Primary OT lead question

[mikepukmel]

I resumed work on one of my amp projects again, now that I have a little time, and read on one of the amp blogs a comment that didn't sound right.

"Never twist the OT primary leads. It will create capacitance and kill tone". So I started digging through the bazillion photos I have collected of mostly home made but a handful of real JTM45, various Fender, etc., and something I hadn't noticed before.

The secondary, to the speakers, is twisted. But on many amps, wiring from the tubes to the OT (primary) are bundled and tie wrapped.

I found a very neatly built JTM45 clone, that does have both pri and sec pairs (triple) twisted, but the twisting is "light" its not twisted like they used a 1" chuck power drill.

Thanks!
Hope everyone is well.

[Helmholtz]

"Never twist the OT primary leads. It will create capacitance and kill tone".

Absolute nonsense!

I measured the primary self-capacitances of around 40 different OTs. The values range between 100pF and over 1nF. Twisting the primary wires may add something like 5pF. Go figure.

OTOH, twisting provides effective shielding and helps to prevent instability.
Also keep the primary wires as short as possible and route them close to the chassis.

[Chuck H]

Well I guess I'm no "guru" but if I can counter this nonsense by my input at all I'll say ALWAYS TWIST PRIMARY LEADS.

Because I build amps that are basically vintage/hot rod and intended to be cranked for higher gain I use any oscillation abatement method I can that doesn't involve the dreaded "capacitor to ground" that non tech observers hate even more than twisted wires.

Just try and build a Marshall type circuit (vintage or modern) and, "OH GOD" don't do anything that might cause capacitance between antiphase leads or take any measures that might otherwise "dampen the tone"... Now just see how it sounds cranked...

SQUEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAALLLLLLLLLLLLLLLLLLLLLL!!!!!!! !!!!!!!!!!!!!!!!!!!!!!

[mikepukmel]

Cool, thanks, this is great. I need to sneak a capacitance meter into the household budget Thanks for the measurement data Helmholtz. Twisted, short, and close to the chassis, will do!

Chuck, I thought I got a little knowledge here and there, and it all fell apart after reading some odd comments like that (don't twist). I think i need to stick to MEF for info, and try to ignore the odd comments on weaker blogs/groups.

Would be very nice to have no squeal!

[g1]

I think the highly technical phrase 'kill tone' says as much about that persons tech credentials as I need to know.
I'm sure he meant 'kill yer tonez'.

[Enzo]

Avoid thinking there is a "this is how it has to be" way for everything. These are just guitar amps.

There are amps with long transformer wires, and you can twist them or not. There are amps where the wires come through a hole and go opposite directions and no twisting is possible. People will rationalize most any way to do anything. Should I mount the tube sockets on top of the chassis or the underside? Should I use rivets or screws? It clearly doesn't matter a whit, but people will line up on one side or another. Oops, and they will argue whether it matters a whit or not too. They will tell you right and wrong which way to put the toilet paper on the roll. Loose end out or loose end against the wall. ( I don't know what they think in public rest rooms where the roll is parallel to the wall) The internet is full of advice. I been known to give some, as have most of us. Consider it, don't make it a religion. When there are two competing ways to do something, consider maybe it isn't one is wrong and the other right, consider that maaybe both ways work.

And consider that there are often real phenomena of physics, like say "skin effect", but that in our guitar amps, it is irrelevant. If I use a wirewound resistor in my B+, I am aware it has some tiny inductance. But the amount is negligible and isn't a consideration. Or a gain stage in a tube amp: does it really matter if the cutoff frequency is 4.7Hz or 3.9Hz?

[Chuck H]

I don't know the exact quote from Ken Fischer but somehow, somewhere he implied that even wire insulation color could affect the tone. Well... Ken was absolutely a smart guy. But probably more an observant guy. He did a "s" ton of mods and repairs compared to the number of amps he built. He said that many times. He wasn't high tech, but he wasn't low tech either and I'm sure he believed in the unseen forces that hide in a circuit via inductive and capacitive couplings. I've studied his designs ad nauseam and I'm astounded at how well he arrived at his results. A matter of observance and trial and error as it applies to what guitar amps do when you actually play a guitar through them. He gets big props from me. But...

I once had an odd thing happen in an amp that I had to fix. It took me two full days pulling hair to find the culprit was a four and a half inch piece of lead wire. When I removed this wire I was actually able to measure voltage across it!?! It continued to exhibit this effect until I shorted the ends together. Then I put it back in the circuit (oh yes I did) and had the same problem, where a new piece of lead wire didn't. Removing the offending lead I was once again able to measure a small voltage from end to end. My conclusion was that there must be some broken leads and oxidation creating a capacitor within the wire itself. It was amusing. But not so much as it was baffling while I sorted the issue. Ken saw a lot more of this sort of weirdness that I did and so I expect that at some point he determined, due to real life experience that is the only way to observe the out of classroom reality of amplifiers, that some insulation colors seemed to exhibit habitual characteristics. Further, he probably didn't make much of the statement and it was just a glib comment. So do I believe that insulation color can affect tone? Nope. Did Ken? Probably not all that much. But he may have had a mental disposition certain wire colors were to be avoided for certain circuits due to his experiences and openness to the reality that there are unseen (schematically speaking) forces in an amplifier. Where was I going with this??? I can't remember

Don't worry about wire color and don't worry about an extra 5pf to 10pf of capacitance on a circuit that already has ten times that. And don't give in to internet lore. Well, unless you're selling amps to mojo junkies. There are guys that specialize in that and I've had to play into it a little myself on occasion. But here on the forum you're going to get the real scoop every time from many different angles. Best of it's kind on the web.

And if you ever measure voltage across a simple piece of wire don't be surprised or amazed. It's just the normal idiosyncrasies of life messing with your head.

Mic drop.

[Helmholtz]

I need to sneak a capacitance meter into the household budget

To measure self-capacitance of inductors, the meter should allow measurement at 100kHz.

Twisted, short, and close to the chassis, will do!

And don't forget unused primary wires as they carry similarly high signal voltages as the main ones.


Twisting is especially effective with a pair of wires carrying anti-phase signals that are symmetrical wrt ground (heater wires, PI output, PP OT primaries). Twisting reduces radiated electric and magnetic fields. (Strong electric fields are produced by high voltages and magnetic fields are produced by currents.)

The OT CT is an AC ground and doesn't need to be twisted with other wires.