'69 Bassman weird voltages question

[strangedaze]

Hi everyone, I'm new here and after a couple of hours of scouring previous posts, I'm not seeing anything quite like my situation. I'm hoping that some of the tube amp gurus who frequent this board won't mind taking a few minutes to puzzle through a very confusing issue. Many thanks in advance to those who decide to take the plunge! The long version of the problem is detailed in the attached documentation, but here's the short version:

The amp in question is a 1969 Silverface Bassman (AC 568 circuit) which I recently took out of a twenty year basement nap to do an overhaul on. I remember that shortly before I put it in storage it was sounding not so great, even with new tubes. I think in the back of my mind I was hoping/assuming that the overhaul process would cure whatever was going on with it pre-storage.

I replaced all of the electrolytic caps and voltage dropping resistors, several plate resistors, the 6L6 screen grid resistors, the bias circuit cap and diode, and the rectifier diodes (replaced the 6 small stock diodes with 4 1N5408s). I also installed a three wire power cord. After the initial overhaul process, the good news was that there was no snap, crackle or pop, no smoke, etc. The bad news is that the measured plate voltage on the 6L6 output tubes was 327VDC with a plate current of about 5mA, so it looks like the output tubes aren't doing anything. These tests were in play mode with no input and a 4 ohm dummy load on the output.

One weird thing that I observed, and my lack of tube amp theory shows here, is that with no tubes installed in the amp, the B+ voltage measured at the output of the rectifier circuit and on the output tube plates is about 450VDC, pretty much right on. As each of the three preamp tubes and the phase inverter tube are installed one by one, the output tube plate voltage -- and the output of the rectifier circuit measured at the summing point of the rectifier diodes -- drops by 20-30VDC for each tube added, finally ending up under 330VDC with all tubes installed. I don't see how that would be possible, and I definitely don't see how the output voltage of the rectifier circuit can be pulled down when the input AC voltage from the PT remains constant at about 670VAC. There are no obvious shorts or burned components, and all of the remaining original resistors measure in 10% tolerance or very close to it.

After staring at the schematic until I'm blind, I'm just not seeing anything that might cause these symptoms. Even if the output transformer was bad, I'd think that I would never see a good B+ voltage, no? The only thing I could theorize was possibly a leaky cap, so I replaced five more original caps (see attached doc), and the symptoms are unchanged. Well, that's the situation -- has anyone ever run into anything like this? All comments are welcome.

Thanks,
Chris

[sluckey]

You seem to be missing a jumper between the first two caps. Look at your pic of the cap board, top left corner. Should look like this...

jumper.png

[strangedaze]

OMG -- I can't believe I did that! When I moved the underneath jumpers up to the top of the board I must have missed that one. Well, it's a combination of total embarrassment at me being a moron and total thanks for spotting that! I'll make that change and report back. Thanks!!

[strangedaze]

Yup, that was it alright -- forehead, meet workbench! Makes me think of one of the old axioms of troubleshooting, right up there with "Always keep one hand in your pocket"..."Never be so impressed with your own work on a project that you automatically assume that everything you've done so far is right." Thanks to sluckey's eagle eye, I now have good voltages across the board, but I'm wondering about my plate current. My 6L6GC plate voltage is around 440, and the most plate current I can get on the "fixed" tube is 32mA. I tried another pair of output tubes and they maxed out at 27mA.

If my math is correct, depending on whether you consider the 6L6GC to be a 25 watt tube or a 30 watt tube, with a plate voltage of 440 and an intended bias point of around 70%, I should be looking for a plate current somewhere between 38-48mA, so my 32mA seems like the tubes would be biased cold. At this point I'm thinking about installing a true adjustable bias pot mod well described here and elsewhere to be able to adjust the bias on both tubes, not just the one the existing bias "balance" pot controls. Any thoughts?

[sluckey]

At this point I'm thinking about installing a true adjustable bias pot mod well described here and elsewhere to be able to adjust the bias on both tubes, not just the one the existing bias "balance" pot controls. Any thoughts?

This simple mod is exactly what you want...

https://robrobinette.com/AB763_Modifica ... as_Balance

[strangedaze]

Exactly the one I had in mind! Like you, Rob seems to know his way around a tube amp or two...

Thanks again for taking your time to help out -- much appreciated.

[xtian]

Biasing to 70% is OLD advice, when tubes were cheap and plentiful. I target ~50% these days. Good sound, and better tube life.

[pdf64]

If 6L6 triode curves are used to derive idle anode current for eg the -44V bias @ 420V screen of the aa864 Bassman, end up with about 20mA.
https://tubedata.altanatubes.com.br/she ... /6L6GC.pdf
https://el34world.com/charts/Schematics ... _aa864.pdf