PP OT question

[dude]

Is it possible to use a push and pull OT for a single ended amp.

Just using the CT and one plate side?

al

[vibrolax]

No, in general.  A single ended transformer has a gapped core, so that the DC component of the plate voltage will not saturate the core.

A push-pull transformer does not have this magnetic gap.  I suppose it is possible to get away using a larger PP transformer with a low-current output tube that wouldn't saturate the core, but I've never done such a thing.

Some PP transformers might have a spec for how much DC they can tolerate in the primary.

[dude]

You sure I can't use one side of the Primary of a PP OT for a single tube...?

It's a friends amp and I put in a single ended OT, he had a PP using only one plate primary. He complained that he liked the tone better with the PP OT using one side.... I just wanted to make sure it's ok.

Another strange thing is when I looked at the PP OT someone had wired the "two plate primary's and left the CT hanging"...? It worked this way too. If I put it back, do I use the CT and one plate or the way it was (both primary plate wires, one to the tube plate and one to the filter....?) Kind of scary to me....?

al

[vibrolax]

I'm sorry if I wasn't clear in my previous response.  Pp ot's are not gapped, so running them single ended could saturate the core.  I saw another post of yours that said this is a single 6v6.  Maybe 25 mA won't saturate the core, but you haven't supplied any data about your OT.

As to whether you should use half the primary or all of it depends on the impedance ratio you're trying to achieve.  Using half the primary will make the impedance ratio one quarter its pp impedance ratio.
Again, you didn't say what the design specs are for this ot.

One lead of a SE OT does go to b+, the other lead to the plate

If you want a definite technical answer, you will need to supply the technical specifications of your circuit and components.

Sorry I can't give more specific advice.

[dude]

The Amp is a 5F2A, Tweed Princeton exact copy (pretty simple circuit) running one 6V6 all stock . It's a friends amp, he bought a Mojo exact replacement OT and asked me to install it. He bought the amp on ebay and wanted to change the OT. When I was taking out the old OT, I noticed it was a PP with the two primary plate wires, one brown and one blue (std. color code for PP OT) but the CT, red wire was hanging loose and the two plate wires were used, one to the tube's plate the other to the power filter cap at the B+. I've never seen this before. I knew the circuit was single end and need a single ended OT which he bought from Mojo. I installed it just like you mentioned in your last post. The guy didn't like the tone, too much break-up he said. I told him that's a Princeton Tweed, that's what you get on full volume it's only 5 watts. He asked me to put the first OT back, the PP OT. I don't know the specs, or make but looks like a Weber OT.

I went to Weber's site and sure enough it was a Weber OT for a Tweed Princeton and Princeton Reverb, at least that's what Webers says (had Weber's numbers on it), it's a PP OT and not a single ended OT like in a Tweed Princeton, 5F2A. I guess the Princeton Reverb uses a PP (I know a PR has two 6V6s, the Tweed one) OT and this OT is used for both applications...?

Here's the specs:   https://taweber.powweb.com/store/022913sch.jpg 

I just want to give the guy his Amp back with the OT he wants. But don't know how to wire a PP OT in a singled ended amp. I just remembered when I was taking the first OT out,  it was wired as mentioned above.

I appreciate your help.

al
https://taweber.powweb.com/store/5f2a_schem.jpg
EDIT  I just found this schematic on Weber's site for a 5F2A

[vibrolax]

Given the impedance ratio for the full primary of the Weber 022913 is 8000:8 ohms, I'd wire the SE across the full primary, leaving the CT unconnected.  The tube data sheet for 6V6GT specifies a load impedance of 8500 ohms at 315 VDC plate voltage.
http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/6V6-GTA.PDF

This is the way the OT is shown to be connected in the Weber 5F2 schematic.  The Red CT wire isn't used.

Proceed with confidence then.

[vibrolax]

I understand about your friend, but with the opposite complaint.

I have an acquaintance with a Fender silverface twin reverb.  It broke up at low volume, which he loved because he was a blues guy.  Then he had it serviced with new tubes, bias, and a cap job.  It didn't break up any more until ears were bleeding, just the way it was designed to do.  He didn't like it as much any more.

A good friend gave me a Blackface Vibrolux Reverb with badly mismatched output tubes, a partially shorted output transformer, and 2 x 16 ohm speakers.  It also broke up perfectly at low volume (it couldn't produce high volume at all).    I restored it back to stock, so it started breaking up at 6 or 7 on the volume.  Ears not quite bleeding, just as designed.

Maybe your friend would be slightly happier by installing a lower gain preamp tube like a 12AY7.  That's what I would do.

Best regards,
Jon

[dude]

I also noticed that the Mojo 5F2A OT had two secondary taps that were wired with two separate speaker jacks, one 4 ohm, one 8 ohms. I had only installed the one 8 ohm, he had someone else hook up the 4 ohm tap. But the guy used a shorting jack, when he played one tap, the other wasn't just hanging loose, the tip was shorted to ground.

I think that's a no, no and the reason the Mojo OT was sounding weak, and breaking up right away, actually no clean. Now I'll have to change the two speaker jacks to non-shorting and let him hear it.

Thanks for the reasoning behind the strange wiring, I would have wired it wrong. Can't stop learning no matter how many amps you've fixed.

The thing about the Twin and Vibrolux is they make pedals

"People Are Strange"

al

[HotBluePlates]

You sure I can't use one side of the Primary of a PP OT for a single tube...?
...
... when I looked at the PP OT someone had wired the "two plate primary's and left the CT hanging"...? It worked this way too. ...

You can drive a car with 4 flat tires too, but it's not a good idea. Vibrolax has been telling you right.

Could you use a push-pull OT in a single-ended circuit and get sound through it? Yes.
Will you get all the power output from your new SE circuit? Maybe not. You might if you use something like a 30-50w OT in a 5w SE circuit.

The idle current in a SE output stage flows through the OT in one direction only, partially magnetizing it. This doesn't contribute to output power but does eat up a significant portion of the OT's "magnetic capacity."* The a.c. signal also creates a changing magnetic field at the core, and the core's ability to support that field with its "magnetic capacity" is what assists transferring power from the primary to the secondary. For a given power through-put, a SE OT is big, and of course adds that air-gap which helps offset the effect of the standing direct current and prevent early core saturation (a point where you simply can't get more transfer of power from the primary to the secondary).

Push-pull OT's have an arrangement of winding and physical connections such that the d.c. of each half of the output stage flows in opposite directions through the primary winding, while the desired a.c. signal currents flow in the same direction. So the end result is the impact of the d.c. on core magnetization is cancelled out as long as the two halves of the output stage are reasonably balanced at idle, but the desired audio output of the two halves add. Because the idle direct current is effectively cancelled as far as core magnetization is concerned, the core doesn't have to support that anymore and can be sized only for the desired audio output. As a result, push-pull OT's have smaller cores than SE OT's of the same power rating, and of course don't need air gaps.

Taken all together, this means that while you might get away with using a push-pull OT in a SE circuit (assuming you even get close to the correct primary impedance to allow the output tube to make maximum power), you won't be able to push as much audio power from primary to secondary as you would had you used the same transformer in its intended push-pull configuration.

... he had someone else hook up the 4 ohm tap. But the guy used a shorting jack, when he played one tap ... the tip was shorted to ground.

... the ... OT was sounding weak, and breaking up right away, actually no clean. ...

Yeah, people make that mistake all the time, not knowing the speaker jacks aren't exactly the same as the input jacks. Or they get an old Fender combo, plug the internal speaker into the external speaker jack instead of the correct one; now the original internal speaker jack is shorting the signal, and very weak distorted output is the result. Good catch on your part, as that's simply an issue of the jacks rather than the performance of the OT.

* "Magnetic Capacity" is not the correct technical term, but I'm using it to help you visualize what's happening.