Twisted PT pairs

[Tony Bones]

Proximity to other conductors is important, but the thing to keep in mind with heater wires is to keep them close together. I mean the the two heater wires themselves. As R. G. said in post #2, the main reason to twist 60Hz wires is to keep them close together.

Think about it: any current going 'up' one wire is going 'down' the other. It's always going in opposite directions. When you separate the wires - anywhere along their path - it leaves what looks like a loop of current, just like what's inside a transformer. That open loop transmits a magnetic field, the amplitude of which is proportional to both the current and the size of the loop (a.k.a. loop area.) Transformers have many more loops, and usually larger loops (unless you really screw up your heater wiring! ) but they also have an iron core that captures most of the magnetic field and keeps it confined. Without the iron core, loops in heater wiring radiate into the air and any wire that happens to be nearby gets to act like the secondary of the air-cored transformer.

So, yeah, keep your heater wires separated from signal wires, but also try to keep them as close together as reasonable to minimize loop area. That's not easy to do at tube sockets where it's necessary to separate them, but be conscious of the loop. Makes me wonder why they put the heater pins so far apart on most tubes.

[pjd3]

Thanks Tony Bones.

Yip, I definately get the theory of magnetic fields sort of cancelling from having a twisted pair happening. But I'll certainly hear out anyone that has worked it from a practical standpoint that has made it really work for them. It seems like there is always some practical subtely of technique that optimizes the theory and gets you good results. Two things I have going for me is that the amp is to serve as a clean pedal platform at low to medium volumes. I don't think I'll ever find the amp in a state of high gain or volume. And if my best efforts still end in some noise or buzz that I wasn't able to recfity myself, its going to Stan Day in Wilmington for a health check. He's decades into vintage design of guitar tube amps and what can go wrong with them. In the end I should be covered one way or another. But thats not going to stop me from picking up any and all wisdom and knowlege from those that know better and then try to apply to the best of my ability. Thats the challenge.

This week I order an Allen TO35MT output transformer and their choke for big Fenders. 3H 100ohms I believe. Then I will do the OT headphone trick to find the optimum location for the OT. Thats sould like a bit of fun, Looking forward to that part of it.

And havent yet totally committed the fully loaded eyelet board to a position in chassis as of yet. Wanted to find the OT home first, then the eyelet board and choke.
The PT is in and wired up to the Rect tube, power, standby and fuse, and many of the preamp wires are soldered in with an extra inch or two. A few more pieces of hardware and the chassis will be close to good to go. Go where, that is the question.

Thanks alot everyone for taking time. Means much!

Phil

[xtian]

But I'll certainly hear out anyone that has worked it from a practical standpoint

As an experiment, I modified an otherwise healthy #183 by adding a 10" extra length of heater wire on V1. And then, with amp running and volume up, I moved the long heater wire around, trying to get some hum to appear. I could not!

Short story, don't stress about heater wiring. Worry about your grounding scheme!

[dorrisant]

In my experience, twisting the wires doesn't seem to be what is responsible for the reduction of hum... Rather, it is the proximity of the two heater wires in physical relation to each other. I would think that 1 twist or 20 doesn't make much difference. It does however, hold the wires in close enough proximity along the runs of filament lead to reduce the EMF cross-sectional area by cancelation.

I have seen high-end amps (feel free to suggest models) with solid core wire, layed out so that they are right next to each other. Absolutely no discernible hum at all. I can think of a quite a few PCB based amps that use board traces to route the filament supply that are obviously not twisted and they sound dead quiet when all else is correct.

Also, let's not forget the lazy loops you see in the Express amps. If ever there was a circuit that would punish the builder for anything incorrect... Sidepoint, Ken would never do it if it wasn't going to give the best results.

I am willing to take it on the chin for the above statements... If I'm wrong, then I will learn something new. That is fine by me.

I totally agree with all of the others that indicate that the grounding scheme has many more pitfalls than filament wiring.

[pjd3]

Hey thanks, guys, I wont sweat it then, just keep them in proximity and probably away from preamp signal wiring. that shouldn't be too much to ask for.

And yup, I'm sure to be back for some advice and direction come the grounding of it all. I've been made aware of that and will heed good advice. The fella who designed out the single channel amp I'm building does offer some specific guidlines on the grounding of the amp, what should be wired to power ground, what should be wired to preamp ground. When its time to commit to grounding I'll throw it out there what I think I know and see what you guys think.

Thanks again!

Phil