uneven current draw

[joshdfrazier]

aka "bias". Same Univox, but experiencing a problem after the fixed bias conversion. The left tube always draw more current than the right. 32ma, and 23ma respectively. Those are approximations, as it changes with different matched pairs, so it is not a tube specific problem. Thought about increasing the value of one of the grid resistors, but that doesn't seem like the right way to solve the problem.

[matt h]

(deleted)

[gingertube]

When converting from cathode bias to auto bias the max Rg1 value drops. Check that the Rg1 values are appropriate. If this was a problem it would usually be so for BOTH tubes.

Other things which can cause this are leaky coupling caps (as Matt H says)
The fact that it is one tube socket which has the higher current is suggestive of leaky coupling caps.

Measure the output tube grid 1 voltage with respsect to 0V (no signal). They should be no more than +0.2V
If the voltages are more than say +1.0 Volts you definitely have a problem.

If that fails to show up a problem the next thing to check is screen voltages being the same. Old screen resistors which have drifted high in resistance could cause this too.

All of these things would have been partially compensated for with cathode bias BUT are no longer.

After that you are on your own - no more ideas.

Cheers,
Ian

[joshdfrazier]

Couple of issues resolved. Some old carbon resistors had shifted by more than 60k, and also, one of the 1ohm resistors grounding the cathode measured in at 4ohms. I guess i will be testing resistors before i add them from now on. there is about a 1.5-2ma discrepancy between the two tubes now, which is fine by me. thanks guys