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In Reply to: RE: debugging time posted by vinnie2 on June 21, 2017 at 07:30:49
Check the voltages and did not see any that were way off from what is on the schem, but they were bouncing around a bit instead of holding steady. I think I will have to assume I have something hooked up wrong and strip it down and redo the wiring unless someone has some other idea to check out. At least with the breadboard it will take less than an hour to redo the wiring. Love those breadboards!
I've not done breadboarding with tubes, but I did learn when constructing a pin-changing tube adapter that one can easily make tube-pin-numbering mistakes when viewing the tube socket from above rather than below. If you are viewing the tube sockets from above when wiring them on the breadboard, I can imagine the probability of making a mistake is non-trivial.
Been there myself, so I checked them with a continuity meter this morning and everything is as it should be.
Measure in sections: Get the power supply stable, then install and measure the the front-end/driver section, then plug in and measure the output tubes. It will help you nail down the problem.
Did like you suggested. PSU is ok, nice and steady by itself. Then I installed just the driver 12at7 tube and it's voltages are pretty much in the ballpark. I then plugged in the 6bq5's. I was getting around the 13 volts at the cathode resistor with the 400 ohm resistor in place that the schem calls for, for about 33 mA for cathode current if I am doing the calcs right. The tube manual says it should be about 65 mA, but would that be for a shared cathode resistor too? Still distorting a lot. The other voltages are in the ballpark too. I am at a loss as to what to try next. Maybe I should test some 6bq5's and !2at7's and make sure I have a good set of tubes? Maybe hook a test signal to it and follow it through on a scope and see if I can see where it starts distorting?
I think the power supply is ok because it is one from another amp, but I will check it. I will try isolating the other sections in the morning. Thanks for the tip.
It is distorting so bad I did not want to leave it on long enough to check the voltages yet. Figured something must be wrong to make all the noise, but maybe I will turn the volume way down and try to get some measurements before I see any blue smoke.
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