Fender Studio Bass - Low Current Draw

[66tele]

I'm working on a 1978 Fender Studio Bass amp. The amp was in very clean condition, only a plate and screen resistor had been replaced on one output tube. All tubes,caps and transformers were original. Complaint was low output and slightly buzzy distortion. As the electrolytic caps were 35 years old, I replaced them (filter, cathode and bias caps). The output tubes tested OK (but not matched), but the owner wanted reliability, so I installed a matched sextet of JJ 6L6Gc's. All voltages checked reasonably close to the schematic. I replaced all drifted resistors and any marginal coupling caps. Output tubes have 510V on the plates and screens and -62V bias on the grids (matches the schematic). I measured 55ma on one side (blue wire) and 25ma on the other side (brown wire). This amp uses 3 pairs of 6L6GC's, so I should be seeing around 35ma per tube or around 100ma per side. Cathode grounds are solid and I cleaned the tube sockets. I even tried a different speaker. Still, I have very low volume and slight buzzy distortion. What would cause the low current draw when there are correct plate and screen voltages and correct bias voltage? I've attached the schematic.
Thanks in advance!

[martin manning]

The absolute bias voltage required may be different with modern tubes, so let that go. It's the current that matters. Does rotating the bias balance pot affect the current side-to-side? If the new tubes are matched, you should be able to equalize the current, and that will confirm that the balance control is working.

Does the OT's DC resistance look reasonable from the plate and screen taps to the CT on both sides?

If you have to adjust the available bias range, you can increase the 100R 1W that is right after the bias rectifier to bring the bias voltage closer to ground.

[66tele]

Thanks for your reply Martin. Yes the bias balance pot adjusts the bias from side-to-side as it should, and I've already adjusted it. I'm getting -62V to the grids of all the 6L6GC tubes. I have the same low output problem with the old, original Sylvania tubes (they have the Fender brand on them), so the problem is not the tubes.
I'm going to run tests on the OT next. It's the only thing left I can think of that could cause this problem.
I can well imagine that I might need to change the bias voltage (not just balance it), but current draw that is 50% to 75% too low tells me there is a problem.....it's not just new vs. old parts or different line voltage. This amp sounds like a 10 watt practice amp!
Thanks again Martin! Keep throwing ideas at me guys! I'll report on the OT tests.

[martin manning]

If you have 510V on the plates and screens, and the cathodes are well-grounded, that is just what the tubes will do at -65V Vg1. Biased that close to cut-off they may not be well matched at all. I suggest raising the bias voltage and get them idling at 35-40mA. Low and distorted output is a different problem, and could be due to a failed OT.

[66tele]

Problem solved! Socket pin grippers were not all making contact with the tube pins. DOH! I re-tensioned all the octal sockets, reinstalled the 6L6GC's and the volume is back up where it should be (loud like a Twin Reverb) and the buzzy distortion is gone.
While I've seen sloppy tube pin grippers before, I've never had some not make any contact at all. Especially odd in an amp that was so physically clean without signs of hard use over the last 35 years.

[martin manning]

Great! What's the idle current looking like now? And, there's another reason to ground each cathode through a 1R resistor so you can monitor individual tube current.