is this a bad PT??

[Alex R]

I have a 50 watt Mesa head brought in blowing fuses. It certainly lights up the old light bulb limiter. Same with all the tubes out. The secondaries are all plugged into the board, so I pulled them out. Out went the bulb, so I wasted quite some time looking for shorts on the board. Then I tried connecting the secondaries back to the board in batches. I found that the bulb would light if I connected only the HT centre tap, which goes to ground on the board.

Bad power transformer? Or something else weird that I'm too tired to think about?

I looked for resistances between that CT and the primaries - nothing. Resistances from the CT to the HT legs look normal.

[Enzo]

Powr off, take an ohm meter to the windings. See if any windings are shorted to other windings. Also check to see if any secondary shorts to the primary. Last, see if any winding shorts to the frame. All windings should be open to frame.

As an example, imagine an end of the HV winding is shorted to frame - which is grounded to the chassis. SO far it won't do anything, but when you connect the CT to ground, now you have the CT AND the end grounded - that is a dead short across the winding. This could happen and the winding itself will still measure OK.

[Alex R]

Thanks Enzo, I'll check it out tomorrow.

[Enzo]

If it does seem to be shorted, it goes into the what have I got to lose mode. Take the end bell off and inspect the wire connections between the winding wire and the external lead wires. You might get lucky and find something touching the inside of the bell.

[Alex R]

Yes - HT winding shorted to the frame. Thank you Enzo! I'll see if I can rescue it.

[Alex R]

OK well I took the PT to bits. The HT winding is not in fact shorted to the frame directly. There's an unsheathed bit of wire coming out from the middle of the windings which is wrapped under one of the bolts and the HT is shorted to that. Some kind of interleaved screen? Anyhow I decided to see if we could do without it, wrapped it in heatshrink and tucked it away. And the amp runs fine without it. A bit hot, but it's small and so probably runs hot anyway...?

So does that screen have some kind of safety function or is it to cut hum - can we really do without it being grounded to the frame?

[Alex R]

...I'll just give this a bump as I'm still confused about what purpose a grounded winding in a PT might have, and whether it might be possible to do without it. I'd be grateful for any educative attempts.

[52 Bill]

A few weeks ago, I got a New Sensor power transformer for a Super Reverb. There is an orange lead that was not shown on the lead diagram. Electronically it was not connected to anything, not to case, or to any other winding. A call to New Sensor revealed that it is an connected to an "electrostatic shield", and should just be connected to ground.

[Bruce / Mission Amps]

A few weeks ago, I got a New Sensor power transformer for a Super Reverb. There is an orange lead that was not shown on the lead diagram. Electronically it was not connected to anything, not to case, or to any other winding. A call to New Sensor revealed that it is an connected to an "electrostatic shield", and should just be connected to ground.

Mostly, those kinds of wires are connected to that copper, single turn sheath around the outside windings... aren't they?

[52 Bill]

Mostly, those kinds of wires are connected to that copper, single turn sheath around the outside windings... aren't they?

That's what I thought, but there was no continuity to anything including the copper band.

[Alex R]

...this was an uninsulated multicore wire coming straight out from the middle of the windings. It had been wrapped around a bolt inside the bell. The HT winding had shorted to it. Odd. Just wondering if it's ok to run the amp without it. Try it and see I guess. It was on a Mesa single rectifier 50w. Tiny little PT for a 50w amp, runs quite hot.

[52 Bill]

If it is running that hot, you may still have a problem with the transformer.

[Bruce / Mission Amps]

...this was an uninsulated multicore wire coming straight out from the middle of the windings. It had been wrapped around a bolt inside the bell. The HT winding had shorted to it. Odd. Just wondering if it's ok to run the amp without it. Try it and see I guess. It was on a Mesa single rectifier 50w. Tiny little PT for a 50w amp, runs quite hot.

Boy, doesn't that sound more like a frame ground then a E-static shield ground?
That PT would scare me... with out seeing it, it really sounds like a dead short to the internal frame ground.

[Alex R]

- yes but no - the HT is not shorted to the iron, just to this tap coming out of the middle of the windings, that used to go to the iron but isn't connected to it any other way... has to be a screen of some sort. No way the HT goes to the chassis without that connection. But keep talking to me Bruce!

I'd replace the transformer but my email to Mesa asking whether they were going to rip me off again like the last time (90 quid for a 2 x EL84 OT) was, I guess, so pissy they didn't respond. Ah well, the customer says he's happy with no guarantees and a nice low price so we'll see what happens. I ran the amp for a day and it was ok, the txr got hot but the damn thing is so tiny it's bound to. I'll try it again. I said to the guy, look if it fails I'll put a nice big Hammond in there, screw Mesa and their prices, won't cost you any more than it would if I did it now, might as well give it a chance to save you money.