Ampeg B15NC & Ampeg B15NF

[boroman]

Thanks a lot guys for all the info I read here and learned, I'm just in the beginning of the game... I've been collecting Ampegs for years and some time ago started to make simple adjustments like adding bias pots, caps change, do grounding etc.

Now, I have a question about 1961 B15N - It's cathode biased amp. It's ~63mA a good value for bias on this model? I checked 250r bias resistor and it's up to specs. Isn't it a bit high? I know that cathode biased amps have more bias, but still, this looks high.

Another one is for B15NF (1970s). The amp sounded great but have some suspicious caps and it was not grounded so in the summer, I have changed 3 prong cable, bias cap, and 30uf/600V cap too + new standby switch and fuse holder. I have just tunred it on, checked bias on the probe and turned off (no sound test). After few months I turned it on and tried to play something but sound is barerly hearable super low level and distorted. Tubes glow blue when I hit the strings. Bias changes, right B+ is on the tubes too. What could be wrong? Pics

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[Helmholtz]

Schematic?

[boroman]

cathode (1st question)
fixed bias (2nd question)

[Helmholtz]

What is the voltage across the 250 Ohm cathode resistor (B15N)?

[Enzo]

Hi boro. Please start separate threads for your two amps. It gets real confusing real fast trying to discuss two separate amp repairs at the same time.

B15NF - low output? The speaker runs THROUGH the external speaker jack, so check the cutout contact in that jack for conduction.

B15N bias? Your schematic shows they expect 36v across the 250 ohm cathode resistor. I = 36/250 = 144mqa, or 72ma per tube. And only about 3ma of that is screens per tube.

[boroman]

Enco, Helmholtz - sorry for 2 topics in one email... and many thanks, I have noted it down, the formula. Still learning.
So it seems quite okay with the B15NC. I measured resistance and it's near 260ohm (slowly raising and stops at 260hm when amp is off, it's because of circuit/cap intereference?), voltage across resistor is 30V, so it could be that it's close to specs.
With the B15NF with EXTAMP jack removed it's all OK but sound is gone when I touch ext. amp jack to the chassis, so the problem might be in the jack itself. Will resolder another one.
Thank you so much again!

[Enzo]

sound is gone when I touch ext. amp jack to the chassis, so the problem might be in the jack itself. Will resolder another one.

No, look at the schematic. Neither side of the external jack is ground, the sleeve side is the hot 16 ohm tap. If you ground that, you have grounded across the output transformer.

[boroman]

Everything is factory soldered on the ext. amp jack. It was bended that hared the chasis bent in near that place - it was shipped with cable put into that jack.

[g1]

That jack is supposed to have insulating washers isolating it from the chassis. The solder may be factory, but that hole in the chassis sure isn't.
Attached is the proper schematic. The stock amp would have had the XLR style 4 pin connector.
The ext.spkr. jack must not touch the chassis like Enzo said, it needs insulating washers.

[boroman]

Ahhhh! Now I get it.

Hole in the chassis is original for ext. amp jack.
The cab has been modded to jack input, so main speaker cable was changed from switchcraft 4pin to 2-pin 1/4". The purpose of 4-pin was the amp not going off standby mode when cab was not connected, but the small pins usually broke fast causing problems. Another thing is you can't hook up any other stuff to that cab if you dont have switchcraft connector, so most B15Ns have been modded for 1/4".

I'll try to isolate it and see how it goes then