Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: whalecalls on January 25, 2021, 05:56:20 pm
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Hello All,
So, I have a fair amount of buzz in my master vol Marshall plexi style amp. I believe I have isolated the buzz to the first preamp stage and I have substituted individual and all the preamp tubes with known good tubes..... Buzz goes away with V1 removed.... The noise is controlled by the preamp volume control and is affected by tone controls, etc.....
I measured the plate resistor and it was spot on at 100k and 220vdc on the plate. When I went to measure vdc across both ends of the .022 cap I got 210vdc. The weird thing is that when I probed the cap with my meter leads the buzz went away! ..... Any insight? I suspect I have a bad/leaky signal cap or.....? Any insight appreciated.
Here is a quick video of the buzz and it going away....
https://www.youtube.com/watch?v=8Nbg26LErXA&feature=youtu.be (https://www.youtube.com/watch?v=8Nbg26LErXA&feature=youtu.be)
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When I went to measure vdc across both ends of the .022 cap I got 210vdc.
if those are you couplers, the right side should have close to 0vdc
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> the buzz went away! .....
So did the signal. In that connection, the meter is a "leak" which pulls the output tube full ON, hard.
Don't do that.