Bias 10mv difference in power tubes

[MickeyB]

Can't find why I have a 10mv difference between the 2 power tubes. They are 6L6GC matched TAD tubes. If I switch sockets, still the same reading on each side. Voltages on pin 3 455v & 452v, pin 4 394v & 394v, but pin 6 I get -46v on V3 and -51v on V4. I checked the 1.5K screen resisitors, the 200k resistors and .1 caps and check values and all good. Not sure if this is even the cause. I am using a different bias circuit than the original schem due to the power tranny not having bias tap. When I adjust bias on V4 to 35mv and check V3 its 25mv. Swap tubes, same thing.

[Jazz P Bass]

What does "same thing" imply?
Is the reading following the tubes or the socket?
If it is following the tubes, then they are not matched.
Let me ask this: how does the amp sound?
If it sounds good, let it be.

How loud do you play the amp?
Do you have a signal generator & a scope?

[MickeyB]

Voltages stay the same in same sockets. Does not follow tubes. I tried a couple of old 6L6 coke bottles I have and they were 6mv difference. I have not plugged into yet. Just fired it up for the first time to set bias. It won't hurt to run it this way? I thought it had to be within a couple mv of each other. No, do not have signal generator or scope.

[defaced]

I'd just flat out swap out the 0.1 caps, 220k resistors, and 1.5k resistors with new parts and see what happens. Did you remove the parts from the circuit to measure their value? If not, your measurements are probably not accurate.

[Albert Kreuzer]

Are the primary windings of your OT exactly matched? If not, this might explain the difference.

[Jazz P Bass]

Is this a new build?
It may be C7 or C8 that is leaking.
Which socket? The top V3 or the bottom V4?
If the amp sounds good I would not fuss over it.

[MickeyB]

That was my next question, would it be in the OT. It is a used PT and OT. I'll check that too. The voltage looked closely matched on pin 3 coming off the OT, so I didnt think that was it. I have another OT that I can try too.

[MickeyB]

Yes, new build. V3 has lower mv than V4. More so with new tubes than old tubes. So If I bias the one at 70%, the other just runs a little cooler, right?. If it sounds good, this won't do any harm?

[riz]

Well, think about it. If one tube is near max safe limit, and the other is running cooler... Where's the problem? It might even sound better. Some people like it that way. I have one amp I have a Sovtek 5881 and a Sylvania 6L6 as a pair, it just sounds best that way. I GUARANTEE you they are not matched! So, have you PLAYED it YET? How'z it SOUND?

[Jazz P Bass]

Have you played the amp yet?
It may be fine.

[Enzo]

With a tiny difference like 10ma, no doubt the amp will work fine, but his question remains - why the difference? And he has demonstrated it is not the tubes.

Power off and measure resistance from OT center to to each plate. If those resistances are not identical, then neither will be the currents.

And since you report a 5v difference between sides in the bias voltage, I have to think that also would unbalance your readings. perhaps one of those 0.1 caps is a bit leaky.

[g1]

Seems unlikely an OT issue/imbalance would cause 5VDC difference at the grids.
As Enzo and others have stated, C7&8 are suspect, if you don't have replacements handy, try swapping them and see if the problem goes to the other grid.
Cap. leakage can be difficult to measure without dedicated equipment.

[Bruce / Mission Amps]

Have you measured the DC resistance from the center tap of the OT to each outside leg? The tubes might be closer then you think if you do the math using the actual DC resistance of the OT leads vs voltage drop.
I have found that many cheap made transformers have different DC resistances, when measured from center to outside leg.
Different enough to cause an apparent slight mismatch in current when doing a shunt method.
Also, the another cause of an apparent imbalance, if everything else is equal, is that one side is drawing a little AC current in an ultra-sonic oscillation that you can not hear. The extra current will make it look like one tube is idling at a different rate then the other.

Wait... you said 10mv... are using a current sensing resistor on the cathodes of the power tubes to determine idle current?
If yes, what value is the resistor, 1 ohm @5% ... 1%?
If yes, how accurate is your resistor or for that matter, the meter you are trying to read it with????
If you are trying to use the resistor method, at least use a 1% resistor and double check your DC resistance of the test leads to zero their factor out... they can be 1/4 of an ohm each, blowing the whole test with a 1 ohm resistor.

[loudthud]

Another possibility is that the two tubes aren't getting equal heater voltage. Check the heater voltage right on the tube's socket pins. Make sure you have good solder connections and that the socket is tight.

[Enzo]

OT imbalance wouldn;t cause the grid voltage difference. That was just starting a list of the various potential causes. The next guy reading this with similar symptoms may not have both issues.