etc.) will easily drive the amp to the rated output power of 1.0 watts/channel. Hotter preamp inputs can also be used to enable class AB operation as well (more on that below).
At the rated output power (1.0 watts/channel) the amp is operating in pure class A, and the total harmonic Distortion is just under 1%. Note that the distortion is almost entirely the second order as associated with single ended topology. You can see that the distortion is very linear with frequency, slowly trending lower with increasing frequency.
Frequency response is surprisingly good given the small size of the output transformers and the fact that no global negative feedback is used. The powerband is nice and flat from around 50Hz out to 10KHz, with the -3dB points at 30Hz and 18KHz.
The sound is very natural and open. The second order single ended harmonic mojo is present but not overwhelming. You do get the impression of depth and presence, but without the expense of softening the sound. The closest comparison that I can come up with is a single ended 2A3 amp, but without the ‘mush’.
The 1626 is rated for grid conduction / class AB operation, and with a sufficiently hot preamp, the 1626s can be driven into class AB operation. Operating as such, it will deliver a rather harsh sounding 2.2 watts at around 3% THD. In this operating mode, the output transformer primaries are seeing current transients of nearly 75ma, well past their saturation point. I suspect this is really the limiting factor for high powered class AB operation. I am very curious as to how this circuit would sound in operating in class AB with a larger output transformer appropriately rated for higher DC current. On paper the 1626 could deliver over 3 watts if driven completely from ‘roof to floor’. The next iteration will address just that… Stay tuned.
The amp chassis itself is heavy gage steel finished in super-tough oven baked textured epoxy paint. The faceplate is nicely figured Ovankol Shedua that was reclaimed from timber supports in an abandoned South African gold mine.
Standard fare on all my amps includes: All stainless steel fastening hardware, silver signal wire, appropriate fusing, grounded chassis with IEC power connection, stake bonded hardware, and 40+ hour burn-in.
Comes with a lifetime guarantee on everything but the glass, and arrives completely turn-key and ready for operation. Everything is cathode (auto) biased, so you don’t have to worry about any adjustments or manual biasing operations.