Help with power transformer selection

[jvm]

I have a Noble M925 tube amp with a shorted power tranny. Tranco A 1069. I am trying to find a suitable replacement. Tube compliment is 2 X 12AX7, 2 X 6BQ5, 1 X 6AU6, and a 6CA4 rectifier. No bias tap. Bias supply is derived from one side of PT secondary. Original tranny has two 6.3 volt windings. One for rectifier and the other for the rest of the tube filaments. Schematic shows 345 VDC at rectifier output. Any ideas for a replacement or how to get around the 6.3 volt rectifier winding?

[Chuck H]

Your HV wind will need to be about 275-0-275 (or 550V ct) (around 100mA). You might find a PT with two 6.3V secondaries. Otherwise you'll need to go with a 5V rectifier. It may be possible to use the filament secondary AC for the rectifier and power tubes, then rectify to DC and decouple AC off the preamp filament supply. Or add a small 6.3V tranny. Or, if you can find a PT with two 6.3V taps that's just a little high in voltage you could use zeners to bring it into spec.

[big_teee]

EDCOR Electronics Corporation. XPWR Series Tube Power Transformers
They have quite a few different sizes.
T

[nashvillebill]

Your HV wind will need to be about 275-0-275 (or 550V ct) (around 100mA). You might find a PT with two 6.3V secondaries. ....

Like this one? http://www.antekinc.com/pdf/AN-3T275.pdf (Had me curious)

[Chuck H]

Yup. Except that it's pretty big for a toroidal guitar amp PT. Probably bigger than the EI lam original. WAAAY over rated on all counts. Not that it's a bad thing, except that it'll be harder to retrofit. If it were my project I'd be looking for a PT closer to the original.

EDIT: Like this:http://www.musicalpowersupplies.com/4.html

Model#pt275.2

It's an import thingy but it has the right specs and the right price.

[nashvillebill]

I didn't look at the size or price of the Antek, the link you posted looks a whole lot more reasonable. In fact I bookmarked it, when I get gainfully employed again I need to pick up a couple of those so I can get back to fooling around with some homebrew projects. Thanks!

[Chuck H]

Thank you! I've been jonesing to build with a toroidal PT for awhile. Antek seems to have it going on.

[Prairie Dawg]

Danelectro and Ampeg built amps for Noble so that's the place to start your research. Judging from the 6AU6 it sounds a lot like a Silvertone. The biggest consideration will be current draw. The 6CA4 draws 1 amp, so add that to your total and see what your replacement transformer can deliver. If it's up to the task, then running everything off the 6v winding presents no problem. You can validate this by checking filament voltage with and without the rectifier tube installed. If it stays above 6.3 loaded for half an hour you're in business. Another consideration is real estate. Is it horizontally or vertically mounted?

[Chuck H]

Maybe it would work "technically" speaking. But why do you think most amps that have the amplifier and rectifier tubes using the same filament voltage use different secondary windings to do it... Hmmmmmm?

[52 Bill]

I'm not a designer or builder, but I imagine that there are many reasons for using a single filament supply and/or a separate rectifier filament supply.

I think that Vox Cambridge Reverb amps used a single 6 volt filament supply.

[Prairie Dawg]

I think the reason is that nobody ever offered a widely used 6V filament rectifier that was highly enough rated. The beefiest ones I know about are the 6AX5 and the 6CA4 and there are a couple loktals that are in the same class-7X7 I think. There's no reason not to do it if the demands aren't that great. All the smaller Silvertones used a 6X4 that ran off of the 6v string.

[jvm]

Thanks for all the suggestions, folks. I had just come across the site that Chuck H recommended. Looks like the 275.2 will be perfect. Even the mounting holes are spaced the same. Thanks again, guys for sharing your expertise and time.