Fender Bassman 50 Blackface AA165 - Normal Channel Hum-Buzz Noise, Help!

[misterc57]

Problem persists.

Observations.

V3 and V4 tube sockets are not designed for shields and are a different type from V1 and V2.
Dog house cover is missing.

Things I have tried. None have changed issue.

I was wondering if V3 socket may be faulty. I moved half the connections (pins 6, 7 8) to V2 half not being used. Still had issue so moved the connections back to V3.
Numerous different pre-amp tubes.
Checked all wiring and grounds.
Checked all pot grounds.
Checked all filter cap grounds.
Replaced nearly every coupling cap. Filter caps had already been replaced.
Replaced numerous resistors.
On the last filter cap tried different values. Currently has a 16 uF.
Tried tube shields.

Questions.

Could the Normal channel volume pot cause an issue? It checks out well on the meter.
AB165 uses 0.022 final coupling caps. AA165 uses 0.1. Both worked, which is better to use and why?
AB165 has a 220K resistor connection between output plate and PI plate. AA165 does not. I had tried it but it did not change anything.
Can anyone think of anything else to try or check?

Thank you.

Latest images.

[mozz]

Have you tried rotating the output transformer since it looks like a replacement?

[Helmholtz]

Main issue is the ultrasonic oscillation...
I see no sense in randomly changing (most likely) good components.
Different coupling cap values neither cause hum nor oscillation.
0.1µ coupling caps pass slightly more bass than 0.022µ caps.

There's a major difference between the AA165 and the AB165 regarding the global NFB and OT primary phase.
The AA165 uses the standard wiring where the NFB is applied to PI tail and the second grid of the PI.
The AB165 applies the NFB to PI input. So OT primary leads need to be reversed.
The 220k resistors between power tube and PI plates provide additional local NFB and should not be used with the AA165 wiring.

Of course an amp having OT phase reversed will like to oscillate.

[misterc57]

Have you tried rotating the output transformer since it looks like a replacement?

Before it cam to me, the PT and OT are new. Do you mean rotate the physical OT 180 degrees?

[misterc57]

Main issue is the ultrasonic oscillation...


The AB165 applies the NFB to PI input. So OT primary leads need to be reversed.
The 220k resistors between power tube and PI plates provide additional local NFB and should not be used with the AA165 wiring.
The 220k resistors between power tube and PI plates provide additional local NFB and should not be used with the AA165 wiring.
Of course an amp having OT phase reversed will like to oscillate.

Intent was to replace all the old caps anyway.

Since we had to reverse phase on the OT primary, this amp wants to oscillate?

I will look again at the differences in NFB. I am uncertain if I should be wiring the NFB per the AB165 or the AA165 in this amp.

It appears I have the NFB setup like the AB165, but just missing the 220K resistor between power tube and PI plates. I will put them back in but I had tried that previously with no changes.

I am uncertain to call this amp either an AB165 or AA165 circuit. Seems like it has become a hybrid.

BTW, I decided to use 0.1 as the final coupling caps.

Thank you

[The Dude]

Regardless of circuit, replacement transformers are not always the same as wire colors relate to phase. It's worth trying reversing OT primary wires to see if it corrects oscillation.

[misterc57]

Regardless of circuit, replacement transformers are not always the same as wire colors relate to phase. It's worth trying reversing OT primary wires to see if it corrects oscillation.

This was done. Originally it was wired per wire color and there was high current draw and all sorts of problems. Reversing those wires cleared those issues. At that time I was connected to a dummy load but if it had been connected to a speaker I would have probably heard the alarm.

[mozz]

Before it came to me, the PT and OT are new. Do you mean rotate the physical OT 180 degrees?

Yes, it may be picking up hum, or a piece of steel in between it and the choke/power transformer.

[Chuck H]

If the OT orientation is actually a problem I would take the trouble to rotate the actual OT rather than install a shield. With the OT physically 180* turned the leads will be routed through the opposite chassis holes. This changes lead dress from stock and increases the possibility of instability.

[mozz]

I meant using the shield as a test. Turning it 90-180 should prove if it's the culprit.

[misterc57]

Progress.

I found a bad connection between the volume pot and the bright switch (at the switch) causing noise on the wire to the grid to V3. That wire seemed to act as an antenna for noise.

After some trials I found things to be less noisy by bypassing the switch. The grid wire now goes from the pot wiper directly to the V3 grid and the noise is barely noticeable at this point.

Interesting that the sweet spot for no noise on the normal channel volume pot is a setting around "4".

Thank you all for the help and putting up with my OCD on this issue.

MarkO

[Chuck H]

Progress.Interesting that the sweet spot for no noise on the normal channel volume pot is a setting around "4".

Your channels are interacting. There is either a flaw in the circuit or the power supply has a higher impedance than it should. The latter is usually due to failing power supply capacitors. I actually identified this in post 4.