Balance on a cathode bias circuit?

[Blair]

On a schematic like or similar to this, what is the best way to install some sort of balance adjustment for the output tubes?

Something like what Bruce Heron does on the Oddwatt amps with a 20-30 ohm pot right in the middle where the cathodes meet? Maybe add a 10 ohm resistor on each leg of the pot for voltage reference?

Thanks!

Blair

[20to20]

Here's how Zenith did it. Test points built-in, too.

K_bias_balance.jpg (76.43 KiB) Viewed 3235 times

[Blair]

Thanks! This is exactly what I was thinking. What are the 47mmf caps for? Shunt any additional DC at the test points?

Is the value there critical? I have some .047uf caps, but no .0047uf caps at the house.

Thanks again!

Blair

[Tom Bavis]

Caps are probably a band-aid for high-frequency stability... I have a pair of those Zenith output transformers and they're pretty bad...

[Blair]

Thanks Tom,

So, assuming better output iron, the caps are probably not needed? I'm using donor iron from a Pioneer SM-Q300. Not sure about the quality, but it's been mentioned on a few forums that they were wound by Tamura back in the day.

I paid next to nothing for the iron, so hopefully the iron is good. It ohm's out fine.

Blair

[DeathRex]

Almost all of the old radios I've seen have that capacitor, even the German radios. It's there to kill hf (>30000hz). Probably don't need them now a days, and don't want them because those caps do kill some higher treble.

[Blair]

Cool. Thanks!