Microphonic noise V1

[ArturoQ]

Good Day,
As an aside issue to a voltage readings I am getting on a Fender Blues Jr. "Mark 1" greenboard, I began tracing down crackling and such. Found that V1 12AX7 was on it's way out. Did a lot of tapping around on the board and noticed that the cabling from V1 was microphonic at high Volume settings. followed it to the board and the C1 Cap according to Rev B is a .0022 microfarad 600V was noticeably microphonic. Wondering is this something apt to cause the issue once I changed the 12AX7 to a reasonable spare I had. It seems overly sensitive but I don't know if this is something that is a known problem. IIRC the preamp is kinda microphonic anyway.
Thanx

[Jon Snell]

All ceramic capacitors can get microphonic over time as they separate internally.
C1 is a known issue on the early models.

[ArturoQ]

All ceramic capacitors can get microphonic over time as they separate internally.
C1 is a known issue on the early models.

Thank again Mr. Snell! That goes on the list for the assist. It really depends on if I want to go and do the full disassembly now or later. Saw some interesting upgrade parts on the Junior coming from the UK, like replacement tube socket board. Considering that as well. Hoping to get myself in gear for building a Fender Princeton head clone.
Thanks from across the much used term "the pond".
Cheers

[Helmholtz]

All Class 2 ceramic caps are more or less piezoelectric, means they suffer from microphonics (or can produce noise sounds).

See here about microphony:
https://en.wikipedia.org/wiki/Ceramic_capacitor

[ArturoQ]

Thank you Sir. I have only noticed this since I ran the tapping test in the attempts to find out a problem I was having with the amp. It turned out to be a bad V1 tube. Majority of the problem was fixed. But I did notice the issue as noted. As the volume is increased a hum increases with the amp. Honestly never recognized if it was normal or not previous to the current investigation. Never use the amp with it pushing the pre amp section that hard. Posted the link to the schematic in reference to the bias question, sorry for my reticence {:-) Thanks for the link to Ceramic Caps.

[pdf64]

If enough gain is applied, almost anything will exhibit microphonics.
Generally it’ll be a bad valve that actually causes problematic symptoms though.

[Chuck H]

And... Don't assume a brand new tube is not noisy or microphonic right out of the box. They aren't always. Or even mostly. Finding suitably quiet and non microphonic tubes for the first triode in modern amps is harder than it should be. The tube for that particular duty usually needs to be selected from several. With the others serving less critical circuits.

[ArturoQ]

If enough gain is applied, almost anything will exhibit microphonics.
Generally it’ll be a bad valve that actually causes problematic symptoms though.

And... Don't assume a brand new tube is not noisy or microphonic right out of the box. They aren't always. Or even mostly. Finding suitably quiet and non microphonic tubes for the first triode in modern amps is harder than it should be. The tube for that particular duty usually needs to be selected from several. With the others serving less critical circuits.

Thanks for the information and confirmation!

[nevetslab]

I've had mechanical stability issues with those cheap pots that Fender uses on that amp and some others built in similar fashion. I often have to removed those, pull the wafer out of the frame to clean, then reassemble them. Of course they don't have mounting bushings as most of our amps have, so you're still stuck with those cheap push-on knobs and have to have the silly centering insert in place to keep them from breaking.

[ArturoQ]

I've had mechanical stability issues with those cheap pots that Fender uses on that amp and some others built in similar fashion. I often have to removed those, pull the wafer out of the frame to clean, then reassemble them. Of course they don't have mounting bushings as most of our amps have, so you're still stuck with those cheap push-on knobs and have to have the silly centering insert in place to keep them from breaking.

Howdy,
Considering the construction of the American made, other than cost cutting, it seems to be pretty well made and are way adequate for my needs. I baby it though.....