Feedback resistor on Baxandall tone stack?

[porkchop61]

Hello all-
I'm still working on my McMartin PA amp to guitar amp conversion, and I saw something curious on the schematic.

There is a 6.8 ohm resistor connecting the screen of the 6AV6 to the coupling capacitor of the Baxandall-type tone stack. I should point out that I have replaced the 6AV6 with a 12AX7 due to noise issues.

Is this resistor a feedback to prevent distortion? I've never seen this used in any guitar amplifier for this type of tone stack.

I did remove it, and I don't hear any difference in the tone of the amp.
Any ideas why this component is there?
Thanks.
Glenn

 

[djmiddelkoop]

Are you sure you put in and removed a 6.8 ohm ?

I think it should read 6.8M ohm and it serves as a feedback resistor to lower the output impedance of the first stage (read driving ability) to drive the network better, no matter how the pots are positioned.
This will also keep the gain of this first stage more constant, independant of the load.

If it suites you, it can be removed indeed.

Dick.

 

[porkchop61]

Thank you for the information Dick.

I didn't put the resistor in, it was there when I purchased the unit. I didn't check when I removed it so see if it was 6.8M or 6.8Ohms so you're probably correct.

I should probably tell you a little more of how I'm using this half of the 12AX7 now. It is actually the third gain stage in the amp now. I have another 12AX7 before this tube and am using both sides for gain with independent volume controls.
I don't know if this makes any difference in the driving impedance as you say.
I'm currently not using the other half of the 12AX7 at the moment, but I may move the tone stack to a cathode follower instead of the plate loaded configuration as it is now. I think this will reduce some of the effects of the tone circuit?

Like I said, it does not seem to make any difference in the tone of the amp, so I'll probably leave it out.

Thanks again!
I'm really learning a lot from all the experts on this forum.
Regards,
Glenn

 

[djmiddelkoop]

you're welcome.

I have another 12AX7 before this tube and am using both sides for gain with independent volume controls.

I don't know if this makes any difference in the driving impedance as you say.

No, it doesn't.

I'm currently not using the other half of the 12AX7 at the moment, but I may move the tone stack to a cathode follower instead of the plate loaded configuration as it is now. I think this will reduce some of the effects of the tone circuit?

Yes, the use of a cathode follower would benefit the gain stage against the loading effects of the tone circuit. As long as you got a spare half, wire it in.

Dick.