Postioning Transformers - Orientation

[rp]

Plan is for a 5G9 Tremolux. Been sitting on the parts for a few years now. This is going to be a board rather than PTP. I have a small 12 1/2 x 8 chassis to fit it all in to. I'm going to use a Marshall JTM45/50 type layout for it figuring it should give no problems.

Regarding orienting the trannies, I'm pretty sure it doesn't matter, half dozen of one half of an other, but you never know, and maybe someone has tried both and had some trouble with one over the other. Reason I ask is the anti-Marshall way, image 2, is less spread out and makes for shorter wires from the choke to the first cap. I'm a big fan of short wires.

[martin manning]

Why don't you power the PT and check? Separate and tape-off the PT secondary leads, connect the primary, and measure the mV on the OT. Put blocks under both transformers to allow the PT leads to come out the bottom, and get the OT to the correct height. Can you squeeze that little choke in between the rectifier and the PT?

[rp]

hey Martin, so I'd actually read an induced tiny voltage on the OT? I guess that's the same as the headphone trick w/o the phones. I was also worried with interaction inside with wires as much as interaction among the transformers.

I could put the choke inside. In fact, I don't plan on drilling for it till I get the board sized up and the main parts all bolted in and see if there's a good place for it. There should be, maybe on the side wall, but I must also have room for a terminal strip for the multi tap PT

[martin manning]

Yes, you'll be able to read a few mV of induced AC voltage, and get quantitative information as you move the OT around. I kind of like the second pic with the PT core going the long way, more compact as you say, but you could verify that it is as good or better than the other way.

If the choke is mounted outside it doesn't restrict anything much on the inside no matter where you put it. Seems to me that you would want it close to the PT and rectifier for shortest leads (and I'm a fan of that too!).

[The New Steve H]

Do the headphone test. It made a real difference in my last amp. It gave me a PT position which was not what theory would have predicted, so you can't rely on traditional practice.

[rp]

double post

[rp]

Well, I'm amazed. I would have bet $20 there wouldn't be a difference. But the Marshall way, PT bells the facing long way, is better. Oddly OT orientation didn't matter, even w/ cores lined up, as long as it wasn't right up against the PT.

I couldn't get a mV read out (Fluke 79) but headphones worked well. If you try this a good pair of lowZ sealed cans like Sony Pros help. I'm going to to do this for every amp from now on.

Thanks MM and ugly Pakled dude.

[Meat&Beer]

I tried the headphone trick with my new build. The "hum" wasn't loud enough through the phones (Beyer DT-770's) for me to really focus on it, and I thought I could do better if it was more audible, if that makes sense.

So the OT got hooked up to a mic preamp, and I watched the meter in a recording program. Ah, visually listening! Turns out, a tiny little twist dropped the noise down about 5 dB!

If you have, or have access to recording software (Lots of free stuff out there) this is a really cool spin on the headphone trick. Although the hum was essentially inaudible with the phones and hard to discern what movements of the OT were beneficial, having it displayed on a screen for visual reference was the cats bumbum.

This may be splitting hairs to some, but for the inner cork sniffer in you, you know you want to!

In the picture, this is a prototype chassis, no need to bust on me about how ugly it is! Gaffer tape on the xformers is to protect from (further) scratches.

[rp]

Turns out, a tiny little twist dropped the noise down about 5 dB!

Are you going to mount that OT cock-eyed? I couldn't bring myself to do it even if that was the quietest position. Some sort of anal/autistic flaw in me and it would drive nuts.

Next time I'm just going to run the OT into a computer speaker or something amplified to hear it better. Or, take your suggestion of recording software and a mike pre, if I can borrow one.

Anyone know if the way I chose was quieter because the bells were doing some shielding, or because the pt's field was at it's widest along the core side? Is it a rule of thumb to point the bells at the OT?

[martin manning]

In the second pic the bobbin axes are in the same plane, I believe, and that is probably the difference. I'm surprised you couldn't get a mV reading, since I have done it that way many times.

[overtone]

There is usually more magnetic field radiated from the the axis of the core (where RP's blue tape is) and less from the bell side and I also prefer not to have the PT core-axis close or "looking" right at the output pentodes.
Are you going to leave your choke way out there to the left and not between the PT and OT?

[rp]

Are you going to leave your choke way out there to the left and not between the PT and OT?

Don't blame me! That's where Marshall always put it, for decades. I've had trouble with turret board builds long ago and since built PTP with minimal wiring, so I'm uptight about this build. I figure I would just follow a known proven layout, though it is a different circuit. I'm not too concerned about the choke and induced noise outside but induced noise from it's wires inside. I'm prolly gonna put the choke inside near the PT. I've got room.

[The New Steve H]

Are you going to mount that OT cock-eyed? I couldn't bring myself to do it even if that was the quietest position. Some sort of anal/autistic flaw in me and it would drive nuts.

Just think of the way John Gielgud wore his hat in "Arthur." Your amp will have more style.

[wyatt]

Turns out, a tiny little twist dropped the noise down about 5 dB!

Are you going to mount that OT cock-eyed? I couldn't bring myself to do it even if that was the quietest position. Some sort of anal/autistic flaw in me and it would drive nuts.

The cleanest layout is not necessarily the best. Function over form.

[Meat&Beer]

Your amp will have more style.

Haha! Yea, that! I tend to believe it'll sound better than it looks. It's gotta, right?

Fact is, the transfo is mounted, yup, I sure am gonna leave it like that. Another fact is, I'll be completely rebuilding this amp as it's in a donor "proto" chassis while this amp is being developed. Depending on what final chassis I go with, the iron may have more room to play so they might end up square.

I didn't notice much induced hum from the choke to speak of. It fit at a 45, so that's how it got bolted. Everything is different though, layouts and types of irons... It'd be best to tailor an amp to what it wants to be location-wise for the lowest noise floor, and go from there. Of course, there will always be compromise as well.