Some things that I want to share with you:
-I ve tested -like Reuben- also a 12AU7 - no problems. But the Outputgain is much lower. I think a 12AT7 will also work. But I ve put a 12AX7 inside and I like it best
-Very important - the two heaters of the 12A..7 Tube are running with only 3,2V because the wiring is serial and that needs usually 12,6V Heater Voltage to get the 6,3 V AC on each heater. That means with the provided 6,3~Volt AC Supply you have on each heater in serial only 3,15~ Volt AC Drop -> so it is necessary to wire it in parallel! Or the 12A..7 will not heat up and stays nearly cold, no glowing... ( like Reuben mentioned, THANKS man!!!) Look at my photo (parallel 6,3V means one Phase on Tube Pin 4+5 ! and the other on Pin 9)
-As Input Transformer I tried Edcors with 600/10k, 10k/10k and Lundahl 1540. But the Tubes are so strongly dominant that there is not any audible difference. You can save your money using a cheap Edcor Input TX
If you want to use an InTX like me with 1:4 600/10k it is necessary to use additionally a12dB Input H-Pad Attenuator or you will get ultra heavy distortion ( that means R14, R15, R16, R17= 180 Ohm, and Bridge R18 =320 Ohm)
If you use the 10k/10k (or I think a 600/600 Ohm Input TX will also work) you can skip the Input H Pad or set it only to 1dB Attenuation.
For a 1:2 TX like I ve tested with the Lundahl 1540 I ve needed a 6dB H Pad attenuation.
The results were all veryyyy close
-I am using a cheap 500uA DC Meter from China, the SD-65 from Ali....
To get that piece properly working you need to adjust R10 and also the Zero Meterpot (1k lin pot + R10 240Ohm standard for 200uA I think). But this also depends strongly from the used 6BA6 Tubes. I ve tested Valvos and GE 6BA6 Tubes. The 6BA6 Tubes from GE are "hotter" for the meter. The sound of the compressor does not really change a lot with the Valvos. For my used 500uA Meter and the GE 6BA6 Tubes I have used for R10 a 5,1K Resistor with 0,6W and as "Meter Zero" Pot a 20K Lin". That works well. But don't expect to much precission.... For testing the Valvo 6BA6 I ve needed for R10 a 12K Resistor and also the 20k Lin Pot for Zero and re-adjusting.....
-The Output TX Edcor XXP1 600/10k works also perfectly ....
-For the Power TX I ve used from Audiophonics in France a standard "stock-TX" (perfectly for Mono!) with sec. 200V 0,1A and 6,3V 2A for about 40 Bucks all....
I have also used the 6,3V heater AC Voltage for the V+ DC Rail to convert it to 5V DC -> Bypass Relays 5V, Lamps all 5V / Leds with 5V (+Pre-Dropresisitor).....
For B+ tweaking I ve needed a 1K and 3,3K Resistors 10W for tweaking down the Voltage to about 210 Volts DC
-I ve done all recommended Mods like mentioned on the first Post from Bernd
Thats all, no Problems....
Thanks to Bernd "Bernbrue" for this super nice sounding build. (And also Reuben) Its really a very, very special compressor....