New build - Torroid layout ok?

[Gainzilla]

Hi all,

I'm starting a new project and due to the small footprint (chassis is 13x7x2), relative complexity, and my choice of a torroidal PT, thought I should get a quick sanity check. Does this layout seem ok? I'm thinking of noise rejection between the two transformers, as well as relays. I tried to lay things out in a fairly logical manner, but LMK what you would do to improve this (or is it great as-is).

Thanks in advance!

Chassis_Layout.png

[pompeiisneaks]

From what I gather reading about Torroids, (no practical experience) they do not have the 'coupling' problem or hum problems (unless poorly made) that the E I type do. Therefore the placement of the OT won't matter as much with respect to the Torroidial PT, OTOH with the OT right over the board, it may need to be a tiny bit more left to avoid any hum coupling into the earlier preamp stages, but it does look like it's more over what should be the PI area and not the preamp.

I've always done fine with even standard PT/OT arrangements like you have so long as I rotated the OT 180 from the PT and have it flat instead of vertical. You don't need to worry about that kind of issue with torroids.

~Phil

[Gainzilla]

Thanks, Phil! One thing I forgot to mention is that while everything in the drawing is to scale, I don't know the exact OT dimensions yet (bought a couple off a friend). I'm looking for 30 or so watts with a pair of EL34's or maybe 6L6's. Assuming the placeholder I added is relatively close, I could rotate it in order to move it further from the preamp and scootch it a bit more to the left. I'm hoping this won't turn out to be a hum generation box. lol

Another thing I'm concerned about are the relays, but I think they'll be ok. I've tried to put them as close to the things they'll be switching as possible. Any thoughts on this? Should I consider using shielded wire?

Cheers!

[lord preset]

I've done two builds with Antek PTs. I think the idea that torroidals don't have hum issues is more theory than reality. Both amps came out fine but in both cases I spent some quality time doing the headphone test to find the best placement. In my AC30 clone build the OT ended up off axis, and in the other ODS-ish build in a small chassis I located the OT on the other end and ran the leads outside the chassis (a la Redplate). OTOH it may be that the headphone test is not a valid predictor of audible hum in the final build but I have never wanted to find out. So in your case I would recommend trying it before you commit on placement.

[pompeiisneaks]

as for the switching of the relays, that's usually pretty easy to handle, if you think it 'may' have hum pickup, just use shielded wires for the runs from and to the relay.

~Phil

[Gainzilla]

Thanks LP, I've been wanting to try the headphone trick for a while. I'm definitely not opposed to a little sideways transformer action either. While I like order/structure, there's something aesthetically pleasing about off-axis. That said, any harm in just moving it all the way to the other end above the preamp? I'd probably twist the wires and run them along the inside edge of the chassis in that case, or under the main board. Thoughts?

Thanks for the relay info, Phil!

[xtian]

it may be that the headphone test is not a valid predictor of audible hum in the final build but I have never wanted to find out.

I can confirm that the headphone trick is valid and useful--the level of hum that is audible in the headphones is a direct indicator of what you'll hear coming out of the speaker when no load is attached (i.e., without power tubes). To look at it another way, your 12" speaker and your headphones are serving exactly the same purpose for this test--the only advantage to the headphones is that they block environmental noise so it's easy to concentrate on the hum.

[Gainzilla]

Definitely going to try the headphone trick this time around. Parts should be here in a couple days. I'll report back with my final positioning.

Cheers!

[bal704]

I've started hooking up to a multimeter (set on AC volts), rather than use actual headphones. That's seemed to work for me so far.

[Gainzilla]

I've started hooking up to a multimeter (set on AC volts), rather than use actual headphones. That's seemed to work for me so far.

Interesting. What are you looking for exactly? Lowest reading VAC?

Cheers!

[martin manning]

Exactly- minimize the milivolts!

[Gainzilla]

Excellent! Thanks! There is a small irrational part of my brain that worries I'll hook the headphones up to wall voltage and Frankenstein myself.

Cheers!

[bal704]

I set it to millivolts AC, and try to get zero. I found it too hard to hear the hum in headphones with all the background noise at my house. I usually experiment and move the OT around just to see what positions will give me a reading on my meter.

[stephenl]

Toroid PT's radiate EM fields, maybe less than EI's but you may still need to adjust your layout.

I think most toroids have a band around the perimeter and a large diameter steel mounting washer/plate on the top and bottom to help - but their is still some leakage in the gaps.

I bought a heavy steel cover from Antek to reduce the radiated noise that was coupling to my reverb tank output transducer (in a recent thread).

If you use a cover, be sure to insulate it from shorting out the transformer. If the cover and mounting hardware make a complete turn around the donut, it's a short...