Solid State Guitar Amp Forum | DIY Guitar Amplifiers

I've been having this problem for a few years, but never took the time to do anything about it.  I have an Orange Rocker 30 and it plays fine on both clean and dirty for a small period of time (usually around ten minutes) before randomly cutting to a to a very loud feedback sound.  If I don't use it for a while I can play it for around ten minutes again, but after the feedback comes in it comes back almost constantly.  Any ideas on what the problem is/how much it would cost to fix it?  Many Thanks.

Strange.
Might be thereverb tank feeding back.
Open the amp, start playing, tap the reverb tank lightly with a plastic pen and see whether it´s unduly sensitive.
If so, pull it and remount it on some sponge pads or similar to isolate it from the vibrating chassis.
If not that, tap the amp inside lightly with said plastic pen (or a drumstick) untilyou find the sensitive spot.

If this is the valve (tube) version of the Rocker, then you could have a microphonic valve.  Its usually the first one but not necessarily so.  If you tap them in turn with an insulated item eg a pen or drumstick as suggested above, then you should find it.
Note there are high voltages present :( be careful!