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Discussion Starter #1
Howdy gang.

My friend has given me his 5150 (original from 1991) to have a look at.

It experiences a huge volume drop after about 15 minutes of loud playing.

This happens both with effects in the loop and without.

I’ve tried different (known to be good) power and preamp tubes - still happens.

I’ve inspected the board visually and haven’t found any cracked solder connections, burnt components, or bubbled capacitors.

Anything else I should look for/at?
 

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Assuming you have plugged your guitar straight to the amp without effects (front input or loop), the huge volume drop still persists.

Since you mentioned there were no visual signs or bad/cold solder joints nor bad caps, I would check the speaker cable for continuity.

Make sure the amp is power off, unplug the cable from the speaker out jack of the amp's end and measure the plug with your multimeter. If necessary, swap the speaker cable with a known good one to confirm if the problem persists.
 

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Discussion Starter #3
Yep. Speaker cable is good as well.

And I’ve tried with nothing in the loop, and with the loop jumpered.

I might start running through the possibilities again from square one. I’ll report back.

Worst case: OT

The fact that it’s good for a while at loud volumes tells me it could be a capacitor not holding charge, a soldier point heating up and being problematic, or the OT starting to crap out.
 

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Plug guitar straight into a pedal, then pedal into the FX return. This will take the preamp out of the equation and you can at least figure out if the problem is in the preamp or power amp (or maybe power supply).
 

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the common things to look for in 5150's and 6505's after the effects loop is the muting circuit transistor. I had a 6505 recently that had really low and distorted output. effects loop jacks were fine and clipping out the muting transistor didnt bring it back. It ended up being a bad plate resistor on the v5 preamp tube. it had gone open and replacing it brought the amp back to life. Clipping the muting transistor may be worth a shot the amp will function fine without it it just will have some channel switching pop. The transistor is common and easy to find. If that doesn't work then I think you may have to start probing and checking voltages which is a huge pain in those amps. the heaters are in series so you cant take measurements with tubes pulled. I had to make alittle jumper wire and jam it in to the socket pin and then insert the tube to take measurements. total PITA
 
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