Marshall JCM 2000 Triple Super Lead
- Thread starter CJ
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CJ
Well-known member
problems:
1) conductive circuit board
2) negative temp coef resistors
3) wrong value resistors
had we not found this on the web, we would have been ready to jump off a cliff after a year or two of torture,
http://www.lynx.bc.ca/~jc/TSL122.html
1) conductive circuit board
2) negative temp coef resistors
3) wrong value resistors
had we not found this on the web, we would have been ready to jump off a cliff after a year or two of torture,
http://www.lynx.bc.ca/~jc/TSL122.html
CJ
Well-known member
rat's nest is a phrase sometimes associated with this amp, here is the chassis with the mother board missing,
in case you did not read the whole link posted above, the situation is this,
they used duff board material so high voltage from the plate and screen grid tube sockets migrates to the negative bias on pin 5, thus red plating the first pair of tubes that gives up first,
sure enuff, this amp had two good tubes, and two baked tubes,
lots af cables, be sure to use a sharpie and mark them, some have the same name, so mark top of bottom depending on which board they go to,
in case you did not read the whole link posted above, the situation is this,
they used duff board material so high voltage from the plate and screen grid tube sockets migrates to the negative bias on pin 5, thus red plating the first pair of tubes that gives up first,
sure enuff, this amp had two good tubes, and two baked tubes,
lots af cables, be sure to use a sharpie and mark them, some have the same name, so mark top of bottom depending on which board they go to,
Attachments
CJ
Well-known member
schematic pages here>
http://www.drtube.com/en/library/schematics/69-marshall-schemas
the bias schemo was hard to trace from the schematics, could not find the aux board with the trim pots, they use two, one for each pair of pwr tubes so you8 can match a set of mis matched tubes,
one way to save drilling out pin 5 on the mother board would be to use self bias (cathode resistor) but this lowers power, generates a lot of heat and there is not much room in the chassis for adding a big pwr resistor,
the bad resistor values were the grid stoppers, they used 220K instead of 6.8 K, you can parallel or stuff new ones,
note the caps feeding the bridge in this bias circuit>
http://www.drtube.com/en/library/schematics/69-marshall-schemas
the bias schemo was hard to trace from the schematics, could not find the aux board with the trim pots, they use two, one for each pair of pwr tubes so you8 can match a set of mis matched tubes,
one way to save drilling out pin 5 on the mother board would be to use self bias (cathode resistor) but this lowers power, generates a lot of heat and there is not much room in the chassis for adding a big pwr resistor,
the bad resistor values were the grid stoppers, they used 220K instead of 6.8 K, you can parallel or stuff new ones,
note the caps feeding the bridge in this bias circuit>
