marshall 50 watt clone voltage drop problem HELP

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marshall 50 watt clone voltage drop problem HELP

Postby cic on Wed Jun 05, 2013 4:20 pm

Is there a legitimate expert here that can help with this problem.

The multimeter readings on the power tubes are fine when the volume is at 0 but change dramatically when the amp volume is turned to 10. Video of the readings is on youtube. http://www.youtube.com/watch?v=29oI5gXdyjQ

Wall voltage at 120 volts. Issue is the same with both new and old el34 tubes.
Volume at 0
Power tube V4 Pin 3 Plate 431dcv, Pin 4 Screens 424dcv, Pin 1 Bias 38mA, Pin 2 Heaters 3.1acv
Power tube V5 Plate 431dcv, Screens 424dcv, Bias 39mA, Heaters 3.1acv
Volume at 10
V4 Plate 377dcv, Screens 362dcv, Bias 40mA, Heaters 3.1acv
V5 Plate 427dcv, Screens 422dcv, Bias 40mA, Heaters 3.1acv

The amp has a PPIMV. The dual pot reads 250k and 230k.
Also the HT fuse blows after 20 minutes of playing. I turned the bias way down and the fuse still breaks.
It sounds thin and goes into distortion when the amp volume is turned above 1.
cic
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Re: marshall 50 watt clone voltage drop problem HELP

Postby gusfinley on Fri Jun 07, 2013 3:38 am

The quick check would be to swap the output valve positions and see if the problem follows the tube. If V4 itself is not the problem then you have an issue in the V4 circuit that is causing excessive current though V4, thus causing the voltage drop.
gusfinley
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Re: marshall 50 watt clone voltage drop problem HELP

Postby gusfinley on Tue Jun 18, 2013 2:25 am

still no update?
gusfinley
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