Peavey VB-2 Heater Voltage is High

[bobloblaws]

Good that bias voltage stable, but bias voltage does not give any clue about bias current, which is what's important.

I've never considered bias current before. Can we assume a fixed resistance? If so, then a stable voltage would mean a stable current too, no?

[Helmholtz]

I've never considered bias current before. Can we assume a fixed resistance? If so, then a stable voltage would mean a stable current too, no?

Seems you missed post #3.
I prefer to use the term idle (or quiescent) current instead of bias current.

Can we assume a fixed resistance? If so, then a stable voltage would mean a stable current too, no?

Idle current needs to be measured in series with each tube.
As the amp doesn't have cathode resistors you should add 1R current sense resistors from cathodes to ground.
Or get a "bias" probe, i.e. a socket adaptor containing the resistor.

[g1]

Another non-invasive method is voltage/resistance measurements of OT primary halves. After measuring resistance and DC idle voltages on each half of OT primary, solve for current.

[Helmholtz]

Another non-invasive method is voltage/resistance measurements of OT primary halves. After measuring resistance and DC idle voltages on each half of OT primary, solve for current.

Yes, but to find individual tube current this method requires to insert only 1 PP pair at a time, so B+ will be increased.

[The Dude]

I don't disagree, but g1's method of measurement would at least tell you if there is excessive current in the circuit. If there is, then you could investigate further.

[Mick Bailey]

The observation that the tubes suddenly red plated suggests that there was a sudden increase in current and to my way of thinking there's possibly an intermittent fault that at the moment is stable. The trick is to be in a position to quickly be able to monitor what's happening to the amp both prior to and during a fault condition, and also to coax into playing up again.

[loudthud]

Note how the push pull "sides" alternate in the layout. V100, V101, V200, V201, V300, V301 and Vxx0 and Vxx1 heaters are in series.

[bobloblaws]

Seems you missed post #3.
I prefer to use the term idle (or quiescent) current instead of bias current.

Idle current needs to be measured in series with each tube.
As the amp doesn't have cathode resistors you should add 1R current sense resistors from cathodes to ground.
Or get a "bias" probe, i.e. a socket adaptor containing the resistor.

Actually, it is in fact a question of semantics. I didn't realize when The Dude said "bias current" that he was talking about idle plate current. Since there is a negative bias voltage on the grid, "bias current" sounded to me like he was taking about current through the grid or something like that. In any case, I did check idle plate current as well, also stable, no issue that I could detect.

I used the shunt method. I know some say it's an inferior method, but It wasn't practical to install resistors and it seems to work better for me than the O.T. resistance measurement method so I usually go with that unless its one of my own amps where I've installed 1 ohm cathode resistors.

Anyway, when I posted I did not know that I could rule out the tubes themselves as a possible cause of red plating, I'll know for the future. And of course I was barking up the wrong tree as well in terms of heater voltage. If there is an intermittent issue that's going to rear its ugly head again I'll have to dig deeper.

Thanks!

[Mick Bailey]

If you noticed which tubes red-plated it may give a clue to what had happened. The bias circuit is common to both sides then splits via R62/R63. A bad joint or resistor problem here could cause one side to red plate. I would think that the momentary loss of all the bias bias voltage (say, from the wiper of the bias pot disconnecting due to a dirty or faulty pot) would cause all tubes to red plate very quickly.

[Helmholtz]

I did check idle plate current as well, also stable, no issue that I could detect.

What idle current per tube did you get?

[bobloblaws]

What idle current per tube did you get?

Hi, sorry for the delay, haven't logged in for awhile. If memory serves it was around 25mA per tube. Incidentally, I talked to the owner of the amp a couple of days ago and he was happy to report there hadn't been any issues to date.