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Old 5th January 2012, 07:07 PM   #1
bubba_b is offline bubba_b  United States
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Default Issue w/ DIY tube amp build

For the life of me, I can't find the cause of this problem! Please take a look at the pics and see what you can find. Thanks guys!
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Old 5th January 2012, 10:11 PM   #2
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Any chance to get a hint of what problem you have?
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Old 6th January 2012, 01:35 AM   #3
bubba_b is offline bubba_b  United States
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I wrote it in the pics, but here's the problem. With the 12AT7 only, everything is fine. Once the 12AX7 is switched in, it still works fine until the gain gets to about 2 o'clock. From that point on, all the voltages (except (-) bias and heaters) change. B+ plates all go down, cathodes on 12AT7 go down slightly, cathodes on 12AX7 go up, screen goes down on the 6V6, and B+ on the OT goes up slightly. Also, DC on the grids go from less than half a mV to nearly 200mV. I've changed out PS caps, tried different values, swapped every coupling cap, tested all resistors, swapped tubes, rewired. No changes.
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Old 6th January 2012, 02:49 AM   #4
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Sorry, it was hard to read in the picture earlier, not sure why.

Do you have access to a scope? You might be getting oscillation at ultrasonic frequencies. High gain amps are tricky. Try a small value cap across a plate resistor or two to see if it helps. You may also need another power supply stage, you have four stages of gain on one node.
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Old 6th January 2012, 01:20 PM   #5
bubba_b is offline bubba_b  United States
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Thanks, I'll try the feedback cap and see what happens. I did try adding another PS cap but it didn't change anything. Still saw the same voltage drops when cranked.

Also, on the first pic, does the V difference between the plate and screen on the 6V6 seem a little big to you? Shouldn't they be fairly equal, screen being a tad lower? They're closer on the second pic readings. Thoughts?
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Old 6th January 2012, 03:00 PM   #6
DF96 is offline DF96  England
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You probably have your input too near your output, so capacitive feedback is making the whole thing oscillate. Try separation or screening. Extra HT/B+ decoupling may help. Not bigger caps, or parallel caps, but extra caps with extra resistors so whatever is happening on one stage can't just feed through to another stage.
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Old 6th January 2012, 09:57 PM   #7
bubba_b is offline bubba_b  United States
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UPDATE: I've installed another 22u cap after the last node feeding the 12AX7 with a 1k/2W resistor between the nodes. Each tube now has it's own B+ node. No change, but I'll leave it in anyway. I've gotten rid of the motor-boating and added a 100p cap across the stage 4 plate resistor. Also added a 220k resistor after the .022u coupling cap to the cathode of stage 4.

The issue seems to be narrowed down to the 4th stage. Measuring the cathode bias voltage on stage 4, it starts at 1.7v (same as stage 3) with gain at zero. As the gain is increased, it steadily rises to 3.8v at full gain. Stages 1-3 bias remains steady throughout. It's causing severe compression and signal cutout with light pick attack and on sustained notes.
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Old 9th January 2012, 07:35 PM   #8
bubba_b is offline bubba_b  United States
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Fixed it! Blocking distortion was the culprit. I installed a grid stopper right after the gain before stage 2 and that did it. Tried doing the same on stages 3 and 4 earlier, which decreased the problem, but didn't cure it. Also helped smooth out some hi end.

Thanks for the input, glad it's all over!
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