weird fixed bias output stage

[electrocutioner]

There's a Sound City 50+ on my workbench that I'm restoring it. After all the appropriate work to bring it back in life the amp is ready for rebiasing and new output tubes.
While I was ready to add the 1R resistors at each cathode and measure the currents I realized that there were allready one resistor at each cathode. But one is 68R and the other one is 22R!
Check the schematic... SoundCity 50+

Any idea why Sound City did this? These resistors are not there in another SC 50+ schematic!
Maybe for balancing the output stage? Like the different resistors that other manufacturers put at the anodes of the phase splitter? Or due to the mismatch of the OPT primary windings?

It's something like a combination of fixed bias with cathode bias!

What do you say? Leave them or take them off? They're adding some feedback over there but maybe they're adding stability, too. I have no idea...

[electrocutioner]

Hey, I'm sure that someone can help on this!

[martin manning]

Combination of fixed and cathode bias yes, seen that before; adds some compression, perhaps. Unbalanced? maybe to add some second-order harmonic distortion to the power stage. Note that the PI already has un-matched plate load resistors in an effort to balance it's output.

As for removing the cathode resistors, my vote would be to leave them if you are "restoring" the amp. It'd be easy (and interesting) to jumper them and see what the effect on the tone is.

MPM

[electrocutioner]

Oups you're right! There are already unmatched resistors at the PI.

Just took them off and connected the cathodes at the ground! I let you know if I'll find any difference at the sound.
Just finished the rebias of the output tubes.... thanks Martin!

[martin manning]

Just looked at the other scheme without the cathode resistors... I see that the PI in that one has matching plate loads.

MPM