I decided to stay with 6AV5GAs, damn the torpedoes, full speed ahead
I wired up the output stages, using the Transcendar 10W, 3K SE OPTs, and 750 ohm 25W "heatsink" type dale cathode resistors.
With 400V B+, the test 6AV5GAs were dissipating about 25 Watts combined plate and G2. Even though these tubes can take a lickin' and keep on tickin' I decided to throttle things back a bot, added an extra C (47 uF / 450V) and R (333 ohms, 30W, three 1K, 10W WW resistors in parallel), before the choke..
That dropped the B+ to 365VDC, before the choke, and 350VDC to the OPT B+ connections.. Instead of 75 to 80 mA plate current, I now get 66 - 68 mA, and a V P-K of about 280V so I'm down to around 18-29W PD, which is closer to reality. Before I throtled it back, I ran tests about three dozen 6AV5GAs and culled out the four weakest ones, and one which oscillated uncontrollably.
I even subjected half a dozen 6AV5GTs to this treatment, and they didn't blow up.either.
I made up another SS 6AX5GT replacement, this time with 50 ohms in series with each uF4007 ,and I added another diode, LED and 680 ohm resistor across 2 and 7 to get a red "filament glow". I tried that out in the "blue plate special" GSG and get the same B+ voltages, though with a slightly lower cathode current, because of its approx 800 ohm vs. 750 ohm RK's.
With this combination, it is noticeably more powerful, better bass, and clear highs, I have run it this way for a few hours, with no ill effects on the tubes, and the PA774 just gets slightly warm. It now has 1.2A less loading on one of the two filament windings, so that saves about 7.5 VA's in the process. It definitely has a much better "Ka-Whump" factor, now
Next, on Thursday or Friday, I'll build up the new amp's 12AT7 driver stages, with SRPP.
And just for ha- ha's, I have also started ANOTHER G*S*G , the chassis is drilled and punched for the PC board, next will be the other two tube socket holes, and mounting holes for the iron, etc. Next year
I'm still deciding whether to use 6B4Gs, 2A3s or 6AV5GAs, or
Photos coming RSN
/ed B in NC