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Hey everybody! Saw something weird today and figured, as I'm out of time for this project today, i'd post my experience to see if anybody had every seen anything like this.
Amp is a... err... 83 fender "rivera" concert, totally icky when my "buddy" (think rat poop- where does he get these things?) brought it over. Cleaned up great and sounded nice but cold with a newish set of GT6L6Bs I had sitting in a drawer. I was lamenting the meager 27 ma of bias current (@470v) I got out of the bias balance and contemplating switching over the circuit and was rockin some chords for about half an hour at pretty close to full output when, from behind the plate of ONE power tube, was a great lil lightning show- not gassy, seemed like there were sparks but I only saw the light, accompanied by a huge crackle, about a second or a half second in duration. Before I had a chance to shut it down (2-3 seconds to reach over and kill the power supply) the decaying chord sounded fine again. Was that possibly a momentary, partial short from plate to ...? It certainly wouldn't have triggered my welding helmet, haha-
I wondered if a grid resistor was damaged and there wasn't any bias on that tube for a flash, but the resistor is mechanically solid and electrically as well, same for the 220k "feed" resistors. The rest of the bias circuit and the sockets seem ok- and, it seems like if it were the back half (rectifier +filter) bias circuit, it would be both tubes, not just one.
I'm nervous to pop another set of tubes in it, but also am not certain the history of those drawer tubes- I don't think they have many hours, but are many years old. The cathode Ecaps are all changed by somebody else who was pretty neat but the power caps are all... original. I'll change them to tighten it up and get the noise down a bit if my bandmate likes the thing. Again, they may not be great, but as it only happened in one tube, it's hard to fault the PSU caps. I was also wondering if there was possibly some strange damage to one side of the OT and I guess I'd be testing for winding leakage? Here's a schematic if you're so inclined-
http://www.stratopastor.org.uk/strat...ingdiagram.pdf
any experience from the bench would be appreciated. I'm gonna ruminate on it a bit and eat a burrito. thanks!
Amp is a... err... 83 fender "rivera" concert, totally icky when my "buddy" (think rat poop- where does he get these things?) brought it over. Cleaned up great and sounded nice but cold with a newish set of GT6L6Bs I had sitting in a drawer. I was lamenting the meager 27 ma of bias current (@470v) I got out of the bias balance and contemplating switching over the circuit and was rockin some chords for about half an hour at pretty close to full output when, from behind the plate of ONE power tube, was a great lil lightning show- not gassy, seemed like there were sparks but I only saw the light, accompanied by a huge crackle, about a second or a half second in duration. Before I had a chance to shut it down (2-3 seconds to reach over and kill the power supply) the decaying chord sounded fine again. Was that possibly a momentary, partial short from plate to ...? It certainly wouldn't have triggered my welding helmet, haha-
I wondered if a grid resistor was damaged and there wasn't any bias on that tube for a flash, but the resistor is mechanically solid and electrically as well, same for the 220k "feed" resistors. The rest of the bias circuit and the sockets seem ok- and, it seems like if it were the back half (rectifier +filter) bias circuit, it would be both tubes, not just one.
I'm nervous to pop another set of tubes in it, but also am not certain the history of those drawer tubes- I don't think they have many hours, but are many years old. The cathode Ecaps are all changed by somebody else who was pretty neat but the power caps are all... original. I'll change them to tighten it up and get the noise down a bit if my bandmate likes the thing. Again, they may not be great, but as it only happened in one tube, it's hard to fault the PSU caps. I was also wondering if there was possibly some strange damage to one side of the OT and I guess I'd be testing for winding leakage? Here's a schematic if you're so inclined-
http://www.stratopastor.org.uk/strat...ingdiagram.pdf
any experience from the bench would be appreciated. I'm gonna ruminate on it a bit and eat a burrito. thanks!
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