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Post by royb3988 on Aug 17, 2020 19:13:42 GMT -5
Hello...
My DUAL Showman is red plating power tubes again. This occured last year and somehow "self corrected" after I swapped the tubes around and substituted a good tube for a known bad tube.
Where...in the chassis...is the "Bias Circuit" located?
Thanks,
RoyB4988
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pdf64
Wholenote
Posts: 556
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Post by pdf64 on Aug 18, 2020 4:02:29 GMT -5
The bias supply rectifier (current limiting resistor, single diode, reservoir ecap) looks to be on a small board beside the pilot lamp assembly, squeezed between the power transformer (PT) and front panel. The high tension oHT rectifier (6 diodes) is also on that board, so lots of voltage and current on offer.
Then there’s the bias adjustment pot, and range limiting resistor, a couple of inches along, mounted on the chassis between the PT and main board.
Do you know how to check the idle plate / cathode current of the power tubes, eg using a ‘bias probe’?
Has your amp ever had its ecaps replaced? That of the bias supply is critical. When it gets tired, the power tubes will draw more current, cooking the transformers.
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Bbendfender
Wholenote
Mostly play Fender guitars and amps. I'm 71 and have had a guitar since 1964. Got serious in 1975.
Posts: 216
Age: 71
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Post by Bbendfender on Dec 2, 2020 12:34:01 GMT -5
If you are asking this question, I'd highly recommend you let a good tube amp tech take a look at your amp. Lots of high voltages and current to deal with in that area of the amp. Stay safe.
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Post by cedarchoper58 on Dec 21, 2020 14:45:47 GMT -5
what are you setting the bias at. I go 36ma and use a weber bias rite
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