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Shorted PT secondary: next debug step?

1/20/2006 5:19 PM
Mark Lavelle	Shorted PT secondary: next debug step? I have a ~1967 Deluxe Reverb here that was blowing fuses, and whose owner thought a bigger fuse would be a good idea (doh!). The 5VAC secondary is now apparently shorted. I'm not at all sure that I'm going to offer to fix this thing (it's a friend-of-a-friend type favor so far), but I'd like to diagnose the original problem. The only thing visibly wrong is that the 12AT7 reverb driver has lost its vacuum, but I don't detect any shorts at the pins. Any thoughts on what else I can fruitfully do without replacing the PT first? I do have an unused PT with similar enough ratings (plenty of current, but only 250-0-250) to use for debugging, but I don't particularly want to go to that much trouble...

1/20/2006 5:19 PM

Mark Lavelle

Shorted PT secondary: next debug step?

I have a ~1967 Deluxe Reverb here that was blowing fuses, and whose owner thought a bigger fuse would be a good idea (doh!). The 5VAC secondary is now apparently shorted.

I'm not at all sure that I'm going to offer to fix this thing (it's a friend-of-a-friend type favor so far), but I'd *like* to diagnose the original problem. The only thing visibly wrong is that the 12AT7 reverb driver has lost its vacuum, but I don't detect any shorts at the pins.

Any thoughts on what else I can fruitfully do without replacing the PT first? I do have an unused PT with similar enough ratings (plenty of current, but only 250-0-250) to use for debugging, but I don't particularly want to go to that much trouble...

1/20/2006 6:50 PM

Ray Ivers

Mark,

quote:
"Any thoughts on what else I can fruitfully do without replacing the PT first?"

Replace the fuse, pull the rectifier tube, and turn it on. If the fuse blows, then either there's a problem with the wiring to the rectifier socket, or the 5V secondary really is shorted across (in which case you'll have to replace the PT anyway). If the fuse doesn't blow, you might be able to get away with using a SS replacement rectifier (I'm not recommending you do this, just giving you a possible option to get this thing off your hands quickly).

Ray

1/20/2006 6:56 PM
Enzo	Be sure about the PT first. COmpletely disconnect the secondaries. Does it still blow fuses? I would not expect to be able to measure a shorted 5v winding, it has such a low resistance to start with. Same with 6v windings. ANd sometimes ther are things like caps across the primary. If that shorts, it blows fuses as well as a bad tranny might. I would disconnect the primary as well as all the secondaries and connect it to the mains by itself to verify excess current. It is my experience that preamp tubes can't easily blow fuses no matter what they do. Even a shorted reverb tranny shouldn't burn up the PT. Other than applying power to the circuit, I don't know what tests you could mount. You can check for shorted filters. If you tack diodes across the recto socket and not use the recto tube during tests, then all you need is HV and 6V applied to chassis. If your spare PT sits on the bench, that is only 5 clip wires over to the chassis. You don't need to install the new PT, just conect it. WHo cares if the test B+ is low. You still ought to get 350VDC out of it, that's enough for testing. Use a primary fuse with your test PT and monitor current.

1/20/2006 6:56 PM

Enzo

Be sure about the PT first. COmpletely disconnect the secondaries. Does it still blow fuses? I would not expect to be able to measure a shorted 5v winding, it has such a low resistance to start with. Same with 6v windings. ANd sometimes ther are things like caps across the primary. If that shorts, it blows fuses as well as a bad tranny might.

I would disconnect the primary as well as all the secondaries and connect it to the mains by itself to verify excess current.

It is my experience that preamp tubes can't easily blow fuses no matter what they do. Even a shorted reverb tranny shouldn't burn up the PT.

Other than applying power to the circuit, I don't know what tests you could mount. You can check for shorted filters.

If you tack diodes across the recto socket and not use the recto tube during tests, then all you need is HV and 6V applied to chassis. If your spare PT sits on the bench, that is only 5 clip wires over to the chassis. You don't need to install the new PT, just conect it. WHo cares if the test B+ is low. You still ought to get 350VDC out of it, that's enough for testing. Use a primary fuse with your test PT and monitor current.

1/21/2006 12:26 AM
R.G.	Do yourself a favor - get a light bulb limiter in front of this thing first. That will keep you from either blowing more fuses or burning out more of the amp. Then run through the "blows fuses" section of the Tube Amp Debugging Page. If the power tranny has a shorted secondary, then you should use the second PT for debugging, or just get a replacement. The likeliest cause of the shorted secondary is a shorted tube powered by the 5V. A short on a filament line will usually burn out the secondary winding, and only then pop the primary fuse.

1/21/2006 12:26 AM

R.G.

Do yourself a favor - get a light bulb limiter in front of this thing first. That will keep you from either blowing more fuses or burning out more of the amp.

Then run through the "blows fuses" section of the Tube Amp Debugging Page.

If the power tranny has a shorted secondary, then you should use the second PT for debugging, or just get a replacement.

The likeliest cause of the shorted secondary is a shorted tube powered by the 5V. A short on a filament line will usually burn out the secondary winding, and only then pop the primary fuse.

1/21/2006 5:41 PM
Mark Lavelle	Shorted GZ34 I guess I should've mentioned in my original post that I used my light bulb limiter to determine that it was the 5VAC that was shorted (with all tubes out). Now I've checked the GZ34 and there is definitely a short from one of its plates to the filaments. Given the high probability that this is the original problem, I'm going to recommend that this guy just get a new PT, GZ34 and cap job. Maybe I'll even let him talk me into doing it... Thanks, guys!

1/21/2006 5:41 PM

Mark Lavelle

Shorted GZ34

I guess I should've mentioned in my original post that I used my light bulb limiter to determine that it was the 5VAC that was shorted (with all tubes out).

Now I've checked the GZ34 and there is definitely a short from one of its plates to the filaments. Given the high probability that this is the original problem, I'm going to recommend that this guy just get a new PT, GZ34 and cap job. Maybe I'll even let him talk me into doing it...

Thanks, guys!

1/22/2006 8:28 AM
KB	Did you do what ENZO said and disconnect the secondaries totally. It is possible that the tube was bringing it down are even an arched tube socket. It's pretty hard to kill that secondary heater winding even if shorted for long periods of time. KB

1/22/2006 10:24 AM

Mark Lavelle

quote:
"Did you do what ENZO said and disconnect the secondaries totally. It is possible that the tube was bringing it down are even an arched tube socket. It's pretty hard to kill that secondary heater winding even if shorted for long periods of time."

No.

1. There was no evidence of arcing at all (I would expect to see some carbon, and a certain odor, too

).
2. As I've already said, it draws all the current the limiter will let it with no tubes in.
3. The recto definitely has an internal short.
4. I was told that the last fuse the owner used was rated significantly higher than 2A, and I have no idea how many fuses he fried before that.

I may not be in a position to be 100% sure, but I'm not going to start unsoldering things in an amp of this vintage without talking to the owner first and recommending that he take it to a professional repair guy. I have no doubt that I can fix it (and I would certainly test the PT 'solo' before ordering any parts), but at this stage I'm just taking a look at it for a friend of a friend...

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