Please explain change in plate V's

[Randall]

I got a replacement set of JJ6V6S for a set that went noisy after a short time. I put them in my 5E3 and put a DMM on the plate voltage to watch it over time. Last night it stayed very near 364v for 6 - 7 hrs. I switched it off and went to bed. This morning I switched it on again and the initial reading was about 375v, and as it has warmed up over the last 90 mins or so I have watched it slowly climb back down to about 364v. I understand things adjusting as tubes warm up, but this is over 90 mins. Can anyone explain this?

[pdf64]

It's only 11V, likely it's tracking your line voltage.
Pete

[Jazz P Bass]

Even so, what is the point of that particular test?
If you want to track the tubes there are better ways.
Monitoring the mains draw is the simplest.

[Enzo]

The voltages in the amp are all related to the mains. With approximately 360v on the B+ and 120 on the mains, you have a 3 to 1 ratio, so for every volt the mains changes, your B+ will change 3 volts the same direction. SO a 3-4v rise in mains could be responsible for an 11v rise in B+.

But even if your mains is rock steady at 120v, there are other considerations.

90 minutes is not forever, tubes heat up initially fairly fast, but they continue to settle down to their full operating temperature over time. You should always wait until an amp has been burning at least 20-30 minutes before making any final bias adjustments for that reason.

But in addition, even if the tubes are fully warm, the stuff around them also heats up, and that can affect resistances and other things, which can affect current draw. Since none of the supplies in these guitar amps are regulated, any change in current draw will also change the voltages. Those parts do not come up to full temperature anywhere near as fast as the tubes themselves.


Besides, from 364v, 11v is just a 3% variation. Pretty close in my book.

[Randall]

Thank you Enzo, that makes sense. This is my first build, so I am interested in understanding as much as I can about how everything interacts. I used the MM uncompensated PT, and I am trying to decide how I feel about running the higher voltages. I am letting it idle for a couple of days and watching different voltages to see what I might see.

[Randall]

Actually, I built two of these at the same time, and I am tracking them both. I do now see and understand the correlation between a 2 -3 volt fluctuation in line power and the ~7 - 11 volt fluctuations in the HV voltages in the amps. Thanks again guys, this is a great resource. I hope to be on the giving end at some point.