Princeton Reverb PT again?

[Randall]

AA764 Princeton Reverb. Three years ago it came in with a shorted PT, I put in a Fender replacement P-TF22772 from CE Dist, along with new filter cap can and power tubes. I also removed the death cap.

Yesterday it came back with that replacement PT also shorted, although I don't where. All tubes are removed, both filament wires lifted, red center tap lifted, B+ lifted, bias feed lifted, red and yellow rectifier wires still soldered to socket, with only the primary leads connected, it makes my current limiter shine bright.

I see no connections between windings, and my neon bulb will flicker slightly when connected to the red secondary winding, but not the others. My individual windings read:

black primary - 3.7 ohms
red secondary - 345 ohms rectifier pins 4 to 6, pin 4 to CT 167 ohms, pin 6 to CT 178 ohms
yellow secondary - 0.2 ohms
green filament - 0.3 ohms

So it looks like the yellow winding is shorted, no? And maybe the green?

My question is what caused this new PT to short out? I changed out everything except the rectifier, could that be the cause?

BTW, CE Dist no longer sells this, so maybe it was just junk?

http://www.thevintagesound.com/ffg/s...1164_schem.gif

[Enzo]

Yellow is a 5v winding at a couple amps, it is HEAVY wire and few turns. Green is 6v at several amps, again HEAVY wire and few turns. It is perfectly normal for those to measure very low resistance. measure resistance on some other Fenderish transformer you might have to compare.

Windings sometimes short to frame, but often they short in the windings, but rarely do they short end to end. If your HV wires (red) shorted inside say 6 or 8 turns, your meter would never know it. Sometimes it is obvious, when one side of a center tapped winding is half the other, your numbers don't look problematical to me.

And since this thing has been working three years, it may simply be a coincidence.

[Randall]

"It is perfectly normal for those to measure very low resistance. measure resistance on some other Fenderish transformer you might have to compare."

That's the problem, I don't have one to compare to.


"your numbers don't look problematical to me."

Are you suggesting it may not be bad, or that it is bad, but not from those readings?


"And since this thing has been working three years, it may simply be a coincidence"

I am not sure what you mean here, that it was working until it wasn't, but from no fault from outside influence?

[Enzo]

I am suggesting it may or may not be bad, but the readings do not say it is. Running it on the bulb with its secondary wires disconnected and it glowing brightly, THAT tells me it is bad. Once in a while a transformer failure occurs that is detectable on your ohm meter, but most times it is not detectable that way. Your bright bulb test, and the blinking neon light test are way more definitive.

Do you have any amp sitting around that uses a 5v rectifier tube? You could pull that tube and measure resistance at its socket heater pins. Rectifier 5v windings do not feed any other tubes - well unless the model uses two in parallel, in which case pull both. No need to pull the chassis The 5v and 6v windings would have very similar resistance - as in almost none. This won't help you fix this amp, but it might give you a basis to judge future readings.

Coincidence in that it wasn't like a new transformer burnt up three days later or three weeks later. If that happened I'd be suspicious of something. But three years means to me there was not something systematically wrong with the amp or the part. Transformers fail now and then, not often, but when you see as many amps as we do, you get them. SO it could be just a coincidence that you had two transformer failures that just happened to occur in the same amp. Right now you cannot power up the amp, right? SO until a working transformer of some sort is connected, we really have no idea what sort of outside influence might exist in that amp.

[dstrat]

Hi, I have a 40-18019 here not sure its a real princeton transformer but its what "they say"

primary R = 6.4
sec (red) = 370
6.3 (green) = .4
5 (yellow) = .3
keep in mind my fluke is older than dirt and the leads are about as bad.
so is your transformer really bad? maybe but .. different manufacturing I don't know.
Your Primary is half the R that mine is ..
I thought if primary was drawing current or shorted the fuse would go, if it was the correct one.
my transformer is new never used yet.

[Pedro Vecino]

I see on a datasheet that that transformer supplies 2A for the rectifier and 70mA on the high voltage.
http://www.oldguitarparts.com/P-TF22772.html
The Hammond model that I use on the Princeton Reverb (291AEX) supplies 3A and 100mA at high voltage. I have changed many because many here are original from the US with 117V and it is common to transform them to 230V. I have never encountered any problems.

[J M Fahey]

Running it on the bulb with its secondary wires disconnected and it glowing brightly, THAT tells me it is bad.

Same here, no need for further testing.

WHICH secondary shorted ? Dunno, magnetically they are all in parallel and as you know, if you have several things in parallel (tubes, resistors, transistors, caps, diodes, etc.) and one of them shorts, all will read shorted.
Even shorted turns in the primary will do the same.

Why it shorted?

Stuff breaks, that´s why servicing and warranty were invented.

Same reason ship, car, home, health, life insurance exists.

That the transformer worked for 3 years tells me it was NOT a piece of junk, now slap a replacement in and bill your customer.

[vintagekiki]

That's the problem, I don't have one to compare to.

The shortest and best way to check the PT is to take all the tubes out of the amp, connect amp to the AC main voltage and measure the voltages on the secondary in "idle".

[mozz]

Just built a Princeton reverb with a P-TF22772. This is a old stock transformer that has been sitting unused, date code is 08? It measures 6.5ohms primary on my Fluke. So i am gonna say yours has a shorted primary. maybe he was running a 5u4gb? I too noticed they don't have this listed anymore, maybe there was a problem and they dropped that model.

[Randall]

My customer isn't real happy about replacing another PT in the same amp without a clear explanation, and I must say I don't blame him. I wouldn't be either. But, I just ordered a Classic Tone replacement from Magnetic Components, I have always been happy with them.

[dstrat]

Dont forget to look at the tube sockets real close because carbon build up will ARC between pins.

[nickb]

One possibility is if he plays in cranked then the PT will take a beating. A bit late now, but a Deluxe Reverb PT might fit with a little bit of surgery. It has almost 2 x the HT primary current rating.

[Randall]

The sockets are fine.

He does not play it cranked. And I know he is a stickler for originality, so I doubt he would go for a PT meant for anything other than what came in it. The Classic Tone is rated at 100mA, vs the 70mA it came with originally.

[The Dude]

Be sure to double check that the installed mains fuse value is correct and make sure bias isn't out of control when you get it running again. I'm apt to believe an intermittent tube is the most likely cause of this. I am a bit surprised the mains fuse didn't blow before the transformer cooked, though.

[g1]

I am a bit surprised the mains fuse didn't blow before the transformer cooked, though.

There is no way to say it didn't. Or what may have been 'tried' in there before it got delivered to the shop. Though from what Randall says of the owner, he doesn't seem the type to do this. You never know though.
Or he could have lent it out or his kid could have played it when he wasn't home, or ...