Output Tranny Question

[Amp Kat]

Is there a way to use a PP tranny for SE operation. The SE has an air gap right but I thought there was a way to use a xistor to offset it or something ?

[soundguruman]

Of course, use half the primary winding, if the half matches your tube. Or use the whole if it matches.
It may not be the "most ideal" transformer, but you can make it work.
You will need to adjust the load on the secondary, accordingly.
BUT it does work with some experimentation, if you can roughly match it.
I have, personally done this, and made single ended amps using a PP transformer. Why not? Go ahead and try it.

[Jazz P Bass]

Yeah, a PP tranny 'might' work.
But: The core of Single End output transformers have an airgap. The purpose of the airgap is to accept the DC current of the
output tube without saturating the core, leaving enough headroom for the sound signal.
Check out thius link: http://www.valveheart-bg.com/theory/transformer.html

[Chuck H]

Also, The core on SE OT's is typically quite large by comparison to PP OT's for a given wattage. This is also to prevent saturation. Using one half the primary can work fine most of the time. Using both halves means you have a smaller stack AND no air gap. So your going to lose plenty in the way of frequency response and maybe even ideal load on the power tube. That isn't to say it won't "work". But it won't be ideal. I've used PP OT's intended for AB1 with tubes biased into class A PP with good results. But for single ended operation you need a bigger stack to absorb and transfer the offset. I don't think I would even bother using a PP OT for a single ended amp. If it's a matter of using an OT you have on hand, build a PP amp.

[Gtr_tech]

Do it right the first time. Use the right xfmr....

[Amp Kat]

Also, The core on SE OT's is typically quite large by comparison to PP OT's for a given wattage. This is also to prevent saturation. Using one half the primary can work fine most of the time. Using both halves means you have a smaller stack AND no air gap. So your going to lose plenty in the way of frequency response and maybe even ideal load on the power tube. That isn't to say it won't "work". But it won't be ideal. I've used PP OT's intended for AB1 with tubes biased into class A PP with good results. But for single ended operation you need a bigger stack to absorb and transfer the offset. I don't think I would even bother using a PP OT for a single ended amp. If it's a matter of using an OT you have on hand, build a PP amp.

Pretty much what I thought and I ordered a SE from Mercury but was going to try and get by for the weekend but decided not too. Everything you guys said is exactly what I read also so it was all about the large DC and I've also heard they can fry if ran for long periods because of it. Anyway thanks fellows.

[R.G.]

In SE OPTs, the air gap is to help stop saturation from the unbalanced DC currents, as noted. An air gap lowers the primary inductance at the same time it reduces the tendency to saturate, so the core area and number of primary turns must then be increases to restore enough primary inductance to get the desired low frequency response. In addition, only one half of the transformer's total magnetic capability is used, so the core has to get larger yet.

P-P transformers have each half offsetting any net DC for each other, so they sidestep the air gap, lower inductance and magnetic capability problems of SE.

Since the root of the problem is DC offset, one can introduce a compensating DC offset. It's a bit tricky, but can be done. You use one half as the normal SE OPT would be used. Then you use a transistor or MOSFET ( a tube works, but why spend a tube to regulate DC? ) to allow an equal DC bias level through the other half-primary. This corrects the DC offset issue, means an air gap is not needed, no lowered primary inductance happens and uses the full magnetic working ability of the core.

About the only problem of using a PP OPT in "Corrected Single Ended" (a name I just coined for the **Official** name for this process ) is that PP transformers are designed with a primary wire size and hence wire resistance to meet only the power issues involved in PP use. Since PP use is more economical of the transformer's use, this may be smaller than would need be used for full Class A use, which is what SE is. In fact CSE mode is effectively *two* SE transformers running together in the same core, at least as far as power dissipation goes. So the transformer gets hotter than it otherwise might. This is partly offset by PP transformers being used at bigger power levels in general, so it may be that they do OK because the SE circuit doesn't push them as far.

It all depend on the details. Like everything.

[MWJB]

Hammonds 125 series PP transformers are recommended by that company for SE use.

If I had an SE amp that need an OT & I had a suitable PP OT on hand, I wouldn't think twice about giving it a go. Just like I would be happy to run a fixed bias PP amp with one power tube for lower power/fi scenarios.

If I had to buy an OT...then I'd probably buy the correct part.

The best sounding 6V6 SE guitar amp I have ever heard used a PP OT. Bear in mind that most SE amps are built that way to be cheap, rather than adhere to ideal design principles. An expensive SE Hi-fi amp may be different proposition.

[soundguruman]

I built a champ with one 6V6 and a PP tranny, and it worked fine. Whats the big deal?
You can buy an exact match transformer if you really want, but for experiments, I just use whatever is around for free.

[soundguruman]

You can actually compensate for the offset with other components in the circuit.

[Amp Kat]

Ok that explains why the tranny has to be so much larger in the SE design. This is a single 6V6 amp used for lower volumes and Cathode biased but not a Champ or even a Fender.
I might would try the transistor just to experiment for fun had I not got the SE on the way but was worried about the bandwidth as to why I didn't opt for the PP.
Anyway thanks to all you posters for replying as I learned something once again here.

[Chuck H]

You can actually compensate for the offset with other components in the circuit.

OK. Don't pull a Gary. can you explain?

[R.G.]

OK. Don't pull a Gary. can you explain?

ROFLMAO...

It's becoming a part of the tube amp language~!!!

"Wait a minute - don't pull a Gary on us."

[Enzo]

Odd, I sometimes fantasize a debate between the two.

[MWJB]

If you were using a PP OT in a SE design, couldn't you (for some practically valid reason that escapes me at the moment) run one tube at one end of the OT primary SE, then have a dc current path to ground from the other end, via a large hi wattage resistor? Akin to if you are switching a PP cathode biased amp to SE mode by killing the signal at the out of phase tube's grid? Perhaps this is the kind of thing that Soundguruman is hinting at?

Or, if the B+ was low enough & the OT primary DCR suitable (big "IFs" I grant you), have that same SE tube's cathode driving the other half of the primary?

Just thinking out loud...