Bias - Peavey

[Helmholtz]

Please pull the PI tube and repeat scoping and measurements. Does B+ increase?

[g1]

You can also double check your tube idle current reading using volts/resistance method at OT primary.

[Valvehead]

Please pull the PI tube and repeat scoping and measurements. Does B+ increase?

B+ drops 1V and scope wave is flat

[The Dude]

I don't think a volt PP at 48k should increase idle current substantially. Can you check bias with the PI tube out to see if bias has changed? g1's suggestion in post #17 is also a good idea as another method of bias measurement.

[Valvehead]

I don't think a volt PP at 48k should increase idle current substantially. Can you check bias with the PI tube out to see if bias has changed? g1's suggestion in post #17 is also a good idea as another method of bias measurement.

Bias is exactly the same with or without PI

[Valvehead]

You can also double check your tube idle current reading using volts/resistance method at OT primary.

Did that and it gives me 25ma / 33ma ...so why are 2 different meters in shunt method showing 50-ish ma per tube?

[The Dude]

That sounds more like what I would expect for current with your bias voltage. When you used the shunt method, meter was set to DCA and probes were plugged into appropriate jacks? I ask because some meters use different jacks for current.

[Valvehead]

Yes sir...like i always do , but both meters , diff brands, read the same. Strange...

[The Dude]

Not sure why the discrepancy???? Maybe using the shunt method was inducing oscillation??? I would be more apt to believe the OT resistance measurement.

[Valvehead]

Thanks boys ! Ill try a matched pair in there soon

[g1]

I'd be inclined to think the shunt method was causing oscillation. Though the volts/resistance method puts the probes in the same places.
Did you try the shunt method again with the PI tube removed like Helmholtz suggested in post #16 ?
I guess you are normally using shunt method with same meters and have not had the odd readings before?

[Helmholtz]

The OT shunt method is not perfect. Reason is that any ammeter has some resistance ( professional meters often have a DCR of around 2R in the 200mA range, cheaper meters likely have more).
So while the major part of the plate current runs through the meter there's still a bypass by the DCR of the OT.
Means that the reading typically is too low by up to maybe 10%.

So no explanation for high readings here.

Connecting meter leads to a power tube plate may stimulate oscillation but typically only on one of the plates and not on the other (out-of-phase) one.

Now if there was oscillation increasing the plate current, B+ should drop. This wasn't confirmed.
Pulling the PI should stop oscillation but didn't increase B+.

Seems something fundamental went wrong with the shunt method measurements.

I would consider adding 1R cathode resistors or getting "bias" probes.

[Valvehead]

I'd be inclined to think the shunt method was causing oscillation. Though the volts/resistance method puts the probes in the same places.
Did you try the shunt method again with the PI tube removed like Helmholtz suggested in post #16 ?
I guess you are normally using shunt method with same meters and have not had the odd readings before?

Same meter , its never done this so i verified with my other one. PI removed didnt affect it

[The Dude]

I'd be inclined to think the shunt method was causing oscillation. Though the volts/resistance method puts the probes in the same places. ......

Just thinking out loud because I can't come up with any other reason, besides oscillation, that would cause the discrepancy.:
While you're right that probes go in the same place, one method adds a very low resistance to the circuit, while the other method doesn't- even thought probes are connected at the same place. MAYBE this could be why one method causes oscillation and the other doesn't? Of course, this is pure speculation since we haven't even proved there is oscillation.

[Helmholtz]

I don't think a power stage can oscillate without a P