5F1 build developed squeal when on high volume

[northern picker]

Hello all, nice site you have here... lots of great information!

I completed a 5F1 build (weber kit) a few years ago. Recently I put in a Dr Z brake lite attenuator (not sure it is related to my issue today...) and then earlier today when I went to turn the amp up to full it started to squeal/squak when the amp is turned up to over 10 (it goes to 12), and it does this with a guitar plugged in and when no jack is plugged in.
When I remove the 12AX7 tube, there is no squeal/squak... and I've tried about 10 different 12AX7's, all with the same result.

I have since removed the attenuator, again the same result. Did I blow something? the amp still sounds great as long as i don't turn it to 8 or so.

Any suggestions would be appreciated.

Thanks,

[loudthud]

The first thing I would try is a remote speaker. This will tell you if the feedback is acoustic or some kind of purely electronic feedback. Do you have another speaker you can plug into the amp? Move it some distance from the amp and see if it still feeds back. Or you can just remove the amp from the cabinet.

[northern picker]

Thanks loudthud,
I don't have another 4 ohm speaker handy to check and troubleshoot the feedback suggestion. But I did some more testing and i found out that I incorrectlly diagnosed the amp last night.

The amp squeals when no guitar (or cable) is plugged into the amp and the amp is turned to 10 or more.
The amp does not squeal when the guitar is plugged into input 1 (and the amp is very quite when my strat is in 2nd and 4th position (because of the reverse middle pickup)
When a guitar is plugged into input 2 the signal fizzles (and sounds weak) around 10 and then starts to feedback a little bit around 12 (but not at the same level as when nothing is plugged in).

To me, that sounds like it's an issue with the input wiring... and maybe I never noticed it earlier because I usually have a guitar plugged in when I turn the amp up. All of the issues are the same regardless if I use the attenuator of not. (so I think i can rule that piece out).

let me know if you see anything different or have suggestions on anything else I should try.

Cheers,
Ernie

[loudthud]

Well, you still should try removing the amp from the cabinet and extending the speaker cable. Could be as simple as a microphonic tube. Do you have any spare tubes you could try? If you flick the 12AX7 with your finger, do you hear a pinging sound at the same frequency as the squeal? An electronic squeal caused by an oscillation will usually start and stop abruptly. A microphonic squeal has a softer start and stop.