Bassman AA864 hissing

[Leevi]

How typical is the hissing in the bass channel?


https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_aa864.pdf


Normal channel is pretty quiet maybe because there is less amplification.


The bass channel is noisy even with zero volume.
I have done many trails to fix the issue but so far without success.


If I shortcut the 3. stage input (pin #7) of the bass channel to ground the hissing disappears.


/Leevi

[sluckey]

If you pull V1 does the hiss disappear?

[Leevi]

If you pull V1 does the hiss disappear?

Yes

/Leevi

[Leevi]

Timeout


I found an issue I have to fix first.


I'm modifiyng 1965 AA165 to AA864 and noticed that the tones stacks of the bass channels
are different in those circuits. I'm using now AA165's tone stack in AA864 which is probably not a good thing.


/Leevi

[Leevi]

I changed the tone stack but it's still hissing. The tone stack is located before
the volume pot so everything caused by the stack is damped when the volume is off.
It's hissing even when the volume is off.


/Leevi

[sluckey]

Not much between the vol pot and the grid of that third stage.

[Leevi]

That's right. I think it could be a microphonic issue. The V1 is a bit microphonic
but it's not depending on the tube since I have tried with other tubes. Might
be a bad contact somewhere making the circuit microphonic..


/Leevi

[Leevi]

The hissing dropped remarkably when I changed the carbon PSU resistors to metal film resistors.


/Leevi