Biasing A Red Knob Dual Showman

[PepeSilvia]

Fancy little test points, supposed to be easy, right? Well, I'm not getting nearly as much current as expected. The instructions say to bias to 40mV, I'm getting a maximum of 10mV. I figure that can either mean the bias voltage is off, or the plate voltage is off. The full range of the bias pot changes the grid voltage from -67V to -55V. This seems to be correct. The plate voltage is sitting around 415V. The schematic said it should be 470V. Is that that difference enough to cause a 4x difference in bias current?

[MWJB]

"The plate voltage is sitting around 415V. The schematic said it should be 470V. Is that that difference enough to cause a 4x difference in bias current?" You could have a 4x difference in plate current at the same B+ voltage from one brand of tube to another, so "Yes", being 60vdc down on normal operating voltage can seriously affect plate current (e.g. you might see a dip of 5mA for every 7 or 8 volts dropped at the plate).

You need to ascertain why your plate voltage is low. With a lower than normal current draw it should be high, if anything.

[PepeSilvia]

Being that there's not much between the plate and the transformer, my first inclination is to cry capacitor. Seem likely?

[MWJB]

First off I'd compare all PT secondary voltages with the schem, an old cap might drop a little B+ but 60v seems a lot, a significant failure would have been accompanied with obvious symptoms...like smoke & goo coming from the cap (or caps, does the amp have series main filters?).

What voltage & current did the old tubes run at?

[PepeSilvia]

I'll check the PT voltages when I get home. There was nothing obvious from looking at the caps.

I got the amp without any tubes in it, so I'm not sure what they were running at. The new tubes I purchased are supposed to be pretty cold, but I didn't think it would make THAT much difference. Could I be wrong?

[MWJB]

Well, if the B+ went up to 475vdc, an increase of 12%, then so would your negative bias voltage up to a minimum of -63vdc...this sounds like it would be too cold for most current production tubes.

Differing tubes & brands of tube (of the same broad type) might vary by 30-40mA per tube when installed in an identical circuit, without any bias adjustment.

[pdf64]

Resolve the plate voltage discrepancy first, but, I've had this issue 'of insufficient bias range on red knob twin', such that the tubes were cold at any setting. I added resistors in parallel with the 100k resistors (R212?, R213) at the ground side of the bias pots. Possibly 220k. I was aiming to get 60mV per side. Pete.

[trem]

Have you looked at B+ without any power tubes installed.?
Best

[g1]

Have you checked the diodes across the cathode resistors? If they shorted you could get a low reading there and maybe high idle current causing low plate voltage.
However I would think your plates may be a little reddish if such were the case.

[PepeSilvia]

Ok, so I took a few voltage readings. These were all taken without the power tubes installed. Note the 5 red numbers:



1. 364VAC
2. 434VDC (I took an AC reading also, got 54VAC, not sure if that's significant, but it seems like there shouldn't be that much wiggle there)
3. 355VDC
4. 337VDC
5. 284VDC

3, 4, and 5 are all off by 25-40V. I'm not sure what the AC voltage is supposed to be off the transformer, does 364 sound right?

2, 3, 4, 5 were all measured with respect to ground, 1 was measured between the transformer output (cathode side of 302 to cathode side of 304).

[PepeSilvia]

Have you checked the diodes across the cathode resistors? If they shorted you could get a low reading there and maybe high idle current causing low plate voltage.
However I would think your plates may be a little reddish if such were the case.

Do you know where on the board these are? I can't find them...

[MWJB]

Aha! It would have been more useful to have the loaded voltages (tubes installed), plus the AC heater voltages, plus the AC wall voltage & the nominal AC wall voltage for your country (when export taps are employed, there can be a discrepancy vs. the amp's domestic voltages).

[PepeSilvia]

Well, when I had the tubes and and went to make the measurements I was getting a nasty hum whenever I connected the meter across the caps. Is that safe for the amp?

[MWJB]

Were you getting a hum with all the measurements? This doesn't sound normal (but might be layout/circuit related). Are there any dc voltages you can take without inducing this hum? Is there any particular tube/socket that is installed when the amp hums. You don't need to install all 4 power tubes to take voltages, you could start with one, then 2 then 3 power tubes...?

364VAC feeding the rectifier, unloaded, doesn't seem out of whack. But we really need to see the loaded voltages.

Other than the bias being cold (for which PDF64 has a solution), does the amp display any unwanted symptoms?

[Jazz P Bass]

I beleive the reference is to D104 & D105.
6L6 cathodes to ground.