What Makes PT Windings Open?

[R.G.]

Maybe someone else can shed light on this. Based on my reading of the only web references I could find, which included a Chinese patent, PT's with copper shielding straps have drain wires soldered to the straps for the purpose of preventing them from acting as capacitors.

Some PTs have shield layers inside between the primary and secondary. These layers need to be grounded to prevent capacitive coupling of noise from the primary AC wires from being coupled into the secondary, and vice versa. It doesn't keep the capacitances from being there, but it does shunt the noise to ground between primary and secondary, and keep them from communicating through the capacitances.

[The New Steve H]

I appreciate all the info I got here since it blew up. Now at least I have an amp that powers up for diagnostics.

[Firestorm]

Transformers are a whole different world. If you thought tubes were complicated...

They look idiot simple, but aren't.

Yes, some modern ones have thermal fuses. A lot have that extra ground wire.

And Fender used to throw a resistor in the primary under the tape wraps to protect the rectifier tube. Never saw one like this myself, but HAD swears it's there in some. Who am I to argue?

If you're up and running, put limiter in, or better still a Variac with current monitoring.

Parts fail, especially first generation parts

[The New Steve H]

Here's what will drive me nuts: if it works fine, will I be able to make myself blow $60 on a new one? And I don't understand why the transformer body wants to be 19V above ground.

Weird, quasi-related news: I bought some used motors from some guy on Craigslist, and I'm pretty sure two of them are for doing windings on motors or transformers or something. I only took them because they were dirt-cheap and came with nice speed-control pedals. They have counters and everything.

I didn't want them all that much, but they were bundled with a stationary Themac grinder, which is a very expensive tool, not to mention a mandrel for making a bench grinder with a drive belt.

Having thoughts about redoing my annoying guitar pickups, and maybe even transformers.

[R.G.]

Here's what will drive me nuts: if it works fine, will I be able to make myself blow $60 on a new one?

Aye, there's the rub. It works fine - doesn't it??

And I don't understand why the transformer body wants to be 19V above ground.

That's scary. If it were mine, I'd unplug it, and use my ohmmeter to test for continuity from the transformer body to every lead. If the resistance is under 1M, Houston, we have a problem. You might try connecting the transformer body to "ground" (how do you have this hooked up that you can measure it to ground?) with a 20K resistor and measuring across that. 20K is about the resistance of dry skin connections, so if you get more than a volt or two across that, it's not a capacitance phantom current, it's a leak. And that could in turn be why it overheated if connected to a grounded chassis. Just guessing.

Having thoughts about redoing my annoying guitar pickups, and maybe even transformers.

Go do some reading about safety before winding AC power transformers. I hate to lose you guys once you get all trained up about which end of the soldering iron to hold.

[The New Steve H]

And I don't understand why the transformer body wants to be 19V above ground.

That's scary. If it were mine, I'd unplug it, and use my ohmmeter to test for continuity from the transformer body to every lead. If the resistance is under 1M, Houston, we have a problem. You might try connecting the transformer body to "ground" (how do you have this hooked up that you can measure it to ground?) with a 20K resistor and measuring across that. 20K is about the resistance of dry skin connections, so if you get more than a volt or two across that, it's not a capacitance phantom current, it's a leak. And that could in turn be why it overheated if connected to a grounded chassis. Just guessing.

Having thoughts about redoing my annoying guitar pickups, and maybe even transformers.

Go do some reading about safety before winding AC power transformers. I hate to lose you guys once you get all trained up about which end of the soldering iron to hold.

Believe it or not, I did think to test the resistance between the leads and the transformer. Too high to register on my meter.

As for ground, I had the transformer connected to the hot and neutral connections (switch and male plug on the PCB), and the transformer itself was resting on a piece of wood, in case there was a short. I put one test lead on the chassis and the other on the transformer, and I got 19V.

The only connection to the PCB was the black lead. Also, the tubes were not in the amp.

You hold the big end of the soldering iron, right?

I'm not planning on winding anything tonight, but it looks like I have the machinery to do it at some point.

[R.G.]

Believe it or not, I did think to test the resistance between the leads and the transformer. Too high to register on my meter.

Good. And good on you for thinking to test.

As for ground, I had the transformer connected to the hot and neutral connections (switch and male plug on the PCB), and the transformer itself was resting on a piece of wood, in case there was a short. I put one test lead on the chassis and the other on the transformer, and I got 19V.

OK. In that case, you're probably reading capacitive voltages. The 20K-to-ground trick would probably make that disappear.

You hold the big end of the soldering iron, right?

Depends on the iron.

[The New Steve H]

Now I'm wondering if it's worth it to get an oversize PT to replace the one that betrayed me. Bill M the Blues Jr. guy seems to like them, although he is selling them to people who use bigger output tubes.

The amp sounds great the way it is, by Blues Jr. standards, but I would like to avoid new overheating issues. Sooner or later I'll want to replace the PT I put back together with no provision for thermal protection, so the question is not whether I want to get a new PT but whether I want to spend 15 more bucks on a bigger one.

I suppose I could put a new fusible link in the old PT, if I knew what to wrap it in. The little wrapper that came with the old one is not coming off.

I want sag, so I don't want endless juice on tap, but I built an amp with a PT that was pretty small, and it seemed to lack testosterone. I got a bigger PT for it, but I never got around to putting it in. Maybe I'll do that this week.

[tubeswell]

Some xformers are undersized for the operating conditions and they run hotter than desired, which shortens their life. For example, undersized PTs made by manufacturers who design a PT using typical 60Hz mains parts when they don't appreciate the inductance experienced with 50Hz mains. Watch where you buy your PTs from.

Another way PTs can fail is being fried by an AC short to ground on one of the secondaries, caused by something such as a shorted reservoir cap, or a shorted tube rectifier causing a shorted cap… Look for evidence of a short (like a burnt supply resistor and/or a shorted cap), or an unwanted contact between the HT supply and the ground return.

[R.G.]

Now I'm wondering if it's worth it to get an oversize PT to replace the one that betrayed me.

Depends on your scale of "worth" and how you score it. Oversize carries a lot of things along with it besides lower temps, as you note below. However, simply getting one that is the same size but wound with higher temperature rated insulation will let the same transformer just run hotter.

This is apart from the issue of whether you had a random failure on the thermal fuse, or a systemic (i.e. it worked like it was supposed to) failure. Without more data, you still don't know whether the PT actually was too hot or the fuse was defective.

My own personal scoring polynomial for "worth" includes a term that says that something suddenly failing to operate makes the worth drop to zero, so I tend to make things that will withstand nearby detonations, then make them "softer" or "sweeter" by other means than making them just fragile enough to be soft and sweet as a byproduct of being fragile.

I want sag, so I don't want endless juice on tap, but I built an amp with a PT that was pretty small, and it seemed to lack testosterone.

It is very easy to add any amount of sag that you like to a bigger, stiffer PT's output. External power resistors to fake added resistance that's not in the windings do this nicely. You can even put the resistance in the primary wires to make all outputs sag, or in one or more secondaries to affect only that one.

It might be worth playing with some sag resistors to tune your test amp.

[Structo]

I have only fried one PT, which was a freak thing due to a piece of a screw that had shorted out the 12v transformer in my ODS.

The filament winding in a PT is usually the highest current winding on a power transformer.

So if something demands more than that for very long, you will get smoke.

Transformers have a very unique smell when burning.
Once smelled you will remember that.

Using an underrated power transformer for that vintage sag is dangerous.
Fender's transformers were heavy duty and can take abuse.

But when you exceed the current specs, things will get warm.

[The New Steve H]

I'll keep playing it until it blows up again, and if it does, I'll probably get the bigger PT just to make problems less likely.

Then if it's not sagging the way I want, I'll come back and pick everyone's brains so I can fix it.