Quote:
Originally Posted by Stan Martin
I have a Fender Blues Deville that is driving me nuts. The output voltages appear to be fine, however according to the Fender Schematic I have too much Ac voltage at TP2, it should read 222mV, but reads 1.5V. The plate resistors read fine the preamp tubes have been replaced and the problem persists.
Tonally the amp sounds great through the whole sweep of the volume, just low output. FYI the previous power tubes shorted and took out the screen resistors. Could a possible faulty volume pot cause this problem?
Also how would I test for a partially shorted OT, if the OT were the problem?
Any help would be greatly appreciated.
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Stan,
TP2 is the first plate of the initial gain stage - the 222mVAC figure is supposed to be the "response" to the specified input signal of 4mVAC, in other words a gain factor of 55 (or about 35dB, which is correct). If you are really getting 1.5VAC there, the only way for that to happen (assuming all else is correct) would be if the plate resistor was WAY out of wack - like about 600K ohms or so.
Of course, that would not result in low output, just a horribly distorted (and LOUD) sound all the time. And that is not what you are hearing, so check your input assumptions.
If the preamp out sounds good to you, then the problem is probably in the power amp. A blown OT would sound awful - check your suppressor grid resistors (the 470 ohm ones) and the limiting diodes that are attached to each power tube plate. Heck, just replace them all, since you report that bad power tubes ate your screen grid resistors.
Also, you can try feeding a different source into the power amp to see what it sounds like.
Finally, the gain switching in this amp is all done with FETs. Make sure they are all alive.
February 4th, 2008, 10:41 AM
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