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Re: JCM800 2203 does it need a new OT?

3/25/2004 8:28 AM
Rick Erickson	Re: JCM800 2203 does it need a new OT? I don't know where Marshall is getting their PI voltages from but there is at least 300V on the supply side of the PI plate resistors. Doing the math for 210V (300V-90V) across an 82K resistor gives a figure of over .5W, meaning they would be burning up the .5W plate resistors! I think that 90V figure is probably a mis-print that didn't get corrected. You really need an o-scope, dummy load and signal generator if you are going to try to fix amps. You might look at each tube stages plate, grid and cathode voltages for starters - write this info down and look for anything that looks odd. If pin one gives you a loud signal when probed then pin 2 should give an even louder signal, since it's the grid. One other thing to check - In order for the common cathode preamp stages in Marshall amps to self-bias you need a DC path to ground on every preamp tube grid, with the exception of the CF stage which is direct coupled to the previous stage's plate. With the amp off use an ohm meter to check each 12AX7 for some resistance (1M or less) from pins 2 & 7 to ground. The distortion you describe sounds like a stage is not biased correctly. I just had this happen to me on a mod I was doing on a 1959 100W 4 input model - I forgot to ground the other side of a 470k divider resistor after converting it to a MV style amp per customer request. Since your amp has been messed with I wouldn't assume anything has been done correctly until it's all been traced out & verified. Happy hunting! RE

3/25/2004 8:28 AM

Rick Erickson

Re: JCM800 2203 does it need a new OT?

I don't know where Marshall is getting their PI voltages from but there is at least 300V on the supply side of the PI plate resistors. Doing the math for 210V (300V-90V) across an 82K resistor gives a figure of over .5W, meaning they would be burning up the .5W plate resistors! I think that 90V figure is probably a mis-print that didn't get corrected. You really need an o-scope, dummy load and signal generator if you are going to try to fix amps. You might look at each tube stages plate, grid and cathode voltages for starters - write this info down and look for anything that looks odd. If pin one gives you a loud signal when probed then pin 2 should give an even louder signal, since it's the grid.
One other thing to check - In order for the common cathode preamp stages in Marshall amps to self-bias you need a DC path to ground on every preamp tube grid, with the exception of the CF stage which is direct coupled to the previous stage's plate. With the amp off use an ohm meter to check each 12AX7 for some resistance (1M or less) from pins 2 & 7 to ground. The distortion you describe sounds like a stage is not biased correctly. I just had this happen to me on a mod I was doing on a 1959 100W 4 input model - I forgot to ground the other side of a 470k divider resistor after converting it to a MV style amp per customer request. Since your amp has been messed with I wouldn't assume anything has been done correctly until it's all been traced out & verified. Happy hunting!

RE