Voltage jump

[tsutt]

This has to do with my scott hifi amp which now working sounding good. has been running pretty good. for a day had a week channel found the problem, fixed it. i have it hooked to a variac with dmm monitoring line v and using another to measure dc inside. it has a bias pot and a balance pot for each channel. i was adjusting the balance on one side. when in an instant my meter started acting up i knew something was wrong took a quick look at the power tubes and 2 were starting to redplate so i shut it off all told maybe 3 seconds. let it cool for a minute turned down the variac and and the bias post just to be safe, powered back up to find it took 7 less line volts to get back to my B+ v so i turned it back up to where i was just to do a quick check and B+ had jumped 20volts. SO this was about 20 minutes of being on at idle so something is changing when it gets warm is my guess but what? there is a power resister that is supposed to be 8k 20 w that is measuring 8.9k cold and 10.5 k hot all the others, there is 5 power resisters measure small change one being a 750 that is actually 740 cold 736 hot Any idea what would cause this kind of jump? and the line voltage hadn't changed at all.


Thanks
Todd

[tsutt]

well that was 1oclock in the morning. only the PT rectifier and 20 ohm resister involved in B+ at the OT. Would a rectifier tube act like this? the 20 ohm seems stable so that leaves the pt it does get warm, ot's stay cool. I guess i'll do another power up test to see if it does it again.

[xtian]

Bias pot may be dirty. Sudden jump in current might have caused this when u were messing with bias.

[Reeltarded]

Fer sure! I'd expect it.

[Structo]

How old is this amp?

Did you replace all the electrolytic caps, particularly the filter caps and bias caps?

A bad cap can make the voltage go all over the place.

[tsutt]

It dates to 1964, All E caps replaced all pots have been cleaned best i can and sweep checked with the ohm meter for consistency.