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[grimoire]

My guitar tube amp has a disease of some sort

When I turn it on everything works fine, but after heating up (whan I've played for 15-30 minutes) it starts crackling and buzzing, probably a short circuit.

I've had a guitar technician look at it, but he couldn't find anything. Obviously that isn't really weird since the amp only does it when it heats up after a while of playing. I'm not an amp-tech by any means, but from what I've read it could be the old tube sockets that can short circuit when they heat up and need to be replaced. Could this be right, or could it be something else?


here are some details

-it's an Ampeg VL-503 (designed by lee jackson)
-combo guitar amp (mono)
-tubes are new (JJ E34L) but first I did something wrong with the bias so maybe they're broken? *
-it does the crackling buzzing thing all the time after 15-30 minutes of playing (when the amp is heated more I guess). In the first 15-30 minutes it sounds good! Could this be the tubes, or most likely some short circuit?



* First the B+ fuse (1A slo blo) blew (first great sound, then after a while crackling/buzzing (also while not playing) for a few minutes, then no sound anymore-->the fuse blew). According to forums and a call to a guitar technician 90% chance it was bad end-tubes. so I ordered a fresh set of tubes (also preamp tubes since I figured they were old too) and put them in. The amp can be biased on your own since it has small knobs and leds that light up green when properly biased. So I did that, even better sound than before! but after a while bzzcrchhhtsskkk... same *Golly Gee* fuse blew. Now I couldn't figure it out by myself easily so I brough the amp to a guitar/amp technician. And gave him the schematics which he found very helpfull. He looked at it (measured it trough and stuff) and called me on the phone saying nothing was wrong, but that there was something weird with the bias system. When you turn the bias knobs from down to up the voltage was way too high (36 mA in stead of 26 or so, which probably blew the fuse. When you tusned the bias knobs from up to down the voltage would be good. He thought it was a design flaw or something.
well I picked it up and played for a while, everything worked fine (also put the pre-tubes in I bought). Now I would hear a certain nice timbre in my guitar tone that I didn't hear before, I adjusted the EQ to see where it was 'located' because I was curious. Well it stayed and I suddenly noticed that I was hearing the sound of the wood my guitar was made of, amazing! (never had a tube amp before)
Is it possible that the tubes got ruined because of the too high voltage? In a way that they now work fine, until they heat up and start buzzing and crackling?



I hope someone here can help me out!

thanks in advance:)

[MikeM]

My experience with crackling and buzzing after warming up has been either something arcing (cap, resistor or ??) or bad solder trace. I would have mentioned tubes but they are new? Pre amp tubes also?

Sounds like you found your amp's sweet spot! As for the wood of your guitar being the source of the tone I respectfully disagree. The tone wood debate has been around for a looooonnnggg time. While it may hold true for acoustic guitars, I feel it has very little merit when it comes to electric solid body guitars. The ONLY things that generate the ac signal to the amp are the strings and pickups.