Ampeg B-15n Power Transformer

[twilightofthedogs]

Howdy folks. Recently acquired a very nice B-15N with a blown power transformer. As I have been researching the best way to replace it (I am going to have a new PT potted in the original can) I have come to the realization that the majority of the B-15Ns that you see online have had the PT replaced. I am wondering what is up with that? Anyone have any insight? Bad PT design? Something about the circuit? Bad research on my part?

[Enzo]

The ability of amp owners to stick 30 amp fuses in the holder. Or wrapping blown fuses with aluminum foil. "Hey, I have a fuse in my car, I'll go get it..." Famous last words. I'd bet on that factor.


Always good to put it in context. How many B15Ns have we seen? Like how many of all B15Ns do NOT have a new PT?

[Tom Phillips]

...I am going to have a new PT potted in the original can...

That's a noble repair approach but if you find out that the overall cost is too much then there are drop in replacements available. Just in case you are not aware one source is Power Transformer PT108 for Ampeg B15N/B12N - Product Details
Regards,
Tom

[loudthud]

My old B15N has a replaced transformer. Any idea what the specs of the original or replacements are?

[pontiacpete]

I have B15 transformer that I bought from fliptops and never used that I'm looking to sell, in case you are interested.

[twilightofthedogs]

@ Tom: The prices for new ones and having it repotted were actually pretty comparable. A few more dollars to get it repotted, but not a deal breaker. Seems like the only way to save money on this is to source a transformer and repot yourself, which seemed a little too nasty for me to undertake, but plenty of folks do. @loudthud: check here: Mojo Transformers » Ampeg B-15 Power (Single Primary) Transformer go to where it says "more info" that should bring up a data sheet for there replacement. 375-0-375 with taps for filaments (no CT) and rectifier filaments. @Pontiacpete: thanks, but I have already shipped out the old PT.

I have been in discussion with the transformer guys (you know, the guys out of grand haven, MI) and they confirmed that they do in fact see a high prorportion of B-15 PTs. they were of the opinion that having the PT potted was certainly not helping anything. Too bad it looks so COOL up there on that B-15.

[JoeM]

I have B15 transformer that I bought from fliptops and never used that I'm looking to sell, in case you are interested.

Still available? I have a friends B-15 here that needs a new one.

[pontiacpete]

yes, PM me.

[Enzo]

The B15N used power transformer PT108. Alternate was PT157 with apparently same specs. PT108 specs:

750v CT @ 160ma
5v @ 3A
6.3v @ 5A

[jlaber]

I am repairing an Ampeg B15 circa 1960. There is no 'N' suffix, it has a single set of tone controls, and a single rotary switch for Off/Stby/+/-(polarity). Rated 25 watts. Tube compliment is typical 3x6SL7, 2x6L6, 1x5U4. This seems to be quite a rare model. Sounds incredible. I believe this is the very first B15 design.

The power transformer had recently leaked some potting compound, and it showed up with 5881s in the 6SL7 sockets, so it appears the heater winding was overloaded and led to overheating the transformer.

I corrected the tube situation and let it idle for 16 hours, and the tranny didn't overheat, but it feels a bit on the warm side. I'd estimate 110 degrees F. in a 65 degree room. While there probably aren't a lot of owners of this particular version B15, it is probably very similar to the models that span the early '60s, so the question is whether this seems to be a normal operating temperature for this transformer.

Anyone out there with a B15-x, if you can post how warm the power transformer runs, I'd be grateful. A strip-thermometer or IR thermometer would ease the guessing a bit.

Also the B+ at the rectifier runs a bit high. The schematic specs 400 volts if I recall correctly, and actual reading as about 440. The 6L6s are idling at about 50 watts each, and the voltage across the cathode resistor is pretty close to spec. I think it was 32 volts.

Thanks in advance,
Jeff

[JoeM]

.....
The 6L6s are idling at about 50 watts each, and the voltage across the cathode resistor is pretty close to spec. I think it was 32 volts.

Thanks in advance,
Jeff

Jeff, can you recheck that? 50W each, or is that for the 2 tubes? (Typo maybe?) What's the value of the cathode resistor?

[jlaber]

Jeff, can you recheck that? 50W each, or is that for the 2 tubes? (Typo maybe?) What's the value of the cathode resistor?

Sorry, more like 25 watts each. I was looking at cathode current as being the total through the cathode bias resistor, forgetting to divide it by two. So it sounds like it's class-A biased if the idle dissipation equals the power output rating. Or at least on the heavy side of AB.

I also noticed in the B15N Heritage schematic, which in 1964 mode is pretty close to the same design, it looks like the B+ gets dragged down from 450v to 430v, a good indicator that the bias current is significantly higher than in the fixed-bias mode. So maybe up to 1964 the power trannys would run warmer than later models.

http://www.ampeg.com/pdf/Heritage_B-15_OM.PDF

The main differences are that the B+ at the rectifier is stated to be 400v on the 1960 schematic, and it uses resistors instead of an inductor for the downstream B+ filter. Had I realized the 1960 schematic isn't plastered all over the web I would have take a photo of it while I was in the shop. I'll be sure to get one the next time I'm there.

A bigger question at this point is whether the transformers should have PVC insulation on their wires. I've seen plenty of other gear later than 1960 that has fabric-covered insulation. The leaking potting compound also seems to have a very new look to it, but I suppose that's possible if it got hot enough. I'm starting to wonder if the iron was replaced in this unit, producing the higher 1964 voltage.

[twilightofthedogs]

Never really had mine up and running to see how hot the pt gets. Still waiting for that PT to come back! I did, however, take some pics before I sent it off. I will post them later for your reference. If I remember correctly, it was PVC though, and it looked pretty new and clean in there, I can only assume that it was the original PT. Then again it was also a 64 model.

[olddawg]

Do you have a variac on your bench to calibrate the mains voltage? Mains voltages were typically lower in 1960. If you are using a modern 120v+, your B+ is going to be higher.

[J M Fahey]

And 110 Deg F (43ºC) is *nothing*, even more so in a tube amp.
Start worrying at 158/160 Deg F and scream for help at 200 Deg F (at which point potting compound will be oozing from every hole).