Ampeg B15- plate voltages way too high?!?

OK, not DIY but I suspect this is the best place for an answer.

I've recapped a 70's Ampeg B15N, and while it works OK it's extremely microphonic (even the power tubes, and different power tubes were tried) and also (I suspect related) the plate voltages are all 30-40 volts too high. I'm stumped - the only clue is that the 5AR4 rectifier tested "really really good" on my tube tester. The bias is right where it's supposed to be, and the output tube current is right where it's supposed to be. I would have guessed that there's a missing voltage drop resistor, but the circuit matches the schematic inside the lid while the voltages are way off. So either somebody swapped in a hot-rod power transformer (unlikely), or I've got an ultra-rare rectifier tube with Kryptonite plates.

One actual change made in the recapping was to replace the 30uF/600v electrolytic with two (parallel) 20uF/600v, resulting in 40uF instead of the original 30uF. I can't see this having any affect on the voltage.

A close-enough schematic can be found here:
http://members.aol.com/portaflex/schems/b-15-n68.gif

These Guys are true tube-amp nuts.

That sounds about right.
Transformer variations and power changing from 110 of old to 120 of new.
B 15 N pretty old.
Bias on pwr tuubes mite be cold.
That is a killer amp! Used by Jamerson, among other amps.
How does it sound?
Microphonics normal for old tube amps. Swap tubes.
Watch the ground. I think that flip-o-matic amp head feature relies on the hold down screws for ground.

What kind of speaker in that thing?

[quote author="CJ"]
Bias on pwr tuubes mite be cold.
[/quote]
Nope, I've been through that - measured the plate current directly, and it's right where it ought to be. The waveform from a sine wave input is a bit funky, though not crossover distortion.

That is a killer amp! Used by Jamerson, among other amps.

Click to expand...

Not heard on Motown records - they recorded direct.

How does it sound?

Click to expand...

OK, but too microphonic.

Microphonics normal for old tube amps. Swap tubes.

Click to expand...

I did swap tubes! I spent a fair amount of time tapping various things and couldn't find anything else making noise. Very puzzling. Even with the input tubes out it's microphonic. I tightened all the tube socket contacts, freshened the solder joints, etc.

Only thing I haven't tried is a different rectifier tube, because I don't have another 5AR4. Hmmm... I do have one of those little evil silicon replacements, though.

First Like CJ posted the line voltage is most likly the reason for the higher voltages inside. Remember the transfomer steps up the line voltage so you numbers might be "correct" for the line voltage.

Be carefull if you use the Si the voltages will go up even more.

What is the fil voltage at 7.2VAC?

Do you have a variac? If so turn adjust it for the rated line voltage

6SL7s can be microphonic When I built a small all octal guitar amp I had problems finding 6sl7s that were usable because of microphonics. I ended up with russian 6sl7 types

Might be a microphonic part like a cap. Check the ones by the PI and output grid coupling caps

Did the power supply caps measure bad?

Jamerson used to tour.
Try the chop stick test. Tap on the components with a pencil or chop stick one at a time to see if you can isolate the area.
Bass really tears up a chassis, so maybe a loose connection somewhere.

Hi,

Old B-15's did not short the input jacks. It will be more microphonic like this with an open input grid. Run a shorting plug in the input or a bass with the vol turned down and try.

It could also just be microphonic tubes, octals are typically worse than the 9 pin. Swap...

I have seen plate voltages on B-15's up to 520V with the rectifier tube. The later ones usually were higher. Only the very early (cathode bias)versions were around 425-450V, but they also only put out 25W. Most after that point (B-15N, B-18N) were around 500V or more, and consequently put out 35W. Jess Oliver told me once to load the transformer with 4 ohms instead of 8 and it would be better.....that works out to 3300 pri imp vs. 6600, I tried it, it worked, same power output, cleaner, tighter sound, slightly more notch.....either way you want to try it.

As long as you are not seeing significant crossover notch and the amp sounds good I would not worry about it. Just use good power tubes, JJ would be my choice for high B+.

600V cap is not a bad idea with those amps either.

Best of luck.

Power tubes can be microphonic too .... sorry I just re-read your post.

Trying to remember if rectifiers can be micro.
Guess I'll go home and hammer on the SOB and find out, eh?

It's difficult - I used to own an older B-15, and don't recall it being microphonic. Nor the one my brother owns. This one is, though. I spent several minutes with the chopstick banging on various components, nothing stood out. Even swapping in a different pair of power tubes along with a couple different sets of 6SL7 didn't seem to change anything.

I'm OK with the higher voltages (even if they contradict the schematic printed on the bottom plate), but I don't think the microphonic stuff can be ignored. Weird... never seen anything like it.

Hmmm... unless maybe there's too much gain somewhere in the output circuits. Maybe I should check for resistors drifting high.