I had an issue last week after running the amp for a long period, where I heard a quiet "pop" and then the amp slowly became noisy over the next 5-10 seconds. I let it run for about 3 minutes as it was still playing, then shut the amp off and took out the multimeter, turned it on and monitored for stability. Bias voltages on all 4 points were 100-150 mV, far from the 400mV they should be. However the amp still semi-worked even at that, just was noisy.
My first assumption is the quad cap, and I found this post: http://dynacotubeaudio.forumotion.com/t1053-blowing-rectifier-tubes and tried to measure capacitance on the quad sections based on that with the multimeter (which does have a capacitance feature) but results were inconclusive or low, in the range of 10-50 nF. This either means that the capacitance is actually very low, or I did it wrong. If someone could explain the procedure (which points to test, I just went from ground to the leads) for this then that might help.
The other possibility is the rectifier tube, but I figured that would make more of a show if it went and I didn't see anything, nor do I see anything when it's turned on again, seems normal. However the rectifier is the original Mullard 5AR4 still, so I wouldn't be surprised.
Thanks for any help diagnosing this!
Last edited by calinet6 on Sat Feb 11, 2012 1:01 am; edited 1 time in total