Blues Jr bias running 25-30ma apart

[pontiacpete]

After quite a bit of testing and inspection and then the changing of several parts and tubes, the el84s are running 22-25mAs apart @ 54ma and 32ma. Resoldered both boards, replaced the two sockets, changed out C15 and C16. R31,R32,R33,R34 all test good.
I'm at a loss at this point.
Has anyone come across a problem like this or can offer some suggestions as to where to take it from here?
Always much appreciated.
pete

[Jazz P Bass]

Output transformer problem?

[Jag]

+1 on JPB's reply.

What's the bias voltage at pin 2 on each of the EL84s? Have the bias feed resistors drifted? What about the coupling caps from the PI?

[pontiacpete]

I swapped out the OT transformer with one from a princeton, but nothing new. 64ma on V5 and 31ma on V4. Reversed the tubes and it's 60ma on V5 and 34ma on V4.

[pontiacpete]

Bias voltage -10.1 on each el84. R31-2 are measured at 216-8K for each. The coupling caps, C15 and C16 have been replaced. I also removed C14 that is across the plates of the PI triodes.

[Jazz P Bass]

How does the amp sound?
I have seen the exact same thing on Mesa Nomads.
The why of it I do not know.

[pontiacpete]

The amp sounds good so far. Apparently it ate one of a set of new tubes, the owners says. I'm going to play it for a long while and see how it does. I hate to see eat up another tube but here it goes.

[g1]

Sometimes those amps can be very sensitive to meter lead/probe placement. Seems to have to do with the ribbon cable near the power tubes. Does moving the meter leads around cause any fluctuations? If so it could be a measurement issue rather than a fault.

[pontiacpete]

Thanks g-one, I was measuring current with the shunt method. When I use the method of measuring the resistance from CT to each end of the OT and dividing the voltage across by that, I found a totally different result, 31mAs for both tubes. I spend a lot of time to learn this lesson.
Thank You,
pete

[Robert M. Martinelli]

I recently modified a BJ for a friend of mine, I altered the bias circuit making it adjustable, then I cut the two cathode conductors on the ribbon cables in two points half an inch apart, and put in a couple of 1 Ohm 1% resistors for bias reading purposes. I then biased the whole thing a little cooler, because these amps (at least the ones I've been working on) in their stock configuration are biased a little too hot IMHO, and tend to be rough on the output tubes (valves on this side of the pond).

JM2CW

Best regards

Bob

[pontiacpete]

what size wattage were those 1 ohm resistors, Bob?

[Robert M. Martinelli]

I used 0,25 W 1% metal film resistors. The voltage developed across the resistors is very low and equals the cathode current in mAmps, so the power the resistors have to dissipate is minimal, at 35mA (35mV) the dissipated power is slightly above 1 mW.

Cheers

Bob