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

[misterc57]

Hey Folks

I have a hum-buzz noise on the Normal channel only. With volume pot at zero, no inputs connected, you hear it in the background.

Dial the volume pot up and it gets much louder in the volume "3" to "7" range. Past "7" to "10", it gets quieter as it was at the "0" position.

Also, I notice that if I tap on the bright switch wire (anywhere from the switch all the way to pin 7 of V3) I have an additional crackle noise that should not be there. I have a good connection and continuity on the wire from the switch to pin 7.

Hum-buzz is present regardless of bright switch position setting.

I have tried 4 different tubes at V3 and the hum-buzz is present.

I have also replace coupling caps. The hum-buzz is still present.

If I remove V3 there is no hum-buzz noise. Bass channel works fine with no noise.

I have taken some scope images at the output with the Normal channel volume set around "5".

V3 Pin 1 239 VDC
V3 Pin 3 2.1 VDC
V3 Pin 6 264 VDC
V3 Pin 8 2.08 VDC


Thank you in advance for your help.

MarkO

Fender_bassman_ab165_schematic.pdf

[Helmholtz]

Your scope pics show 2 overlaid traces indicating a trigger problem and making analysis impossible.
Don't rely on auto-set.
Select only one channel and adjust the trigger for a stable single trace.

[g1]

Why start a new thread? Things can crop up that could be relevant from one fault to the other.
If the normal vol. won't kill the hum, but pulling the normal ch. tube (V3) does, that implies the triode after the volume. Is the cathode cap at pin 8 good?

[Chuck H]

I think the hum/buzz diminishing with the volume control above seven is interesting. This is actually common to the signal chain in similar amps when channels are jumpered. Since the two channels are out of phase it's almost certainly the coupling of the two channels causing the change in amplitude. Meaning that the two channels are likely communicating with each other through the power supply (so filter caps may need replacement). And that the problem exists on both channels when it's manifesting in the normal channel at higher volume settings.

[misterc57]

I debated starting a new thread.

All cathode caps were replaced before I got the amp. Also as seen in the pic there is a new "blue" wire connecting the cathode cap to pin 8.

I just tried it with V1 pulled (bass channel) and V3 inserted. I still have the hum-buzz as described.

Will try more tomorrow. Thank you

[Chuck H]

Rethinking my post above...

A shared ground lead that isn't making contact can also couple the channels.

[Delta362]

I recently had one of these in with very similar issues. I ended up replacing the 0.1 coupling cap to V2, plus a few resistors in that circuit. See attached image showing them on your amp. It's now completely silent.

[misterc57]

I recently had one of these in with very similar issues. I ended up replacing the 0.1 coupling cap to V2, plus a few resistors in that circuit. See attached image showing them on your amp. It's now completely silent.

I will check that out.

I am also seeing perhaps weird readings at 12AT7 V4 PI tube

V4 pin 1 222 VDC
V4 pin 2 58 VDC
V4 pin 3 90 VDC

V4 pin 6 216 VDC
V4 pin 7 60 VC
V4 pin 8 90 VDC

Should pins 2 and 7 have these high VDC?

There are three older caps around V4 feeding the grids all 0.1 but in the schematic one looks like it should be 500 pF. And the third 0.1 cap in that location I have not found on the schematic yet. I need to do more digging to understand this section.

I have pulled these caps and will replace then once I understand what should and should not be there.

[Helmholtz]

I am also seeing perhaps weird readings at 12AT7 V4 PI tube

V4 pin 1 222 VDC
V4 pin 2 58 VDC
V4 pin 3 90 VDC

V4 pin 6 216 VDC
V4 pin 7 60 VC
V4 pin 8 90 VDC

Should pins 2 and 7 have these high VDC?

With a LTPI the grids sit a voltage just below the cathode voltage.
So I would expect something like 88V here.
Seems your meter input resistance pulls down the voltage.
As grid voltage readings vary with the meter resistance, they are typically not specified on a schematic.
But as cathode and plate voltages look good, grid voltages will be fine (without the meter).
So no PI problem.

[g1]

There are three older caps around V4 feeding the grids all 0.1 but in the schematic one looks like it should be 500 pF. And the third 0.1 cap in that location I have not found on the schematic yet. I need to do more digging to understand this section.

Are you sure this is an AA165 or are you just going by the sticker? The layout drawing for the AA165 does not match at all. (attached).
If you do not have the correct schematic that would explain the component discrepancies.

[misterc57]

Are you sure this is an AA165 or are you just going by the sticker? The layout drawing for the AA165 does not match at all. (attached).
If you do not have the correct schematic that would explain the component discrepancies.

I was going by the sticker in the cabinet. Looks like I have a different version chassis which I am trying to identify. Those 3 0.1 caps feeding V4 is my current clue to look for.

[g1]

It's possible others have previously changed values not having the correct schematic as well.
But that 47K looks like a stock part and I think it is the feedback resistor?

[g1]

How about AB165, attached.

[misterc57]

Looks like it may be a AB165. But I also found two other models with the 3 0.1 caps; AA270 and AC568

Edit. Just saw last post from g1 on AB165

bassman_ab165_layout.pdf

Fender-Bassman-AB165-Schematic.pdf

[misterc57]

I have verified all the components/connections related to V4 per the AB165 model. So going with this amp being a AB165.

AB165 has 0.022 coupling caps to the output tubes. I installed 0.1 caps there (per AA165). Going back to 0.022 but wondering what effect this would have and could it cause the hum-buzz noise.

Thank you