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Hi Guys:
A friend gave me a JMP Marshall Mark 2 Lead 50 watt head built in 1979 to fix. I replaced the input and output jacks for him. While I was in there I checked out the tubes. The 6550’s checked strong and were well matched (tested on my Hickok). I measured the plate voltage before biasing and came up with 353 volts which struck me as rather low for an amp running 6550s.
At first I thought the electros were weak but they also checked out good. Then I disconnected the secondary of the PT to read an unloaded voltage and came up with 268V AC. I was about to condemn the PT but what bothered me was that it looked fine visually and was obviously original. The part number is 1202 164. I did a little research online and found out to my surprise that for this particular transformer those voltages seem to be normal.
My question is what do I bias this amp at? If I plug those plate voltages into a bias calculator I get a suggested bias MA number of 83.2 for 70% AB1 operation. The highest numbers I can get are between 30-40 MA on the existing setup. What should I set it at?
BTW when I opened this amp up the mains AC was soldered to the 100V tap on the transformer. It looked that way from the factory. The resulting voltages were a little higher—289VAC and 381 VDC on the plates. I found a chart of this particular transformer on line and moved the AC entry on to the (correct?) 120v tap thus the lower output voltages that I noted above.
A friend gave me a JMP Marshall Mark 2 Lead 50 watt head built in 1979 to fix. I replaced the input and output jacks for him. While I was in there I checked out the tubes. The 6550’s checked strong and were well matched (tested on my Hickok). I measured the plate voltage before biasing and came up with 353 volts which struck me as rather low for an amp running 6550s.
At first I thought the electros were weak but they also checked out good. Then I disconnected the secondary of the PT to read an unloaded voltage and came up with 268V AC. I was about to condemn the PT but what bothered me was that it looked fine visually and was obviously original. The part number is 1202 164. I did a little research online and found out to my surprise that for this particular transformer those voltages seem to be normal.
My question is what do I bias this amp at? If I plug those plate voltages into a bias calculator I get a suggested bias MA number of 83.2 for 70% AB1 operation. The highest numbers I can get are between 30-40 MA on the existing setup. What should I set it at?
BTW when I opened this amp up the mains AC was soldered to the 100V tap on the transformer. It looked that way from the factory. The resulting voltages were a little higher—289VAC and 381 VDC on the plates. I found a chart of this particular transformer on line and moved the AC entry on to the (correct?) 120v tap thus the lower output voltages that I noted above.
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