#1
Hi all, my bassist (who is a bit of a gear collector) has a Fender Red-knob "evil" twin that has not been used for a while and needs some attention.

We're trying to figure out what likely needs replacing/repairing following the below event:

Amp is plugged in an turned on, there is a quiet hum, then a steadily increasing screech quickly powered off.

Waited a few minutes, power it back on, screeches again, power off. Wait again, on again, no screech, but is humming again, quieter than before.

Plugged a lead into Ch.1 with no guitar, and touched the free end with finger, no sound from the amp (both before and after the screeching). The light for Ch.1 stayed lit even when unplugged.

Plugged the lead into Ch.2 and the light for 2 lit up (after the screech)


Any idea on what might be happening? Not looking for a new amp, would like to get this one fixed first.

I'm thinking the tubes have gone, but want to check
#2
The screeching sound slike maybe the pre amp tubes might have gone microphonic? I had the same happen on my AC30 a few years back, pre-amp ended up being at fault there. As for the other bits, sorry i can't be much help there.
#3
Its the evil twin coming out. Your amp is possessed, send it to me for...ummm...repair.
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#4
Quote by Sephiroth976
Hi all, my bassist (who is a bit of a gear collector) has a Fender Red-knob "evil" twin that has not been used for a while and needs some attention.

We're trying to figure out what likely needs replacing/repairing following the below event:

Amp is plugged in an turned on, there is a quiet hum, then a steadily increasing screech quickly powered off.

Waited a few minutes, power it back on, screeches again, power off. Wait again, on again, no screech, but is humming again, quieter than before.

Plugged a lead into Ch.1 with no guitar, and touched the free end with finger, no sound from the amp (both before and after the screeching). The light for Ch.1 stayed lit even when unplugged.

Plugged the lead into Ch.2 and the light for 2 lit up (after the screech)


Any idea on what might be happening? Not looking for a new amp, would like to get this one fixed first.

I'm thinking the tubes have gone, but want to check



to me it sounds like a filter cap has gone bad, but that is kinda a shot in the dark. i have also had something similar happen to my THD when a resistor went bad on it.

if you want to isolate a preamp tube you think has gone bad then get one or two 12AX7's and individually replace each preamp tube with the new one until you find the problem 'fixed'. i doubt it is a preamp tube but it's worth the effort to be sure.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
#5
Thanks for the quick replies. I'll have a look and see what happens with the tubes. and we'll try to strip it down to have a look at the board (I've heard they can be a pig to work on). My bassist (who owns it) is thinking of replacing all the tubes in any case, as he's not sure when it was last done, and they may have been knocked about when he moved.

I think the connector problems may be a bit of corrosion (we're on an island where there is fairly high humidity, and you're pretty much never more than a mile from the sea). so Will try cleaning them, and checking the joins

RBM01991 - I'll try whacking it with a bible a few times, failing that I'll give you a bell