Fender Bassman 50 Blackface AA165 - Bias Problems

[misterc57]

As it is a new OT I suspect the phasing is reversed, causing the oscillation.

As in blue and brown are reversed at the plates?

[The Dude]

Quite possibly, yes. Swap them and see if it takes care of your problem.

[Helmholtz]

As it is a new OT I suspect the phasing is reversed, causing the oscillation.

This is a possibility, but in my experience oscillation caused by inverse OT phase is typically an audible frequency.

[misterc57]

Hey Folks

Reversing the brown and blue at the plates did the trick! Amp is working now. I will put in a matched pair and re bias and calc the plate dissipation, and report back.

New question on NFB. The new OT has 3 secondary taps, 4, 8 and 16 ohm. The NFB is connected to the 8 ohm. Wondering if this is correct or if it should be attached to the 16 ohm tap

Thank you all for the great help!

[misterc57]

new measurements

[Helmholtz]

New question on NFB. The new OT has 3 secondary taps, 4, 8 and 16 ohm. The NFB is connected to the 8 ohm. Wondering if this is correct or if it should be attached to the 16 ohm tap

Original circuit uses the 4 Ohm tap.
Connecting to a higher impedance tap increases NFB and can cause instability.

[Tom Phillips]

Original circuit uses the 4 Ohm tap.Connecting to a higher impedance tap increases NFB and can cause instability.

And... I'll add a comment directed at the OP.
In case it's not clear, the feedback connection should not be changed when you connect the speakers to different secondary taps. In the case of your amp, just leave it connected to the 4 Ohm tap.

[g1]

This is a possibility, but in my experience oscillation caused by inverse OT phase is typically an audible frequency.

Originally that was my experience as well. But occasionally over the years there has been HF causing 'crackling' in the audio, LF or midrange oscillation, hum, or other just odd symptoms due to various levels of instability. Enough that if there is any evidence of OT re-wiring or replacement, verifying phasing is one of the first suggestions.

[misterc57]

I will connect NFB to the 4 ohm tap.

I had been using a dummy load (which is why the oscillation was not audible to me).