Ampeg B15NF Fliptop Blows Fuses

[earache]

Hello All

I am working on an Ampeg B15NF that blows fuses.

I opened it up when I first got it and found that the 100uf/100V bias capacitor was blistering and leaking goo so I replaced it.

This amp has likely never been serviced, as it has all the original Ampeg-branded preamp tubes (6SL7's), Sylvania 6L6s and a Telefunken 5ARZ recto. It also has a two prong plug.

The speaker's ok, as is the speaker cable and all the switches.

It still blows fuses, even with all the tubes out.

Here's the schematic:b15nf_schematic.pdf

What to test next?

[earache]

Another version of the schematic via Joe Piazza

ampeg-b12nf-1968-portaflex-amplifier-schematic.pdf

[g1]

I guess you will have to disconnect the secondary wires of the power transformer and see if it still blows the fuse.

[R.G.]

"Blows fuses" is going to be an exercise in temporarily disconnecting things until fuses stop blowing.

And fuse blowing is expensive and frustrating, so you really ought to be using something like a light bulb limiter to stop wasting a fuse with each test.

You can work the debug process from the AC wall plug toward the speakers, or from the speakers back to the wall plug, as you wish. You've started already with removing tubes. That stops the output tubes from overcurrent conduction (they're not there to conduct!) so the problem is toward the AC wall plug from the output tubes.

Next stop is filter caps. Old filter caps simply die, as you found with the bias cap. But you said you pulled all tubes out - does that include the rectifier tube? If it does, the filter caps and rectifier are already disconnected; those were the next two steps toward the AC plug.

Next up then is, as g1 said, the transformer secondaries. Temporarily disconnect and insulate ***all*** of the secondary windings. If the fuse blowing stops, it's in the wiring or parts downstream toward the speaker from the secondaries. If not, it's the transformer or primary wiring.

In that case, temporarily disconnect both primary wires. Still blow? If no, the transformer has a problem and needs to be tested for internal shorts. If yes, the problem is in the AC wiring, including possible the death cap and/or the AC mains wires themselves. Grommets for the AC mains wires were not a strong point back in the day, and the AC mains wires may be shorting to the chassis.

[earache]

RG

Is it possible that the Hum Balance pot could be the problem? It is seriously "crusty" looking with aluminum oxide dust all over it. I measured it at the leads with a meter and it seems to work OK, but it looks bad...

I am still looking over the amp and thinking about how I will attack the job of removing the transformer leads.

[Leo_Gnardo]

Is it possible that the Hum Balance pot could be the problem?

Not likely to be the hum balance pot. I've worked on hundreds of B15 of all varieties and never found the humdinger to be a fuse blowing problem, OTOH many have been damaged due to shorts in output tubes from plate or screen grid to filament, resulting in open not shorted elements inside the hum pot. A crusty looking cover is par for the course in a 50 year old amp.

I am still looking over the amp and thinking about how I will attack the job of removing the transformer leads.

Here also I've never found desoldering the transformer secondaries to reveal anything of interest. It is a good start on removing the power transformer though, all too common for B15 (and similar Ampegs) PT to go bad in recent years. Also test the "death cap", these also aren't very reliable 50 years on. I've had some of those shoot across the chassis on application of power, leaving a spiral of sparking aluminum foil and a cloud of smoke. Spectacular! Now I just cut 'em out and don't replace them as they really don't have a function in amps with a grounded chassis.

[Dave H]

I am still looking over the amp and thinking about how I will attack the job of removing the transformer leads.

R.G. has already asked this but just to be sure before you remove any wires did removing all the tubes include the rectifier tube?

[catalin gramada]

Hello. Think is a constantly error into Piaza drawings regards the tone stack. Those PEC module never had 47pF/470pF into treble section. The correct values are 470pF/4.7nF as I seen from all original schematics, even from old B15 circuits which used 4M pot for treble adjustment.

[Randall]

I've had two B-15s with shorted power transformers. I had them both rewound. heating the square can they are in will soften the potting enough to remove the transformer. If that is your problem, it will keep the amp looking the same.

[TimmyP1955]

I too had one with a shorted transformer. I got a look-alike from FlipTops.net

[earache]

LeoGnardo

Are you saying that, with all the tubes out (including rectifier), if the amp is blowing fuses, that conclusively determines that the power transformer is kaput? A bad "death cap" couldn't cause it to blow fuses, could it?

[earache]

Dave - yes even the rectifier tube is removed and it blows fuses.

[g1]

Even if the rectifier tube is out, it is not 100% guaranteed that the PT is the fault. There could still be a wiring fault or defective tube socket, or even the hum pot, though as Leo mentioned, not likely.
So you really should disconnect it's wires and check before you order a replacement.

[Leo_Gnardo]

Are you saying that, with all the tubes out (including rectifier), if the amp is blowing fuses, that conclusively determines that the power transformer is kaput?

With tubes removed, there's very little left that might cause the amp to draw fuse-popping current. 1: a short within the hum balance pot, a condition I've never seen. 2: some unexpected short across the PC board, the sort of thing you might encounter if metallic debris has gotten wedged into just the perfect position, say a piece of guitar string, broken-off guitar cable plug, a screw or bolt, coin. Rare but worth looking for. 3: IF the ac cable has been changed to a grounded type, and the chassis IS grounded, and you have a shorted "death cap," that will pop the fuse.

I go through some trouble to prove that the problem really can't be anything but the power transformer. Then and only then do I unsolder the PT connections, clip in leads from any competent similar PT, and then apply power. If things start to work right then, there's no other conclusion is there?

Like Randall I've removed the PT from its cover, tar-like potting goop is a pain to deal with so I use a mixture of white modeling sand and 50/50 beeswax & candle paraffin to repot. Instead of rewinding (expensive, plus shipping) I use a PT intended for Fender Vibrolux & similar amps. With the bells removed it fits neatly into the transformer box. For the standard B15N I measure just below 30 watts at clip, close enough for rock n roll. When I heat the transformer box the paint gets damaged and I just knock off the rest with a drill equipped with wire-wheel then repaint with VHT spray wrinkle paint, available at auto parts stores.

If you don't want to go the whole mad-scientist route, a properly potted PT is available from FlipTops but costs a couple hundred bucks. In one case a customer brought me an open-frame transformer & he didn't care at all about the looks, just bolted that to chassis in place of the original, so there's another option.

Randall if you have a method for heating up that transformer box so it doesn't shed its paint I'd sure like to find out, TIA.

In one very lucky instance the trans box innards, transformer + potting goop, were already loosened from the box & simply shook out. No muss no fuss, what a miracle. Wish they were all so easy.

edit: As G1 says, defective tube socket wiring, a short across a tube socket, a solder drip in a bad place, any of those could also be the culprit.

Something else to consider: given the age of your amp it would be an excellent idea to replace filter caps at this time too. Many Ampegs of that era show up in my shop with bad PT's driven that way due to leaky or shorted old filter caps. It would be a shame to replace the PT then wreck the new one because of defective electrolytic filter caps.

For curiosity where exactly are you in Mass.?

[earache]

Leo - I'm in Walpole, Mass. And thank you for taking the time for such a detailed and concise answer. Actually everyone who has responded to the thread has been great, I sincerely appreciate it!