Separate heater filament transformer position in relation to PT and OT

[thehoj]

So on this little homebrew amp I'm working on, I decided to go with tubes that use a 12V heater supply. 12SF5GT's in the preamp, and 12V6GT's in the power section.

I have a little 120V->12.6VCT transformer that I need to fit somewhere on top of the chassis. There's plenty of space for it, I just don't know how it should be placed in relation to the PT and OT. I know that the OT should be at a 90degree angle to the PT to reduce interference, but how should the 12V tran be placed? I was thinking that it could be positioned right next to the PT, positioned in the same way (90degrees to the OT). But would the two power transformers interfere with eachother? Or cause any other issues?



Also, on a less important note, if I end up using a choke for this amp as well, how does it's positioning play into this? Should it be placed similarly to the PT's, 90degrees to the OT?

[eschertron]

I have a little 120V->12.6VCT transformer that I need to fit somewhere on top of the chassis. There's plenty of space for it, I just don't know how it should be placed in relation to the PT and OT. I know that the OT should be at a 90degree angle to the PT to reduce interference, but how should the 12V tran be placed? I was thinking that it could be positioned right next to the PT, positioned in the same way (90degrees to the OT). But would the two power transformers interfere with eachother? Or cause any other issues?

Since the PT and the heater PT both run off the 50/60Hz mains, there'd be no 'funny modulation' from PT to the heaters that could affect the amp. If the trannys did inductively couple, then you'd get a little more or a little less than 12.6v out of the heater tranny. I'm not sure that you'd see a difference - especially under load - but if you're concerned, set up the two trannys, apply mains power to both, and see! IMHO don't worry about spacing the heater PT away from the PT. Keeping BOTH away from the OT would be more important. JM2C

[thehoj]

Since the PT and the heater PT both run off the 50/60Hz mains, there'd be no 'funny modulation' from PT to the heaters that could affect the amp. If the trannys did inductively couple, then you'd get a little more or a little less than 12.6v out of the heater tranny. I'm not sure that you'd see a difference - especially under load - but if you're concerned, set up the two trannys, apply mains power to both, and see! IMHO don't worry about spacing the heater PT away from the PT. Keeping BOTH away from the OT would be more important. JM2C

This is what I was thinking.

Now related to this, my OT is a laydown style, while the PT is a standup style. Initially I had trouble figuring out if it was 90 degrees to the PT, but then I realized since it's flat it'll be 90 degrees out from the PT no matter how it's positioned.. I think.. Is that right?

Any thoughts on choke positioning?

[eschertron]

This is what I was thinking.

Now related to this, my OT is a laydown style, while the PT is a standup style. Initially I had trouble figuring out if it was 90 degrees to the PT, but then I realized since it's flat it'll be 90 degrees out from the PT no matter how it's positioned.. I think.. Is that right?

Any thoughts on choke positioning?

The laydown style, commonly called 'Z-plane orientation' I think (??), is terrific for reducing magnetic coupling.

I haven't built (or owned) an amp with a choke yet, so I can't speak to that issue. Without any knowledge to base this on, I'd suggest that you treat to heater PT just like the PT in reference to the choke. Other than that, look at the various layout drawing there are for ideas on how the choke is placed in amps of your size and style.

[eschertron]

I'm not familiar with 12SF5GT tubes. Are they special, rare, or simply available? You are aware that any of the common 12A.7 tubes can be run off a 12v ac supply?

[thehoj]

I'm not familiar with 12SF5GT tubes. Are they special, rare, or simply available? You are aware that any of the common 12A.7 tubes can be run off a 12v ac supply?

It's mostly about availability / cost. Where I might pay $40 or more for a decent NOS 12ax7, I can get a 12sf5gt for $5 or less.
Same thing with 6V6 vs 12V6.`

I initially got playing around with 6SF5's which I really enjoy, They've got the same gain factor as a 12ax7, but they're a single triode. I had a handful of these metal 6SF5's and wanted to get some 6SF5GT's, but they're not as common as I thought they might be.. As it turns out though nice NOS 12SF5GT's are pretty dang cheap and easy to get ahold of. I found a guy that sold me 4 of them for $15. I also liked the idea of being able to get quality NOS 12V6GT's for much cheaper than 6V6GT counterparts. I also got 4 of these for another $15.

I guess I was looking for something a bit different, and I like the idea of being able to get these old unused tubes very cheap.

I put together a little amp (with 6SF5's and 6V6's) based on the Magnatone 213 / Tonemaster 214 (minus vibrato) and it's just awesome. I'm about to put together a final build with these 12V tubes.