P2P 5F2-A smoked my OT

andrewRneumann

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I put this test circuit in LTSpice to simulate the architecture. Current Flows in the primary, the secondary cancels. If you lift the ground on the secondary center tap and place it on one end of the speaker load it works. No telling if the Tube will like the load, but feeding both ends of a center-tapped primary into the same tube seems to work. I would have thought it would not.

View attachment 988226

I think the individual legs of this OT are in anti-phase. Try it again with the upper inductors rotated 180 degrees.
 

ArcticWhite

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I think the individual legs of this OT are in anti-phase. Try it again with the upper inductors rotated 180 degrees.
I mapped the transformer.
- First measured resistances with the meter.
- Then used the heater wires connected at the primaries. Heater current is 6.98 VAC.
- Then connected the heater wires to the secondaries and read VAC from the primaries. (The results surprised me because the highest voltage reading came from between what I assumed was the center tap and the brown primary leg. Do I have the center tap wrong?)

20220528_174151.jpg
 

andrewRneumann

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I mapped the transformer.
- First measured resistances with the meter.
- Then used the heater wires connected at the primaries. Heater current is 6.98 VAC.
- Then connected the heater wires to the secondaries and read VAC from the primaries. (The results surprised me because the highest voltage reading came from between what I assumed was the center tap and the brown primary leg. Do I have the center tap wrong?)

View attachment 988264

Is it possible that Brown-Blue was 0.47kOhm? Then orange would be the center tap on the primary.
 

andrewRneumann

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I'm not sure what I did is the best way to simulate this. It really doesn't represent a tube. That and I goofed up the secondary inductance values.

I bet you did it right. I just think this OT is not the typical "push-pull" OT. It think it's more like "push-push" or "pull-pull"... if that makes any sense.

But I'm having doubts now, because @ArcticWhite 's tests should not have worked the way they did if I'm right about that.
 

2L man

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When you measure the winding ratio applying 6,98VAC to secondary you can't assume it is still 6,98VAC because secondary resistance is very low! It is wrong way to apply voltage anyway!

I think if OT fails when Mains voltage is applied to the primary it is not useable for tube amplifiers anyhow. I always test OTs feeding European 230VAC mains to the primary and winding ratios come exactly what OT datasheets show. About 115VAC Mains in NA also produce accurate winding ratios too.

When OT is a push pull and has the Center Tab the voltege there is EXACTLY half what is inputted to the primary coil ends. And if OT is Ultra Linear their voltages come exactly what they are wound to be.

Sometimes Single Ended OT has few primary inputs but their voltages should also come where they are suppose to be!

The OT resistance measures are just estimates and they can not trust because there are different lengths of wire on separate coil wounds although wound count are right! Even 25% difference between PP primary resistances can be fine.

And now when all primary and secondary voltages show good values I know OT is safe to install!

Of about 30 OTs I have measured one had different winding ratio, sold as 8k but it was 6k which would have not been critical. But one had primary CT and Anode wire colors crossed which would have been critical for operation.
 

ArcticWhite

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@ArcticWhite I was probably wrong on my guess about this stereo OT. Don't hook it up like I suggested. Who knows what might happen if you did. I can't wrap my mind around about how this thing worked in mono...

That said, you probably can't use this for single-ended.
You were correct, I measured wrong!

The Blue/Brown resistance is actually 470 ohms. So I had the center tap wrong. Also, my secondary readings were wrong as well - I must have had a wire grounded inadvertently.

Anyway, here is a revised schematic
.

[EDIT - I DIDN'T REVISE THE VOLTAGE READING.]
See my next post.
20220529_065157.jpg
 
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Phrygian77

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@ArcticWhite at this point, I don't know why you don't get a new appropriate output transformer.

If you're voltage numbers are correct now, that's a really low turns ratio, to the point that I still think something isn't right.
 

andrewRneumann

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Shame that lower transformer is now blown. I think you could have got it to work if you used both just like in the original circuit. Without it, I think you're looking at a push-pull OT that's not going to be happy wired up as SE.
 

ArcticWhite

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@ArcticWhite at this point, I don't know why you don't get a new appropriate output transformer.

If you're voltage numbers are correct now, that's a really low turns ratio, to the point that I still think something isn't right.
Yeah, that is probably the best plan. I do like to build these things out of old amps, and so far I've done five or six without having to buy transformers.

I think if I'd done as Andrew suggested, and followed the original scheme with both OTs working together it would have been okay.

So now I will have to spend fifty bucks on a new transformer. Kinda disappointing. Like buying a new engine, for a '73 Datsun B210.
 

ArcticWhite

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Shame that lower transformer is now blown. I think you could have got it to work if you used both just like in the original circuit. Without it, I think you're looking at a push-pull OT that's not going to be happy wired up as SE.
Okay. But...hmm.

If you look at the original Westinghouse schematic, the second OT is switched out of the circuit, when the amp is in mono mode. And the main OT has the secondary center tap grounded through the other OT secondary.
So it does seem to be operating alone in mono SE.

IMG_20210720_001114 (1).jpg
 

andrewRneumann

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Okay. But...hmm.

If you look at the original Westinghouse schematic, the second OT is switched out of the circuit, when the amp is in mono mode. And the main OT has the secondary center tap grounded through the other OT secondary.
So it does seem to be operating alone in mono SE.

View attachment 988466

That's what I originally thought, but there's something I don't understand about the "mono" mode. Both output tubes are amplifying the same signal and are in phase with each other. In a regular push-pull, the signals have to be inverted copies of each other. But in this case, they are not and it still works. How do they not cancel each other out in mono? That's why I originally thought the windings must be in anti-phase. But then stereo doesn't work. So truly, I'm confused but I'm fairly confident that running it with a single tube isn't going to work and may overheat both the tube and OT if you tried it the way I suggested. But if you are feeling like experimenting, be my guest!
 

ArcticWhite

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Here is what @mrriggs said about this stereo amp a while back.
https://www.tdpri.com/threads/new-old-amp-to-convert-into-something.1077143/post-10804433

Edit: my initial interpretation (below) was about ninety percent wrong.

So here is my understanding: The stereo signal is reproduced out of phase on the vinyl. So a phase inverter isn't necessary. The two signals are already out of phase.
A mono pressing won't have any right channel info at all, so only one of the channels will be getting any signal at all. (Also, the cartridge has a stereo and a mono needle.)
Conclusion(?) Half of this OT might be used in SE configuration by using the center tap, and one leg of the primary??

Edit: also, take a look at the switch on the channel 2 OT - where it's tied to a second switch on the grids of the power tubes. In mono the signal has a path to ground, while the channel 2 OT is out of the circuit.

The effect of that is way over my head
 
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andrewRneumann

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Edit: also, take a look at the switch on the channel 2 OT - where it's tied to a second switch on the grids of the power tubes. In mono the signal has a path to ground, while the channel 2 OT is out of the circuit.

For some reason, I thought when switching to mono, it would combine L and R. You are saying it just isolates L and cuts off R?
 




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