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Vintage threads from the first ten years

check the grid resistor

:

12/8/1999 1:57 PM
J Fletcher	check the grid resistor If it's open the tubes will burn up. Could be a poor connection to ground on the end of that 1 meg resistor, or a bad connection at the grid end...Jerry

12/8/1999 4:56 PM

Re: DR Blows Reverb Driver Tubes

jc:

Something is causing the tube to run in overcurrent, and it's eventually failing. You need to carefully measure the DC voltages on the tube socket pins. Sections of the 12AT7 are paralleled, so measuring at pins 1 & 6 are same, pins 2& 7 are same, and pins 3 & 8 are same. What voltages do you read at each of these three points mentioned?

Lift one end of the cathode bypass cap and see if the conditions change. If not, that cap isn't causing the problem.

Any chance the reverb transformer could be bad?

Doc

12/8/1999 7:57 PM
jc	Thanks I took Andrew's suggestion and changed the resistor to a 2.2K and added a 25uf cap in parallel (stock was just a 680 resistor). I put in a new 12AT7 (my last one) and played for about an hour. It sounded much better, didn't get as hot and didn't go haywire. I'll continue monitoring the situation, but I think that that may have fixed it. Thanks guys! Jack

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