Rebuilt V4 power supply, voltages not quite right

[Aleph]

Hey Ampeg folks.

I picked up a '74 V4 (actually a chopped VT22) a few months ago, and while I loved the sound of the amp at first, it seemed to break up really early, and had quite a bit of hum. I popped it open and saw that at some point the power section had been rebuilt with low-quality, 350V electrolytics, and the main filter caps were hot-glued directly on top of the signal wiring to the power tubes! I was determined to rebuild the power section.

Based on several posts from Hangman over on the V4 forums, I ended up using F&T electrolytics: one side of two 100/100 cans for the main filter with the other side of the bottom can and an axial 100uF for the screen supply.

For C17 I used three radial 50uF F&Ts.

I also changed the screen resistors to 1k/5W, removed a variable bias mod,
and replaced:

D1 thru 6

R49
R52
R53
R54
R56
R57
R60

C11
C13
C16

R55 burnt up at some point in the past, and was replaced with a new 10W Xicon. It looked undamaged, so I left it alone.

So I fired the amp back up after all this, and it works, but all the voltages are slightly off. I'm only getting 535V at node A, where I should be seeing 545V; and a full 534V out of 535V at B!

Here's everything I measured:
Primary: 117VAC
Secondary: 395VAC
A 535V
B 534V
C 426V
D 368V
E 335V
BIAS -59V
Bias currents ~ 17mA

So the tubes are running really cold. It sounds fine, but I don't want to keep running it like this until I figure out what's going on.

I tested and inspected my wiring pretty thoroughly for errors a few times, but didn't find anything.

Any ideas? I may just rebuild it with the Orville Amps boards down the line, because the PCBs are pretty warped, but I'd like to figure out what's going on first. Thanks in advance.

[Hangman]

535 at node A doesn't sound bad. I've had V4s range from 535 to 545. you can check the AC voltage from the wall too. you might feel better if the AC voltage is a little low too.

Point B is never a full 10 volts less than A, even if you measured 540 at point A you would likely measure 539 at point B.

point C is high compared to the schematic, but again, I'm not worried about it.

Lets clarify what you mean when you say it breaks up "early". are you saying that it breaks up a really low volumes, or are you saying that it breaks up when you have the volume control set at 11 o'clock and while its loud as hell, you don't expect it to break up when the volume control is set so low?
is it a pleasant breakup? or does it sound like its farting out?

another question : do you have an oscilloscope?

[Aleph]

Thanks for the quick response!

Regarding the "early breakup", that was before I rebuilt the power supply, and it was breaking up at low volumes (before 11). I think that was mostly due to the under-rated filter caps that I replaced, though. It's getting louder and staying smoother now, not to mention a lot quieter!

If A and B are okay, then mostly I'm just worried about the power tubes being biased cold. 17mA is pretty darn cold, right? If it's fine, then I guess I don't have too much to worry about.

I removed the adjustable bias mod because I've seen plenty of posts talking about how adjustable bias is unnecessary in these amps, and they're biased conservatively as part of the design. I can always add it back in though.

I do have a scope, but it's an old Tek 454 I bought to play around with and maybe restore eventually. I doubt it's in spec, and some dingus cut off the ground pin on the power cable, so I'm not sure I'd feel comfortable using it around ~500 volts until I replace that. I can get access to a more trustworthy scope if I need to.

[Hangman]

17ma is pretty cold. generally I would say that 50-60% max pate dissipation is ideal for the v4. which would be 28-33ma

at 17ma I imagine you might be getting some ugly crossover distortion.