P2P 5F2-A smoked my OT

[ArcticWhite]

I built this amp out of junk.

Harvested the chassis and transformers from a Westinghouse single ended stereo record player. Choke from a Hammond chord organ. Pots from some other thing.

Built it point to point in this itty bitty chassis, using the existing terminal strips and making up the layout as I went. I used contemporary grounding practice of running all the power stuff to one spot and everything else to the input jack. So I wound up with ground wires all over the place, and choke wires, and it's kind of a mess, but it worked, until it didn't.

Fired it up after I got the input jack figured out (after doing it wrong) and the amp sounded pretty great, plugged into an 8ohm 1x12 cab. But after twenty minutes or so, it started to make loud pops as I hit loud chords. This got progressively worse, until the OT gave off a few puffs of smoke, and then it started sounding really bad, and very distorted. So I turned it off.

When I built this thing, I used the smaller, simpler OT, because it had just the two primary and two secondary leads, with no center taps. The other one has center taps on both sides. Should I have used the other OT?

As you can see, the Westinghouse schematic is largely unreadable, so maybe these transformers aren't good for an 8 ohm speaker load?

 

[2L man]

Obviously you should have

Hammond 1750C and 1760C which can produce about 5W weighs 260g which is ~0,57lbs. If you did not weight the one which did burn compare its size to Hammond datasheet.

 

[SerpentRuss]

Hard to say what happened and of course speculating without knowing what you truly started with is a bit hard too.

Did the record player use EL84 output tubes for each channel? If not, what tube did it use? Even though you can't read the schematic, there would be a lot you could have figured out just by knowing what power tubes our transformer donor used.

You could also have computed the turns ratio of the transformer you used and figured out if it was a good match. An EL84 can put out close to 10 watts if you don't care how long it's going to last. That's probably more than a record player amp would be built to handle.

Hopefully the OT is the only thing you need to replace.

 

[Phrygian77]

The transformer was designed for push pull, so it may not have been able to handle the full on current of single ended.

 

[Phrygian77]

By the way, you said 'Westinghouse single ended' in your post, but the schematic you posted is push pull.

Edit: Sorry, I see now that it's stereo.

 

[SerpentRuss]

By the way, you said 'Westinghouse single ended' in your post, but the schematic you posted is push pull.

I didn't look that close, but I thought it was singled-ended stereo.

 

[ArcticWhite]

By the way, you said 'Westinghouse single ended' in your post, but the schematic you posted is push pull.

It's actually single ended stereo.

 

[ArcticWhite]

Hard to say what happened and of course speculating without knowing what you truly started with is a bit hard too.

Did the record player use EL84 output tubes for each channel? If not, what tube did it use? Even though you can't read the schematic, there would be a lot you could have figured out just by knowing what power tubes our transformer donor used.

You could also have computed the turns ratio of the transformer you used and figured out if it was a good match. An EL84 can put out close to 10 watts if you don't care how long it's going to last. That's probably more than a record player amp would be built to handle.

Hopefully the OT is the only thing you need to replace.

I did not change the tube lineup.
12AX7 EL84 6AC4

 

[SerpentRuss]

I did not change the tube lineup.
12AX7 EL84 6AC4

It may just be that the magic smoke in the OT was destined to come out. It happens. Shouldn't you have a 2nd OT of the same type laying around?

 

[Phrygian77]

I didn't look that close, but I thought it was singled-ended stereo.

I see that now. However, the B+ goes to the center tap, so the current is offset in the same way it is in a push pull output transformer. That's what keeps the core from saturating.

 

[ArcticWhite]

It may just be that the magic smoke in the OT was destined to come out. It happens. Shouldn't you have a 2nd OT of the same type laying around?

The other one has a center tap. That's why I didn't use it.

 

[SerpentRuss]

The other one has a center tap. That's why I didn't use it.

Officially confused. SOP for me. I should have looked harder at the schematic. If you didn't use the center tapped transformer that is shown, what role did the transformer you did use have in the record player amp, or did it come from somewhere else?

 

[ArcticWhite]

I see that now. However, the B+ goes to the center tap, so the current is offset in the same way it is in a push pull output transformer. That's what keeps the core from saturating.

I used the one with no center tap, but it appears that it wasn't up to the job.

Officially confused. SOP for me. I should have looked harder at the schematic. If you didn't use the center tapped transformer that is shown, what role did the transformer you did use have in the record player amp, or did it come from somewhere else?

The record player amp had a second OT because it is a stereo amp. One OT per channel.

 

[Phrygian77]

I used the one with no center tap, but it appears that it wasn't up to the job.

The record player amp had a second OT because it is a stereo amp. One OT per channel.

I can see now what you're talking about, but looking at the schematic, it doesn't quite make sense. That second transformer doesn't look like it's used on it's own, but it looks like it's switched in parallel (or series maybe) with the center tapped transformer.

 

[andrewRneumann]

I can see now what you're talking about, but looking at the schematic, it doesn't quite make sense. That second transformer doesn't look like it's used on it's own, but it looks like it's switched in parallel (or series maybe) with the center tapped transformer.

It’s pretty ingenious. The lower transformer, when not bypassed, transforms the difference signal between the L and R. Then recombines it with the upper transformer to separate out L and R on the upper transformer secondary. Bypass the lower transformer and you get a combined signal on both L and R (mono) on the upper transformer secondary. Or at least I think that’s what it does. Ha.

 

[Phrygian77]

@andrewRneumann I'm sure most seasoned old-timers would look at that and tell you exactly what it was, and wonder why it took us 15 posts to figure it out. Admittedly, my mind is focused on tube guitar amp circuits. While I know a bit about hi-fi, I certainly haven't studied much of the older hi-fi stuff, let alone any folding compact suitcase type phonograph systems.

 

[ArcticWhite]

@andrewRneumann I'm sure most seasoned old-timers would look at that and tell you exactly what it was, and wonder why it took us 15 posts to figure it out. Admittedly, my mind is focused on tube guitar amp circuits. While I know a bit about hi-fi, I certainly haven't studied much of the older hi-fi stuff, let alone any folding compact suitcase type phonograph systems.

@mrriggs figured it out last year when I first tore into this amp.
https://www.tdpri.com/threads/new-old-amp-to-convert-into-something.1077143/post-10804433

I'm still confused about this center tapped OT. Can I use it for an SE amp, or no?

 

[andrewRneumann]

@mrriggs figured it out last year when I first tore into this amp.
https://www.tdpri.com/threads/new-old-amp-to-convert-into-something.1077143/post-10804433

I'm still confused about this center tapped OT. Can I use it for an SE amp, or no?

EDIT: What I wrote below is probably misguided and wrong. Don't try it. It is based on false assumptions about the phasing of the windings.

I'm going to stick my neck out here, because I don't have first hand experience with this kind of OT. I think you can. Connect the plate of the EL84 6V6 to the upper and lower leg (parallel) of the OT primary. Connect B+ to the center tap of the primary. Ground the center tap of the secondary. Connect the speaker to upper and lower leg of the secondary.

Can I get a second on my idea here?

This is essentially running 2 OT’s in parallel. The same as the original schematic when running in mono.

Start with low volumes and check for overheating as you raise the volume.

Also keep in mind I have no idea what impedance this presents to the poor EL84 6V6. Since the original had two power tubes and you are using one, I would guess it’s half the impedance. You might want to double the speaker impedance to keep the current reasonable.

 

[andrewRneumann]

I would also test that OT with a source of AC voltage to figure out the winding ratio. Then you can select the appropriate speaker.

 

[SerpentRuss]

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.

EDIT: I was wrong, moving the ground reference does not keep it from canceling. So while you can move current through the tube with this arrangement, the secondary cancels regardless. That's if you believe LTSpice and they way they handle transformers.