Choosing transformers

[thearmofbarlow]

I ran across a schematic for an old Supro steel guitar amp that I'd like to build due to the ridiculous simplicity. You can see the schematic here: http://schematicheaven.com/bargainbin/supro_1946.pdf

The only question I have is... how do you determine what transformers to use when you have little information to go on? I've seen explanations of how to find something that will work when it comes to specific amps, but never a good (meaning understandable by a non-engineer ) generic explanation.

[jimipage]

Determining the filament power is usually my first step. From the schematic you linked to, there is a 6SQ7 and a 6V6 that need to be powered by the 6.3v winding of a PT. Their combined amps are .75A, so you'd need something with at least 1A of 6.3v power. The 5y3 takes 2A of 5v power, which, of course, uses a separate winding. The schematic also calls for the HV power of 270-0-270V (540V, center-tapped). With a 5Y3 you'll end up with 300-310V on the plate of the 6V6. You could also approximate that by using a transformer rated at 250-0-250V (500V, center-tapped) in conjunction with using a 5AR4/GZ34. It will be a little higher than spec (say, 320-330v), but it will be fine. (Even closer would be to use a transformer with 240-0-240V, but that might be harder to find) Also note that the schem specifies the plate voltage and bias voltage (40mA and 11.5v, respectively). A transformer with 50mA should work. Hammond makes a decent and affordable variety of PT's. For this amp I would probably go with the 270BX. It's spec'd at:
Secondary: 275-0-275 V, 50 mA
Filament winding 1: 5 V, 2 A
Filament winding 2: 6.3 V, 2 A

I've found that it's not necessary to be "dead-on" when it comes to figuring out what PT to use. Get some approximate figures according to the tubes that will be used (consult tube data sheets when in doubt), and make sure the transformer has a little more to give than what's needed to ensure cooler and safer operation.
and by all means, don't be afraid to ask questions -- we're all here to learn and help one another.

hope this helps

EDIT: just looked at the schem again -- I meant to say 11.5W dissipation, not 11.5v bias

[thearmofbarlow]

Thanks for the help. How would you go about choosing an output transformer?

In case you can't tell, transformers are a total mystery to me.

*EDIT*
Now that I think about it...
6V6GT in SE operation. 40ma at idle. 11.5W dissipation. This is similar to a Valve Junior. Hammond 125ESE?

[jimipage]

Actually, I think the Hammond 125CSE at 5k is perfect for one 6V6. (that particular OT always seems to get overlooked when people talk about building single-ended circuits. I don't know why -- that and the DSE sound really good) It even works great with two 6V6's in parallel, though it will start to creep into saturation, but some, including myself, like that in certain amps. Keep in mind that Hammond underrates their iron, often by as much as 15%.
I know what you mean about understanding transformers. Took me a while to wrap my head around it all. I personally suggest getting Dan Torres' book "Inside Tube Amps" (it's on sale for $33 at www.tubesandmore.com right now) if you're still in the beginning stages of learning all of this. Some people don't like him, but that book is an excellent tutorial-styled overview an look into how each component and stage of an amp work and work together. Lots of great info -- You will learn a lot. But, like any book, don't take the whole thing as gospel (he has his opinions and such, but at least throughout the book he continually emphasizes that he wants you to learn).