[EdHolland]

Thank you gentlemen for posting the circuit details, and persevering through everything else to get them into this thread

Next, an apology for not keeping up with the thread when folks were trying to offer help and advice

Is there a preferred value specified for the potentiometers? I can certainly calculate what is needed, based on the target output and range of voltage adjustment needed.

Today, I took the first step in implementing bias control, and fitted a 10 ohm, 1% tolerance resistor in the cathode circuit of each 7591 valve. Also added was a twist of wire on each solder tag to make it easier to hold a meter probe for measurements. I'll order parts for the rest of the bias modification this week. However, it was still possible to operate the receiver, and measure the voltages to determine the corresponding quiescent currents in the output stage when running with the old valves.

I have V8: 35 mA, V9: 35 mA, V10: 24 mA, -V11: 42mA, give or take a few minutes warm-up..

Given that B+ measured at the anodes is about 438 V (430 V specified on the schematic) the valves are working a bit hard, except for V10 which must be quite tired. It is perhaps not a coincidence that this low current tube appears to be an original Fisher branded part, whereas the remaining three are RCA branded. Switching V10 and V11 proved that the low current followed the Fisher valve.

Grid bias voltage at this point was measured at approx -17.6 V relative to chassis - it seems the factory circuit is working as intended, but could use some help, and the independent adjustments will be added, along with lower value (220K) resistors and 0.1 uF coupling caps. This will enable me to save up for, and fit a new set of valves.

Having fun with this!

Ed