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Howdy,
new to this site but seen some good threads that have helped me troubleshoot here in my shop.
So a customer brought a JCM 2000 DSL 50w head in complaining about a squeal noise. He said he replaced the power tubes and fuses popped and after replacing several times the squeal persisted. Note: he replaced them with non-matched pair svetlana tubes. I have swapped every tube, pre amp and power, with new or nearly new tubes. Nothing of real interest to speak of, squeal remains although the fuses no longer pop.
I deal more with peavy and fender so a marshall is a bit beyond my fishbowl.
I've done a bit of research and turned up few leads. I have a schematic from drtube yet I've found a few mistakes... Also bias pots have an extremely low mA reading. Somewhere around 17 mA. Using EL34's the plate voltage is 475V, so i usually set the bias at about 60% (roughly 32mA). I cant get these pots to come anywhere close to that using a bias probe or the three prongs next to the pots.
Any suggestions or prior knowledge of dealing with these amps and maybe a reliable schematic would be very appreciated... it has sat in the corner of my shop for 2 and a half weeks... far too long.
new to this site but seen some good threads that have helped me troubleshoot here in my shop.
So a customer brought a JCM 2000 DSL 50w head in complaining about a squeal noise. He said he replaced the power tubes and fuses popped and after replacing several times the squeal persisted. Note: he replaced them with non-matched pair svetlana tubes. I have swapped every tube, pre amp and power, with new or nearly new tubes. Nothing of real interest to speak of, squeal remains although the fuses no longer pop.
I deal more with peavy and fender so a marshall is a bit beyond my fishbowl.
I've done a bit of research and turned up few leads. I have a schematic from drtube yet I've found a few mistakes... Also bias pots have an extremely low mA reading. Somewhere around 17 mA. Using EL34's the plate voltage is 475V, so i usually set the bias at about 60% (roughly 32mA). I cant get these pots to come anywhere close to that using a bias probe or the three prongs next to the pots.
Any suggestions or prior knowledge of dealing with these amps and maybe a reliable schematic would be very appreciated... it has sat in the corner of my shop for 2 and a half weeks... far too long.
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