'65 TRRI squeals when input is unplugged

[afm_380]

My '65 Twin Reverb Re-Issue squeals loudly if I unplug an input while it's live, sometimes it happens if I throw it into standby before unplugging. Not sure where to look first. I notice that the RI has a 1500pF bypass cap across the NFB resistor where the originals didn't. Any ideas ???

[Jazz P Bass]

My first thought is "Why are you unplugging the input with the amp on?"
Does the amp sound o/k when you are playing it?
If not, the squeal could be that the output transformer secondary polarity may be incorrect.
The best test that I have used is to monitor the output sine wave.
Then disconnect the feedback resistor.
If the amplitude goes up, it is wired properly.(negative feedback)
If the amplitude goes down, then you had positive feedback, which is incorrect.
The solution then would be to reverse the output transformer connections at the plates.

[afm_380]

Yes, the amp sounds great if I'm plugged into either input of either channel, until of course I unplug to plug into one of the other inputs. Should I be powering the amp down between input changes? (fwiw ... my other amps don't complain like this)

[Jazz P Bass]

I don't know about your other amps.
I would not recommend plugging in an input with the amp volume turned on.
( I read somewhere of a customer that bought real high end speakers.
Brought them back blown. The sales guy is like WTF.
So he says "come in & show me how you operate your equipment"
The customer shows up with a microphone, turns the amp all the way up & plugs in the mic.
POW go the speakers.
The customer looks at the salesman & says: "Cool, huh")

[Enzo]

This squeal, is it continuous with nothing plugged in? Or is it a momentary noise as the plug leaves the jack? Does the channel volume control turn the noise down?

[afm_380]

yes, the squeal is continuous and can be turned down with the channel volume which sometimes removes the squeal, but more often the amp has to be powered off.

[afm_380]

... Continuous with nothing plugged in. I'd like to put an A/B foot switch into the two channels and use it for channel switching.

[Enzo]

OK, but once a guitar is plugged in it works OK, right? Just sounds to me like the grounding contact on the jack is not doing its job. That leaves the input unterminated when the jack is empty, so it amplifies whatever it can from ambient noise.