OT question

[Justin Thomas]

I have no idea what the experts are talking about, exactly, when they say it "won't work." Maybe it won't work WELL, but tone and performance are subjective, right? I am up for some good old-fashioned "try-it-and-see" if anyone else is. Only caveat being, don't do it with any vintage parts. Smoke, fire, exploding caps, melting glass and grids, the Thermionic Apocalypse... let's have some fun! Don't forget at least part of the reason we got into this hobby to begin with - surprises and danger!

Justin

[nickb]

I have no idea what the experts are talking about, exactly, when they say it "won't work." Maybe it won't work WELL, but tone and performance are subjective, right? I am up for some good old-fashioned "try-it-and-see" if anyone else is. Only caveat being, don't do it with any vintage parts. Smoke, fire, exploding caps, melting glass and grids, the Thermionic Apocalypse... let's have some fun! Don't forget at least part of the reason we got into this hobby to begin with - surprises and danger!

Justin

Ah the good old practical approach

"Thermionic Apocalypse" I wish I had chosen that for my handle.

[Justin Thomas]

Is it possible to change it? You can take it if you can - call it compensation for the schematic resource!

Justin

[Chuck H]

FWIW I did use a small P/P 20W OT (small even for a guitar amp) as a sub for a crappy little 80's Champ OT and it sounded fine with ample bottom end. Better than the stock one. I had it on the Bench and I wasn't seeing any performance anomalies that I could relate to the OT. So there's that. I still wouldn't fail to idealize with a proper, gapped OT if I were putting my time into a build. Build time is more valuable than components.

[Enzo]

I think in general though, the message I would deliver is that getting away with something doesn;t make it a good idea.

I know people who successfully drive home drunk from the bar every night. Good idea?

[R.G.]

Wow. Whatta ride.

There's an old saying that just because you can do something doesn't mean you should do it. Exactly what "should" implies depends a lot on what you wanted to do. If what you wanted to do was go wandering about looking for something unexpectedly good in terms of sound, you may find something unexpectedly GREAT.

On the other hand, as the ladies say, if you want to find a prince, you're going to have to kiss a lot of frogs.

If what you're trying to do is use up parts in hand, you can go ahead and try it, even if most of the time it will return a result that's wildly different from what "normal" practices would expect. It is entirely >> possible << that this neato looking mushroom you found nestled in the grass on your walk will not only be edible, but taste GREAT. But it also might be a great idea to test it out quietly before wolfing a batch down. Just in case the new find is actually amanita ocreata - otherwise know by its common name, "death angel mushroom".

Hmm. I hadn't thought of it that way. Amanita ocreata is edible, in the sense that you CAN eat it. But doing so is generally accepted as a bad idea, and maybe you should not, even if you can.

But back with transformers. Single ended trannies diverge from pure-AC transformers because they have to carry a relatively heavy DC current. For mostly non-distorting sound, the DC current has to be such that it sets the core flux about halfway between zero and saturation. There is a long chain of reasoning behind that, but it results in the iron in the core being used for only 1/4 of its possible range of flux excursion. To get better output power for the same given amount of iron and copper, gaps are introduced to help the overall core plus gap not go into gross saturation. As with most transformer design issues, this line of thinking leads rapidly down a path that winds up with a gapped core and lots of copper winding, and about a 2 to 4 times bigger transformer for a similar power through the transformer to the secondary.

The push-pull setup was invented at least partially to better use the transformer iron and copper. with P-P, you can use the whole range of flux in the core before saturation, from nearly saturation positive to nearly saturation negative. That line of thinking leads down the path of one or more pairs of output tubes pulling the core back and forth. Because the core is nearly balanced in terms of resting flux, the transformer gets much smaller for about the same output power, saving a lot of money and weight in iron and copper. This happens because the CT plus P-P setup puts a net current in the primary of ideally zero, but never worse than the imbalance in output tube bias currents. It's generally agreed that balanced idle currents in the output tube pairs is a Good Thing, and that any imbalance ought to be small.

the first P-P amps were biased at what we now call "Class A", meaning each tube was set halfway between zero current and max current at idle. This means that neither tube ever turns off. It was found that if you biased the output tubes at much lower currents, the standing power in the tube was reduced and you could get more power out from the same tubes. This was Class AB, and the sound world now had to cope with crossover distortion. The OT design followed suit and yet a different path resulted in OTs designed for this particular working arrangement.

Hang on, I'm almost there....

With that as background, back to the original question.

Can an 8 ohm center tapped push pull OT be used to drive a single ended 4 ohm load?

The answer is clearly "Yes, but..."

You absolutely CAN do that. Whether you SHOULD do that depends on what you wanted to do, and whether you like what accidentally happened along the way. The "no you can't" answers reflect the limitations you're likely to run into based on the poster's experiences. Those answers are right within the limits of normal guitar amp practices. There are also special circuits designed to make this setup work in some fashion.

You're very likely to find that the mostly-undistorted output power is tiny compared to the nominal rating of the P-P transformer. It will need a LOT of DC current through the primary, and your single ended tube will get pretty hot for the amount of output power you get. DC bias of the output tube will probably be tricky to get the most of the small power available.

In a perversion of Lewis Carroll's exchange between Alice and the Cheshire Cat, which said:
“Would you tell me, please, which way I ought to go from here?”
“That depends a good deal on where you want to get to,” said the Cat.
“I don’t much care where–” said Alice.
“Then it doesn’t matter which way you go,” said the Cat.
“–so long as I get SOMEWHERE,” Alice added as an explanation.
“Oh, you’re sure to do that,” said the Cat, “if you only walk long enough.”

If where you want to get is using a PP OT in a SE circuit and are not fussy about what it sounds like or how much output you get - kewl, what you want IS the road, and you're already there.

[Dave H]

Some time ago I had an Express 25/5 on my bench. Well, it's neither 25 nor 5. The class A power was around 1.5 Watts.

That could be because in SE class A they are using one EL84 to drive one half of the push pull OT primary. Half the turns is a quarter the impedance so the single EL84 is driving something like 2k which is far from optimum.

[Dave H]

In a perversion of Lewis Carroll's exchange between Alice and the Cheshire Cat, which said:
“Would you tell me, please, which way I ought to go from here?”
“That depends a good deal on where you want to get to,” said the Cat.
“I don’t much care where–” said Alice.
“Then it doesn’t matter which way you go,” said the Cat.
“–so long as I get SOMEWHERE,” Alice added as an explanation.
“Oh, you’re sure to do that,” said the Cat, “if you only walk long enough.”

All cats talk like that around here. I live in Northwich Cheshire a few miles south of Lewis Carroll's birthplace of Daresbury.

[Randall]

For the record, I asked this question because I am considering building a Garnett Herzog overdrive unit. it is a tweed champ, basically with a resistive load rather than a speaker. IT runs a single 6V6. I am looking for inexpensive transformers and came across a cheap small push pull one. Just wanted to know if it would work. In this case, it sounds like it might, since distortion wouldn't be a bad thing.

[cjenrick]

how small is the transformer?
your running about 320 VDC B+ if it is the Bachman model.

did some calcs on the subject, computed AC and DC flux for a 6V6GT running at 100 Volts AC-rms using various core sizes.
we used 40 ma DC current.

AC flux formula used was 25,000,000 V/ N A(cm^2) f
and DC flux formula was 0.4Pi N I 10^-4/Lg + Lm/Ur (Lg=.004" natural air gap)

75 EI Lam with 3000 Turns at 20 Hz = 11.5 KG (AC) and 13 KG (DC) = 24.5 KG (12.25 KG at 40 Hz, would be alright)

87 EI Lam with 2500 Turns at 20 Hz = 21.4 KG (would be fine if it uses barn roof lams)

100 EI with 2200 Turns = 18 KG, OK even for GOSS steel.

112 EI 2000 T = 17 KG
125 EI 1800 T = 14.4 KG
150 EI 1600 T = 11.8 KG

a square stack was used for the above calcs, most OPT's have a larger than sq stk so that would bring flux down about 20%

Bottom Line: 75 EI would be the smallest i would use if P-P.
Fender DeLuxe Reverb or Pro Reverb OPT would work but would be a bit mushy.
any 40 Watt Fender or Marshall OPT or bigger would be loafing with that SE 6V6GT.

FTY a 75 EI core measures 1.875" x 2.25"

note that you will have many more Henries than a stock Champ OPT due to the Lap 3 core, so if the DC does not saturate the core you will have killer bottom end over the stock Garnet. (so will work for bass guitar also)

put a speaker jack on that thing.

winding some Tweed Champ OPT's right now.

cj

[Randall]

I am overwhelmed by the physics of this question. This OT I had in question was. On eBay, so is long lost, since I did not buy it. would the champ OT you are building work?