AB165 Fender Bassman 50: multiple issues

[Helmholtz]

I'd expect the peak-to-peak voltage across the 220k resistors to be larger than 600Vpp at full power.

[Chip tele]

I'd expect the peak-to-peak voltage across the 220k resistors to be larger than 600Vpp at full power.

Is this high voltage what caused the values to drift from 220k to 270k? I thought it was curious that both resistors had drifted out almost exactly the same amount--about 50k high.

[Justin Thomas]

Is this high voltage what caused the values to drift from 220k to 270k? I thought it was curious that both resistors had drifted out almost exactly the same amount--about 50k high.

I would just call that "typical carbon comp." I've seen those piggies drift 100-200% high sometimes. ~25%? Typical. Obviously out of spec if 10% tolerance, but not atypical. Usually I'll leave them in if they're 20% tolerance & the amp isn't misbehaving or self-immolating.

Jusrin

[Chuck H]

I would just call that "typical carbon comp." I've seen those piggies drift 100-200% high sometimes. ~25%? Typical. Obviously out of spec if 10% tolerance, but not atypical. Usually I'll leave them in if they're 20% tolerance & the amp isn't misbehaving or self-immolating.

Jusrin

Some artists have had their favorite old Fenders "blue printed" just to identify the actual component values. I wouldn't be surprised if these amps get 1% metal films of the preferred and measured value as replacements when service is needed.

[Chip tele]

https://www.youtube.com/watch?v=vITMHK_hiW0

This amp looks like a later Bassman with the master control. He's getting some great tone out of it!

[Helmholtz]

Is this high voltage what caused the values to drift from 220k to 270k?

Not necessarily voltage itself.
All CC resistors tend to increase in value over time, even if unused.
Drift rate will increase with resistor temperature, so dissipation matters.
And dissipation is a consequence of the voltage load.
I'd expect those 220ks to dissipate between 0.3W and 0.4W at full output.

[Chip tele]

Heads up if anyone else needs Caig Deoxit! Musicians Friend has it on sale for 25% off. Only requires a membership with them. I just ordered a can and need to clean the contacts on my Bassman with it. I had already removed all the tubes and sprayed the contacts with CRC Contact Cleaner before the new noise issue developed. The consensus seems to be that Caig is more effective. If that doesn't work, I'll try pulling those 220k resistors that Greg mentioned in an earlier post. FWIW I'm not sure if this is a new noise issue or a continuation of the original noise issue that I originally had before I replaced the PI tube. Although it did play okay for about the first 30 minutes or so initially. Once the Caig product arrives, I will keep y'all posted. Thanks!

[Justin Thomas]

If you're still talking TV static, or maybe bacon frying, or some other "sizzly" noises, carbon comp resistors can do that. They may just make noise after getting warmed up. Aside from the 220k, plate load resistors can do it. And in the case of MY Bassman years ago, it was the channel mixing resistors. I reckon anything in the signal chain could contribute.

It's tedious to check them for noise one by one but I like to keep stuff as original as possible... If you can't get satisfaction with easier suspects, time to go there I think.

Jusrin