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I installed new 6550 power tubes a few weeks ago, biased 40mA @ 510 Volts. customer comes back yesterday, wants more clean headroom. I check the bias...43 mA @ 610 volts. after a few minutes, I measure the plate voltage again...640 volts and rising (measured 650 volts a few minutes after). why is this happening? the filter cap cans are all original, bias caps too. thanks!
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Not likely that there is really 650V DC there, I think those caps would blow up, what voltage are they rated for?
Possibly a problem with grounding or your meter. Where are you connecting your black probe? Try at the neg. end of the main cap.
Do those caps ground to a lug screwed to the chassis? Check the solder at the lug, and check where the lug meets the chassis for corrosion.
Other than that, can you verify your meter is good for plate voltage in some other amp? (never hurts to check the battery either).
Are you getting any hum or AC ripple on the plate supply? Some meters may give an erroneous reading in such cases.Originally posted by Enzo
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That seems really high. Is your mains voltage selector in the right spot?"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22Comment
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That doesn't sound right. There is no practical way for the amp to increase its voltage in amounts like that. So next time you find such elevated readings, check the mains voltage from th wall outlet. Is your 240v mains up to 320v? Measure the AC voltage from the power transformer into your B+ rectifier. How does it compare to when it is cold.Education is what you're left with after you have forgotten what you have learned.Comment
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*embarrassed*
my meter's 9v battery measured 5.5V...my badComment
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At least it was an easy fix!
"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22Comment
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