Ampeg B15N power trasformer failure?

[boroman]

My '69 B15N is blowing fuses - I have already unsoldered all the wires from it, HV, filament etc. Still, just after powering it on, it blows a fuse. Does it mean that PT is 100% shot?

[Enzo]

Disconnect the primary wires too, now blows fuses still? It is possible there is a short in the primary side mains wiring. Otherwise, it sounds like a bad tranny, yes.

[SoulFetish]

Disconnect the primary wires too, now blows fuses still? ...

for a second I thought you were being a smart arse

[Enzo]

Me?

[Ted]

B15s eat power transformers. Its really common. If as Enzo says, it still blows fuses with primary disconnected, you have got off lightly. If its teh transformer, you can either bin the whole assembly and buy a complete potted replacement from Fliptops or Mercury Magnetics. If money is tight you can unpot the original and get it rewound and repot it either using epoxy potting compound or the original bitumen. Its a lot of aggravation and to be avoided if possible. I have a small electric oven I use for B15 transformers after an incident with the kitchen cooker led to some domestic disharmony a few years back.

[Randall]

My personal B15N blew its PT years ago. I had it rewound at a shop in Orono, Maine for a reasonable price. Transformer Rewinding Service. Not sure of his current status, but it is good as new and you couldn't tell it had been done.

[nevetslab]

B15s eat power transformers. Its really common. If as Enzo says, it still blows fuses with primary disconnected, you have got off lightly. If its teh transformer, you can either bin the whole assembly and buy a complete potted replacement from Fliptops or Mercury Magnetics. If money is tight you can unpot the original and get it rewound and repot it either using epoxy potting compound or the original bitumen. Its a lot of aggravation and to be avoided if possible. I have a small electric oven I use for B15 transformers after an incident with the kitchen cooker led to some domestic disharmony a few years back.

I'd never heard that comment before...B15's commonly 'eat' power xfmrs. I can't imagine Ampeg would have designed the xfmr for 60Hz operation only (as an oversight). Running a design like that on 50Hz Mains WOULD tend to do as commented on. I just recently found an Ampeg SVT-CL whose power xfmr had several shorts in both the primary and secondary windings. I've never unwound any xfmrs to find the fault(s) and try to re-wind them. Life is too short to move into that area.

[nickb]

I've never unwound any xfmrs to find the fault(s) and try to re-wind them. Life is too short to move into that area.

Coincidentally, I had to do exactly that on a super high end Hi-Fi system for which no spares or service info was available just last week. I took me four and half hours. I have definitely had more fun times but on the other hand I am enjoying an wonderful sense of victory

[Ted]

I'd never heard that comment before...B15's commonly 'eat' power xfmrs. I can't imagine Ampeg would have designed the xfmr for 60Hz operation only (as an oversight). Running a design like that on 50Hz Mains WOULD tend to do as commented on.

There are several factors

a) The change from 60 to 50Hz - I'm in the UK so 50 is what we have. Although I see plenty of US imports with transformers that have been replaced.
b) Modern 6L6s bias hotter than old ones - these amps are either cathode bias or fixed bias with no bias adjustment.
c) Modern GZ34/5AR4s have less voltage drop than old ones
d) I measured the voltage from my alleged 240/110 step down transformer - its 125V

Some or all of these factors occurring together will overheat the transformer to the point of insulation failure. A short in the rectifier tube is another regular cause.

Like all the best disasters its a multi-factored failure.

[Leo_Gnardo]

B15's commonly 'eat' power xfmrs.

Seems true. I've seen lots of B15 & B12 with roasted PT's. Mostly due to failed filter caps. I've been working on one last couple of days. Its multisection cap had been replaced years ago with a CE brand unit rated 525V. About a year ago the main filter cap failed, and I replaced it with a film cap, 30 uF at 800 or 900V. That - isnt' gonna fail. But a year later the PT finally packed it in, popping fuses with an internal short.

Time was, you'd have to pay $300+ and shipping besides for a replacement. Looked at Fliptops, now they have theirs made by Heyboer and only (!) $225 + shipping. In spite of the price reduction, I did it the way I have for decades now. Melt out the original PT (or OT if that's what's needed), clean the can, repaint it, stuff it with a PT intended for Fender Vibrolux plus a mix of beeswax and paraffin wax like what's used for pickup dipping. Haven't had one melt the potting wax yet, and I have that repaired B15 warming up in my living room right now, to prove it once again.

Bias? If it's running too hot or too cold, we know what to do about that. Right?

With the Vlux PT, the B15 in my LR is running about 460V B+, delivers 33W @ clip to 8 ohms. Healthy statistics, and the good ol' fashioned tone is right there where it belongs.

[Helmholtz]

.., stuff it with a PT intended for Fender Vibrolux plus a mix of beeswax and paraffin wax like what's used for pickup dipping.

Not something I would recommend.
Paraffin melts around 65°C, beeswax around 70°C. Internal winding wire temperature can get much higher than that, sometimes approaching 100°C, even higher in fault condition.
And wax/paraffin vapour is highly flammable.

A suitable potting material could be bitumen (tar, asphalt), but surely no fun to work with.