Adjusting output of Driver stage in 5F2A

[funkgang49]

Can some of the learned and astute amp techs here at the forum help me to understand the function of the 220K resistor after the driver stage in a 5F2A type circuit?
Does it impact driver stage output to power tube?

[martin manning]

Its primary function is to insure that the power tube's grid stays at ground potential. It does affect the drive voltage as well as the frequency content getting to the power stage (in combination with the coupling capacitor). It's value is chosen to be low enough to insure that the power tube grid stays at ground (and recovers quickly if it is driven above ground), but no so low as to reduce the drive signal level too much. If you wanted to control the drive signal level, you could replace that 220k resistor with a 250k pot (wiper connected to the power tube grid) to create a master volume. Overdriving the power stage is important to the tone of the circuit, however.

[funkgang49]

Thanks for the explaination!

If you wanted to control the drive signal level, you could replace that 220k resistor with a 250k pot (wiper connected to the power tube grid) to create a master volume. Overdriving the power stage is important to the tone of the circuit, however.

Here's my problem:
My Preamp section (6SL7-twin triode wired as one V1a & V1b: setup clean controlled by Vol. 1 & 12AX7-V2a wired slightly hotter controlled by Vol. 2 w/ shared Tone) sounds good but the output is a little low so I tried a full frequency (25uf@25v) bypass cap on the Driver stage (V2b) Cathode and now it roars! So much so that I can't get the cleans I had before on V1. The Power tube (5881/6L6) is getting slammed! With the Volumes cranked it's too much. I would be happy if I could back it down to the point where I am getting the extra output without saturation madness.
I currently have a 10K NFB resistor from Driver stage cathode to Speaker Out as well as a 5K Presence control.
Any suggestions?

[Structo]

Isn't the stock NFB resistor 22K?

That may cure the distortion.

[martin manning]

Put a pot in as described above and see if dialing down the drive gets you where you want to go. If you find a setting you like, you can replace it with a fixed divider.

[funkgang49]

Put a pot in as described above and see if dialing down the drive gets you where you want to go. If you find a setting you like, you can replace it with a fixed divider.

So less resistance will allow more signal to dump to ground?

Isn't the stock NFB resistor 22K?

That may cure the distortion.

Yes. I'll try both suggestions and see where that takes me. Thanks much all!

[tubeswell]

Or you could try going to a BF style NFB using a 47R and 2k7 in the NFB loop. (The 47R sits 'underneath' the 1k5 cathode resistor, and the 2k7 goes between the Ot secondary and the junction of the 47R and the cathode resistor). Then you can bypass the cathode resistor for the driver stage with whatever value bypass cap you prefer (to get your desired roll-off).

[funkgang49]

Isn't the stock NFB resistor 22K?

That may cure the distortion.

Is there an upper limit to the value of NFB resistor, where it no longer becomes effective?

[tubeswell]

Isn't the stock NFB resistor 22K?

That may cure the distortion.

Is there an upper limit to the value of NFB resistor, where it no longer becomes effective?

22k is in this circuit is nowhere near high enough for that