Isolating FWB (-) from other ground paths with a PT without CT (tube amp)?

[martin manning]

FWIW, there is a small Hammond upright PT sitting here on the bench. It has nylon washers under the bell attachment bolt heads and nuts (8 total), with no star washers.

[R.G.]

I'd like to think I took the thought and work approach, but part of it could easily have been dipping a toe into the paranoia/superstition pool (even if l was the only one at the shallow end)!

With the rise of instant internet experts it's difficult not to get steered off into the weeds. This stuff was around for a long time in the hifi loony followers, but the internet makes it even easier for some people to proclaim that magic, mystical parts and processes are the One True Way.

I got obsessed trying to figure out why some transformers have different numbers of nylon or fiber washers on the bolts that clamp the laminations and end bells (if present). I read one person's opinion it's worth assuring only one bolt makes contact with the transformer and chassis, and another said it might matter occasionally but how can you know?

Then I saw a photo of a transformer on the Hammond website. 3 of the 4 nuts were visible. 2 had insulating washers under the nuts, one had no insulating washer, and the other was obscured. Couldn't see the screw heads, but I got curious and sent an inquiry to them about what it accomplishes. The application engineer who replied said it was a good question and he remembered reading about it many years ago but didn't remember (apparently not critical).
I've read opinions about avoiding shorted turns, but the bolts thru an EI transformer don't pass thru the main flux like a toroid mounting bolt. There is no question what happens when allowing a toroid mounting bolt to clamp an uninsulated steel mounting washer thru the core and chassis.

The bolt insulators are there to keep the bolts and outer laminations from forming a shorted electrical turn that has high currents from magnetic leakage. Long ago when we made trial/test trannies in the lab, we didn't bother with them, but nearly all bolted production EIs have insulators. Depending on how conservatively the windings and core stack was designed, the effect of uninsulated bolts can be trivial or it can cause unwanted core heating. I suspect that Hammond transformers are well enough designed that it's not critical. Insulating the bolts and nuts is never a bad idea, but the app engineer could be forgiven for not knowing.

I have also seen transformers with welded laminations. Those inspire separate folklore that all transformers should be that way (well, at least one former co-worker believed that). That's a head-scratcher. It must be for a specific purpose more functional than saving the cost of bolts or (really?) actually stifling magnetostriction (core vibration). Or I hope so.

That it's done deliberately (if infrequently) makes me assume it is done when a need is recognized...but for what purpose?

It's done for electrical safety reasons. The weld electrically connects all the laminations at one place, not forming a loop. If the transformer is then mounted/tied to the presumably safety grounded chassis, then any primary winding that shorts to the laminations is properly safety grounded. It's not for magnetic field reasons.

There are too many variations in transformer design to generalize, but maybe it alters the leakage flux or how it couples (or doesn't) to the chassis. Maybe similar to flux bands outside a transformer core (but I've only seen non-ferrous types with copper or aluminum tape. Or I'm making things up.

Straps and bands are to act as shorted turns to flux that completely escapes the laminations in the core. This has the effect of forcing the leakage back into the core lamination. It eats some power, hence these straps are often fairly thick on bigger EI lamination-type transformers. For higher frequency ferrite types, it helps with cutting down on EMI radiated as leakage flux.

So I jumped into the deep end. I removed the power transformer nuts and bolts (it was varnished so nothing moved), found heat shrink tubing that fit over the bolts and put them back thru the laminations, bought the last nylon washers my local hardware store had, and put steel toothed lockwashers between the transformer and the stack nut and between that nut & the chassis. I'll never know whether it helped but I hope having multiple bolts wasn't shunting stray flux to make the field bloom away from the chassis instead of coupling. (I'll just have to say "what the flux, man?" if I relocated a problem. I have no options for rotating any magnetics now. I squeezed in a larger output transformer and a choke. The choke can go if it ends up being a problem near the speaker magnet (it fits).

I don't remember how long it took to rearrange the power transformer hardware, but it was definitely tedious and had me questioning what I was thinking. I have had bolts shear off instead of the nuts loosening, with tougher-than-varnish potting material (Midwest EIA 549).

It is a deep end. The effects of core laminations and any mounting hardware are complicated. It really is a radiated and leaked magnetic field going off into free space, with a coupling to what goes on inside the windings. And the relative amount of external effects depends on how conservatively the core stack and windings are. A design that runs the iron at low flux levels doesn't have nearly as much leakage to start with. A design that "economizes" on iron and copper runs the iron at high flux densities, so there is more leakage. Transformer iron is several thousand times more "conductive" to magnetic fields than free space, so it sucks in magnetic fields just like a lower resistor in parallel with a larger one sucks in more of the available current. But this magnetic "conductivity" (aka "permeability") changes with how much magnetic field is in the iron, and drops off as the iron approaches magnetic saturation. Drive the iron close to saturation densities, and the conductivity drops off, so more of the magnetic field leaks off into space. Gaps where the I laminations don't quite touch the E laminations also let flux leak into space, so the degree to which the lamination stack is squared up and tapped into contact when the transformer is being constructed matter. Sometimes a lot. Running the primary at a higher voltage or lower frequency matters, and can make saturation and leakage worse, depending on how conservative the original design was.

Changing something like mounting/stacking bolts can matter a lot, but only if the base iron/copper design, stacking and impregnation have left the bolts carrying significant currents. In more conservative designs, the bolts may not be intercepting enough leakage to matter much. Everything interacts, which is why not many people understand transformers.

[Murrayatuptown]

Thank you, R.G., for the replies.

I have been calling myself a 'transformer junkie' since my first job cutting lawns as a teenager, and my bedroom closet shelf came down with the drywall because I overloaded it with transformers & chokes!

I use industrial work surfaces and concrete blocks now.