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Well I have a 68' Fender Twin that after biasing to about 65% max plate dis.. one side (pair) within a few minutes - no load just a large 8 ohm resistor in the output jack- rises to over 60 ma - the other pair rises about 2-3ma then remains stable .... could this be the OT shorting on one side? thanks for any tips ...
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No, I don't think that. One way to read plate current is the shunt method, wherein you connect a current meter from center tap to plate on the power tubes. That effectively shorts across the transformer.
Why does it do it? Isolate the problem.
Swap the tubes. Move the 60ma pair to the other end. Now do the same tubes do the same thing in their new homes? Or does the other pair now do it in the same socket as before? That will tell us if it is the tubes or the amp.
If one tube has a short to grid, it can kill bias to that tube, and since the tubes are wired together, it can kill the bias to its buddy, so they both overheat. So if the pair does this, pull one of them out and seeif the remaining one still rises to 60ma. Then put it back and remove the other one, same test. If there is a bad tube, this will tell us which one.
If the same socket does this no matter what tube is in it, then the amp is at fault. SO when it happens, measure the grid voltage. Does the bias remain teh same, or does the bias voltage fall away? A leaky coupling cap from the PI can let CD+ voltage through to counteract the CD- bias voltage.
And how are you measuring all this?Education is what you're left with after you have forgotten what you have learned. -
Hi Enzo,
thanks for your help. I remembered about swapping tubes after I posted, so I did and the results were different. The bias I did rise up but to a point only 10ma higher than the other pair ... Then I pulled out several other 6l6GC's of various brands and tried several iterations ..closest I could get was three of the 4 within 7ma and the remaining tube 12ma higher than the others on average. All of this I measure using 1% 1 OHM resistors across each power tube cathode with a good quality FLuke DMM. So does all of this mean I can only use Quad matched tubes since I have only ONE bias voltage source and trimmer? And if that is so., the tubes that went off so far as described in my first post are fairly new and were purchased as a 'matched quad,. I only have about 7-8 hours on these tubes (do not use the Twin very often).
Also bias voltage is steady, all caps are in excellent shape. I have read that tubes drift off bias after use or after a break-in period ..but shouldn't after say a break-in period of some time (unknown to me) the tubes remain fairly consistent bias wise?Comment
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