Bandmaster AB763 Problem

[The Dude]

I'm going in circles on a Fender Bandmaster AB763. The basic symptom is that it distorts way before you reach max power and output is low. The schematic is HERE

Here's what I know.

1) Tubes are new and I've subbed several know working sets.
2) Electrolytic caps have been recently replaced and tube voltages are normal.
3) 5V p-p on the output tube grids gets you .5V at the speaker.
4) At 6V p-p at the speaker, you start to see distortion on the top half of the waveform (looks like crossover distortion, but is closer to the peak on the leading edge).
5) At 12V p-p (on speaker jack), clipping starts.
6) I can run the input up substantially higher and still get no distortion or clipping on the grids of the output tubes, so I'm certain the problem is in the output section.
7) OT resistance readings are 100 ohm red-blue, 65 ohm red-brown, and about .5 on secondary.
8) The transformer is stamped P-TF22848 and looks to be a replacement.
9) Screen resistors are new (one was burnt and the other out of tolerance originally).
10) Bias voltage and current are fine- both measured and tested (-47V and 30ma@).

I'm thinkin' OT, but am I missing anything?

[52 Bill]

It sounds like you've covered everything other than the OT. I guess it's time to try swapping one in and see if the sound cleans up.

[pdf64]

Consider RG's transformer tester http://www.geofex.com/FX_images/xform_test.gif and look to eliminate any potential oscillation, eg open the global feedback and tack a 470pF cap across the phase splitter grids.
Pete

[The Dude]

Thanks for your advice fellas. Transformer on order. I'll update when I get'r stuck back together.

[Chuck H]

Interesting that it's a currently available replacement OT and it's suspected of being bad. Do you know if the original was replaced while owned by the same person? Perhaps the owner is using the amp in an unsuitable manner. Just an observation. Also, since the OT is a replacement and is now suspect perhaps there was a mistake in the installation or other changes were made as well. I would also check the NFB loop in case the 820r resistor was omitted somehow or look for other such errors.

[The Dude]

Thanks for the replies, fellas. Just popped in the new xformer and all is good. We're up and running. No more distortion and everything is working fine.

Chuck: I asked the customer about it and he says he had it in another shop and the guy mentioned a potential transformer problem. I'm not sure (nor was he) what that means. It could mean the transformer was replaced or it could have been that he suspected a problem with it in which case it should have been replaced. I also asked the customer what cabinet he is using and it's a Fender dual 15 cab at 4 ohms, so everything is good there.

I did do some research on the bad transformer and found that others have had problems with this particular transformer (THREAD HERE). The part number on it was P-TF22848 and there's even a link to pics of the guts of the thing on that page.

[bob p]

"I've reeled in kites and gotten neater winds than this."

That first post had me laughing.

So who is reselling that transformer? My understanding is that 22848 is a widely used Fender code, and that the 989 EIA code ID's the manufacturer. But all of that doesn't tell me who's distributing that "P-TF22848" product, and I'd need to know that to know where not to shop. Unfortunately the stores don't tell you the manufacturer's EIA code, so you don't know what you're getting until you have it in hand.

It would be good to know whose transformers are the bad ones.

[The Dude]

That post got me, too. In case you missed it, here's a link to a gut shot of the transformer. I'm not sure who sells that transformer since I'm not the one who installed it, but I would definitely not buy one if that's how they're built!

[bob p]

Now this is getting interesting. First we had Bruce's infamous thread about the alleged inferior tube quality and poor customer service from CE Dist. Now we have them selling a transformer with a part number that matches the one getting a bad rap:

Transformer - Fender® Replacement, Output, 35 W, 4 Ohm | CE Distribution

I browsed through their catalog last night. When I looked at it, I recollected the representation of their catalog to be that those were Genuine Fender replacement parts.

Right now I'm looking at page 54 of their 2012 print catalog to verify my recollection. They put a Fender trademark logo on the top of the page for those transformers, and they proclaim that they're made in USA, though they do stop short of saying that they're a Genuine Fender product.

Are they Fender, or are they not? (They're advertised under a Fender logo.)

Who is the mystery supplier that's selling the bad trannies? Is it CE Distribution? Will the bad news never end?

[Chuck H]

I think that green insulation indicates it's that "no strip" wire that Marshall had OT problems with. Thinner, weaker and less heat and solvent resistance. Fine for pickups. Bad for transformers.

[Bruce / Mission Amps]

I don't buy my transformers from CE Dist but they used to buy their "generic" transformers from one of my winders. Marvel.
To be 100% honest, over the many, many years I've been doing this, none of the major winders I've worked with have escaped from shipping me a few clams here and there.
It's just part of the cost of doing business.....

[bob p]

That green no-strip coating is probably kapton. Nothing wrong with Kapton. To remove the insulation you have to use a torch. At least that's what I had to do when hand winding a motorcycle stator.

Interesting that the discussions at TGP mention Marvel as being the bad guy, but they mention that Triode's transformers are better. I guess these guys don't know that Triode sells ClassicTone and that ClassicTone is a product of Magnetic Components Inc., and that Magnetic Components is a subsidiary of Marvel Electric Corp.

Marvel Electric Corporation Home Page

ClassicTone has a good reptuation here. Maybe this is much ado about nothing.

[J M Fahey]

No, that's not Kapton at all but "self stripping enamel" wire, absolutely unsuitable for tube related transformers.

And that bobbin example is about the worst I've ever seen.

Looks wound by (un)trained monkeys, it's scatter wound and so uneven that they didn't even get uniform thickness, it bulges in the middle.

So CE is clearly getting the lowest bidder, no matter what.

And if the makers are supposed to be (no real proof of that, just hearsay) a subsidiary of a subsidiary of a subsidiary of a (reputedly) "good" Company , well, don't know how well quality would transfer through all those layers.

FWIW reputed Drake, Marshall transformer supplier, has sinned by also using self stripping enamel (think JCM900 and Valvestate amps) .... and Mesa Boogie too, by the way, but at least their windings are smooth and even



Notice the telltale "pink fluo" enamel, meant to avoid using it by mistake to wind a motor or any other critical application.

The other telltale color is the bright green one shown in the CE transformer. Ugh!!

And it's a fraud showing it in a catalog page sporting a Fender logo, without explaining it is "FOR Fender" (which is also a lie ).

[Leo_Gnardo]

No, that's not Kapton at all but "self stripping enamel" wire, absolutely unsuitable for tube related transformers.

I KNEW that green segmented-looking wire looked goofy. Good comment too about getting a neater wind while pulling in a kite... thanks for passing that along bob p.

Bruce, do you know who makes transformers for Magic/Ruby? I get some from them, never a problem.

In fact can't say I've ever had a problem with replacement iron. Just lucky I guess. So far haven't gotten any from Triode, Magnetic Components, Marvel. (At least not to my knowledge.) OTOH had a brand-new 2000-series Marshall come in with a shorted secondary, right from the factory. Replacement from CE worked perfectly. Mostly I buy iron from Magic Parts and CE.

Whistling thru the graveyard, on El Dia de los Muertos...

[g1]

Guys, the names being thrown around here are not from the original linked thread but only speculation. We should be a bit more careful as it is a bit like the old "telephone" game.
The original thread specified the EIA code of Marvel, but mentioned it was not mag comp but the parent company. It was fairly vague. CE was never mentioned except it was noted that their part number matched the one in question.
The linked thread was several years old and I got the idea the problem units were discontinued?