Harvard Question

[lpd]

Hi guys,
I have a harvard build (built by the late David Root) using 7 pin tubes (6At6 front end 6AV6 driver and phase inverter and 6AQ5 output tubes). He wanted to attempt a build with all 7 pin tubes. I added a small .0056 cap piggybacked onto the input coupling stage to add a little warmth and bass, however I've noticed the amp has a very weird overdrive with humbuckers. It's fine sounding with a strat but the humbuckers sound overdriven and the bass sounds spacey. Any ideas where in the circuit I begin to look to resolve?

[Stevem]

The Fender model 5f10 that uses a 6at6 as a first gain stage does not have a input coupling cap from the input jack to the first gain stage, so what is the value of the as built cap that you jumped with that small .00056 cap?

Does the humbucker sound crappy at any volume output level set from the guitar?

[sluckey]

Maybe the cap is just parallel to one of the 68Ks. Sunn did that with the Bright inputs on their amps.

[lpd]

The Fender model 5f10 that uses a 6at6 as a first gain stage does not have a input coupling cap from the input jack to the first gain stage, so what is the value of the as built cap that you jumped with that small .00056 cap?

Does the humbucker sound crappy at any volume output level set from the guitar?

Hi correct. It has a .022 for tone stack and I strapped a .0056 across to raise the value. Input is a 1.5k resistor and 25uf filter cap. Guitar is ok sounding with volume lowered to 7ish. Changed the Mallory 150s to Sprague brown drops that improved the tone a lot but still has a weird space like bass full volume.

[sluckey]

Replace that 25µF cathode bypass cap with a 5µF or even .68µF. Maybe use a SPDT switch to change cap values.

[lpd]

Replace that 25µF cathode bypass cap with a 5µF or even .68µF. Maybe use a SPDT switch to change cap values.

I tried removing the cap and no noise but lowered gain way to much. Tried higher and it worsened and lower no avail.

[Stevem]

What is the plate voltage on that first gain stage?
I bet it’s pretty low which is why you loose so much gain without that cap there, and thats likely also why the sound browns out so much with the added mid’s of a humbucker!