1961 Fender Bassman 6G6 Bass Channel

[Steelwitch]

Hey y’all,

I’m blessed enough to work on a sweet 6G6 blonde bassman. It’s almost completely done but I’m having a difficult time tracking down some microphonic noise. Bass channel sounds terrible, the bass control is not present and the treble control is noisy as all hell! So, it’s in the tonestack somewhere presumably, I checked the grounding and it’s solid. It is most sensitive around the tonestack. It’s just so sensitive it’s difficult to pinpoint. If anyone has any suggestions of how to track down this nasty old hiss and get some low end back into this channel please share!!

[Randall]

I would verify that the bass pot is good, and that none of your tone caps are leaky. If the treble pot is noisy, look at the .1 cap it is connected to. For that matter, is there any voltage on any pots?

[g1]

Check that it measures 820ohm from the low end of the bass pot to chassis ground.

[Steelwitch]

Yup, the resistor going to ground from the bass pot is good, I just replaced the tone caps with a pair of Jupiters, the astrons didn’t age well but that’s not the problem exactly. It definitely has to be somewhere in the bass channel preamp since the normal channel sounds like it should. There is DC on the treble and bass pot, I can see it draining to ground on my meter. Is it possible the .0001 cap isn’t blocking DC? Maybe a source of my problems here

[Randall]

The treble and bass pots have nothing in common. It is certainly possible the .0001 cap is leaking DC, but it would be on the vol pot as well as the bass. If the treble pot has DC, it is likely either from the .1 cap up top, or the .05 off the second plate of the normal side.

[Steelwitch]

I’m gonna have to check tomorrow when I’m back in the shop. But DC leaking to pots would be noisy but wouldn’t stop the bass control from filtering at all right? The circuit is most microphonic around the tonestack, like, the .25 caps are amplified like crazy. Basically everywhere around the first gain stage will provide a nice “thud” when tapped. That seems like an indication of something other than leaky caps

[nosaj]

I’m gonna have to check tomorrow when I’m back in the shop. But DC leaking to pots would be noisy but wouldn’t stop the bass control from filtering at all right? The circuit is most microphonic around the tonestack, like, the .25 caps are amplified like crazy. Basically everywhere around the first gain stage will provide a nice “thud” when tapped. That seems like an indication of something other than leaky caps

Like a solder joint that is not good.
nosaj

[g1]

Like a solder joint that is not good.

That's why I asked that he measure not the 820R, but the resistance from the low side of the bass pot to ground. Not sure which way he did it. Measuring from the pot to ground will help find bad solder connections.
I guess if it's mounted right on the pot, bad solder would be fairly obvious. But if it's relying on the pot casing, you have to make sure the pot nut is tight and making good ground contact to chassis.

[nosaj]

That's why I asked that he measure not the 820R, but the resistance from the low side of the bass pot to ground. Not sure which way he did it. Measuring from the pot to ground will help find bad solder connections.
I guess if it's mounted right on the pot, bad solder would be fairly obvious. But if it's relying on the pot casing, you have to make sure the pot nut is tight and making good ground contact to chassis.

For me the key was "microphonic when tapping"

nosaj

[Randall]

How long would it take to hit every solder joint in the bass preamp and pots?

A good doctor once said to me,"you are allowed to have more than one issue at a time". Good advice.

[Steelwitch]

So, I went through this morning and hit all the solder joints in the bass preamp and there’s no difference. The grounds are solid. The pot casings are pretty firm to the chassis, it doesn’t sound to me like a grounding issue specifically or I would be hearing way more hum. This is more of a “hiss” from the treble pot. I measured the coupling caps in the bass preamp and all of them are within spec. Usually if there’s a microphonic component I can isolate it pretty quickly but this... is something else...

[Randall]

Freeze spray?

[Justin Thomas]

Any ceramic disc caps in there?

Justin

[Helmholtz]

What about posting a schematic and some pictures ? I don't like having to search and guess.

[nosaj]

So, I went through this morning and hit all the solder joints in the bass preamp and there’s no difference. The grounds are solid. The pot casings are pretty firm to the chassis, it doesn’t sound to me like a grounding issue specifically or I would be hearing way more hum. This is more of a “hiss” from the treble pot. I measured the coupling caps in the bass preamp and all of them are within spec. Usually if there’s a microphonic component I can isolate it pretty quickly but this... is something else...

Being firm or not. You should steel take the pots off and wire brush the star washers and the backside of the chassis where the star washers go.

Just like a battery cable can look nice an tight yet have a layer of corrosion preventing a GOOD electrical connection versus a GOOD mechanical connection.
Extra work but sometimes you gotta do the hard work.
nosaj