Proper way to test a Power Transformer after an amp meltdown?

[Jonny toetags]

Short story, my friend noticed his Dual Showman Reverb head getting hotter and hotter during the final days of his last tour.
He was playing it at practice, got extremely hot, fuse blew and terrible smell.
He said the heat and smell lingered for hours.
He also mentioned that the tolex on the back panel was literally cooking from the tubes as the tour went on and he had to remove it.

So I opened it up, nothing looks fried but 'that' smell is real strong if you put your nose over the PT.
The tubes are not blown but definitively not healthy anymore. I popped them in my Super Six and the amp starts breaking up at 3...it usually stays clean until 8.
The little matching sticker on the tube bases were fried to ash.

I'd just like to know if there is a way to test the PT?

[Tom Phillips]

...I'd just like to know if there is a way to test the PT?

Disconnect all the secondary leads.
Measure the continuity of all windings to each other and to the case. If any windings show continuity then the transformer is bad. If not then power it up with a light bulb limiter or bring the voltage up slowly with a variac.
If the fuse blows with just the PT primary powered then the transformer is bad
If the transformer gets hot it is bad
If the transformer draws more than ~100ma with full line voltage applied then it is bad

Edit: Now that I have a few more minutes I'll add that, if the transformer doesn't do any of the bad things listed above then you proceed to troubleshoot the circuits connected to the secondary windings. There are several approaches.
Test the circuits with an ohm meter looking for shorts or unusually low readings, check the rectifier diodes, filter caps etc.
Hook up the windings one at a time and operate amp until you verify that circuit is OK. (Use light bulb limiter or variac method)
The hook up order I would use is:
1) Heaters
2) HV center tap
3) Bias supply
4) HV red leads to diode board

All this testing should net you good information but I have to say that I would remain uncomfortable keeping a PT in service that had overheated to the extent and is producing the smell that you described.

[Jonny toetags]

Thanks Tom.
I did the testing as suggested...the fuse blew with only the primaries hooked up.
And I agree with you, I do not want to keep this thing. The smell alone tells me it's time to toss it.
I plan on replacing it with a Classictone PT, more specifically this one http://www.classictone.net/40-18004.pdf
Any tips for wiring it up compared to the original? It will be my first retrofit (new PT into old amp)

Other than the amp going through many shows with questionable power, what could have caused this?

[The Dude]

The wiring diagram and wire colors are right there in the pdf. There's no telling for sure what caused the failure, but if you do a little metering and see which windings are shorted, it might give you a clue as to what happened. Often it's a shorted power tube, but I'm not saying that is the case here.
A couple of things to consider:
1) You may want to build a light bulb limiter when you first fire it up again to prevent further damage and possibly frying a new transformer.
2) Be sure to check the mains fuse before installing the transformer and make sure it's the correct value. The fuse should have protected the transformer. If it didn't, it may not be the right one.

[52 Bill]

Often it's a shorted power tube, but I'm not saying that is the case here.

Shorted diodes and bad filter caps also fit the bill. The real problem is the use of the wrong power fuse. When the problem occurs the fuse should blow saving things like the transformers.

Stick a 20 amp car fuse in there and the transformers can get stressed to the point of failure. I used to see amps come in with dead transformers and find a blown 10-20 amp fuse in place of the correct 2 or 3 amp fuse. At least now cars use those blade type fuses that don't fit in the back of guitar amps. So now you find tin foil wrapped fuses instead.

[Jonny toetags]

Thanks guys.
I'm going to order the replacement PT today so I should get it in about a week.
After that I'm going to start her up section by section with the light bulb limiter (i have one The Dude )
All the filter caps are new ( i changed them when he first got this amp) and they all test ok with my Peak Meter.
What's a good way to test the diodes? Just continuity?
One thing I know for certain is that they played a few shows with questionable power. At one show, he actually blew his speaker cab and had smoke coming out of one speaker.


I'll probably check back in here once the PT comes in.

[Chuck H]

Considering the abuse the amp has suffered ("Wow! The Tolex is melting from the heat of the tubes... Better take that panel off.") and seeing as he blew up a speaker cab there's the possibility of an OT failure also. So do test that as well.

Hard to say what you'll find damaged once you start digging My suspicion is that the bias supply caps have never been replaced, have been failing and that was causing the tubes to melt down, causing all the other stuff. Especially likely if Bill was right about the use of an over rated fuse.

[Jonny toetags]

How would I go about testing the OT?

[Chuck H]

Courtesy of the guy that wrote my sig.

Output Transformers

[Jonny toetags]

So I finally installed my new PT and slowly turned the amp on section by section using a lightbulb limiter.
Everything seemed fine until I installed the tubes and took it OFF standby: after about 1 min, the new fuse blew.
I need a little help figuring out how to troubleshoot.
I have about 460V at the rectifier/diodes.
Heaters read about 3.7V
I haven't changed any parts ON the board. I'm wodering if it could be the 0.1 bias caps? By the look of the eyelet board, it took a lot of heat. (its really really bent)

[Chuck H]

Pull the, tubes, install a new fuse and check for bias voltage.

[Jonny toetags]

Pull the, tubes, install a new fuse and check for bias voltage.

Sorry Chuck, could you clarify what voltage I should check.

I'm getting 456V on pin3 of the power tube sockets

* and what abut the 3.7V on the heaters?

[Mick Bailey]

With older amps the transformers will stand much more abuse before failing. The modern trend is to wind a transformer very close to its rated output with little overload capability. The insulation used on modern transformer windings also appears to stand less stress than it used to. All down to cost-saving (I suspect). My observation is that older amps will cook their transformers under sustained overload for quite a while before showing obvious signs of burning or damage, while more recent models suffer from more abrupt failure - usually a short in the case of a PT.

It's always good to check for earth leakage on a mains transformer - at least at the operating voltage of the unit, or ideally at a higher voltage (500v or so). A multimeter won't necessarily reveal anything unusual due to the low test voltages. I've had transformers that test fine with a DMM, but have leaked badly where the fabric-covered mains leads enter the end-bell and this has only shown up under operating conditions.

In the case of output transformers I use a ring-tester that pulses the winding and converts the number of 'rings' into a bar display. It's a refined version of the neon test, but can show problems up early on that can be interpreted as a 'pass' using a neon. It can also sometimes reveal which winding is shorted - depends on the actual fault. Usually an output transformer problem is fairly self-evident and testing it is just there to confirm suspicions. A multimeter will show an open circuit or dead short, but by far the most common problem I've found is a shorted-turn, which a multimeter won't detect.

[Chuck H]

Sorry Chuck, could you clarify what voltage I should check.

I'm getting 456V on pin3 of the power tube sockets

* and what abut the 3.7V on the heaters?

Oh... Sorry. Check the VDC on pin 5 of each power tube socket. It should be a negative number. Like -50 or so.

You're heaters are AC. I think you probably know that because you put down a number, 3.7. That would be about half of what you should have. Did you measure with the black probe to chassis and the red probe on one tube socket pin? The filaments can be measured on power tube pins 2 and 7 with the meter set to AC. one probe on pin 2 and the other on pin 7.

[km6xz]

The symptoms indicate the problem in the amp output caused the PT to fail. The heat from the tubes would suggest the bias problem, and if so, the tubes are toast also. Does it get hot with the new transformer. Does the fuse blow with no signal? In it only blows with high signal, check the OT but if it it blows with no signal it means the transformer is not the issue. You can short the OT completely on its primary with not signal and the tubes will just idle at whatever current the bias setting establishes and not present a heavy load to the circuit.
Pull the output tubes and make your measurements for plate voltage, grid 1 voltage, screen voltage and heater(AC range) and report back.
Be careful with the light bulb limiter, it will drop the mains voltage with unpredictable results, depending on the bias supply design, lower mains can actually increase plate current because the bias voltage dropping proportionally to a greater extent than the plate voltage.