toroidal OTs

[gdgross]

Anyone ever wind their own toroidal OTs?

I'm researching and trying to find out if I can fit a stereo power amp into a single rackspace. Something in the 10-15W per side range. I thought about SE for simplicity, but the OT size would probably be prohibitive.

So that probably leaves me with a PP EL84 or 6V6 design. I saw a one 20W OT at piltron that was 50mm high, which is already too tall for a single rackspace. And it was over £100!

Anyone ever wind their own? Anyone know what Mesa uses on their 20/20 poweramp? Is there any hope for this or should I just go for the two-space. (I really would like to keep it in 1, if I can.)

I know i can wind my own toroidal PT with no problem, it just takes a long time and is a pain in the arse.

[bob p]

iirc toroidal OTs aren't very tolerant of imbalance in P-P circuits. at least that's why hammond says that they will never make them.

[Steve Conner]

Yes, they are not very tolerant of DC imbalance compared to regular E-I cored transformers. I think some manufacturers saw a gap in the core to get round that, but others don't bother and expect you to use careful bias adjustment with regular checks. Or an op-amp based servo bias circuit.

[Gregg]

I think some manufacturers saw a gap in the core to get round that

I assume the gap should be very insignificant otherwise it will affect the inductance.

Or an op-amp based servo bias circuit.

Can you point to a schematic for that?
How about using a uC unit to do that. I think R.G. mentioned it in one other thread.

[Gregg]

This is how Marshall 2x20W looks like.

[gdgross]

This is how Marshall 2x20W looks like.

Is that a 1U chassis?

I found a website (tubeampdoctor) that sells the output trannies for that unit for 39 euro apiece.

[Tom Phillips]

This is how Marshall 2x20W looks like.

I see that Marshall used a toroidal PT but conventional EI lamination OTs in the 2X20W.

[Steve Conner]

Yes, the marshall 2x20 thingy is a 1U rack.

Norman Koren's TENA uses servo bias, the schematic should be here.
http://www.normankoren.com/Audio/TENA.html

The lovely Velleman K4040 uses toroidal OPTs, and I always thought it had servo bias, but it actually doesn't. Just a cheesy 10 LED bargraph for setting the bias. That suggests toroidal OTs can handle one LED worth of mismatch ;-) The schematic is in the illustrated assembly manual @
http://www.velleman.be/be/en/product/view/?id=48127

[bob p]

I see that Marshall used a toroidal PT but conventional EI lamination OTs in the 2X20W.

i would think it gets damned hot in that chassis.

[Amp Kat]

Even with the servo which is needed for the OT you may have to turn or adjust the Power tranny position to cancel some hum. You can listen to it while turning and really cut back some unwanted noise especially in that application.

[Guitarist]

Did you ever work this out? Man, it would be great to have a 1U power stereo amp. We should start a project to this end.

[Steve Conner]

The only hard thing about making a 1U power amp is finding small flat transformers and tiny 40mm fans that don't scream. The Marshall 20/20 has a fan and I wouldn't care to try making one of these things without forced air cooling.

Apart from that it's no different to any other power amp. Do 6V6s even fit into a 1U chassis?

[Amp Kat]

I'm sure it could be built or designed by maybe someone like TECA in Spain but the problem is quantity as to build a 1RU tranny at those specs would take special tooling and transformer Co. only care about one thing and that's quantity as in sales. What's wrong with a 2RU ?

[gdgross]

What's wrong with a 2RU ?

Nothing, really. It's just that I can fit my rig in a 4-space rack instead of the 6 space I have now if I can get the poweramp down to a 1U.

I did find those Marshall OTs for sale from tubeampdoctor. Looks like a german company? They would work for PP EL84 or 6V6, I think.

I think 6V6s should just fit in a 1U, you just have to mount them sideways. A 6V6 bottle is only about 1.2" wide according the the GE datasheet, and 8 pin sockets are what, an inch in diameter?

Maybe it will just be cheaper to find a used or broken mesa or Marshall poweramp and fix/modify. Anyone know what they go for?