Output Transformer Endbells

[Teleman55]

I see some OTs with endbells and some without,what is the difference?

[Tom Phillips]

Personally, I think that eliminating the end bells is a cost cutting measure. The end bells do provide protection to the core against mechanical damage, especially before the transformer is mounted on the chassis. They would also provide some electrostatic shielding but I'm not sure how significant that is for an OT. They would reduce electrostatic radiation that could be picked up by nearby circuits.

[Chuck H]

Certainly endbells will provide SOME electrostatic shielding. But IMHE this is negligable if good layouts are used. Hyboer sent me some OT's with a two hole, no endbell, design. Their engineer asked if I wanted to do this because it would save $$$. I said I wasn't sure, having always used endbell trannies. But they sent me OT's with no endbells and I used them as I was under the gun with a time crunch. I'm using an aluminum chassis (much lower shielding properties than steel) and I haven't had any noise or hum issues. At this point I will continue to order this OT as it was sent to me (with no end bells) because it sounds sooo good the way it is and I have no added noise issues over the original (with endbells) OT I built the prototype with.

JM2C

Chuck

[Steve Conner]

The ones with endbells look neater.

One annoying thing I discovered is that PTs with endbells can buzz louder under heavy load. The leakage flux makes the thin sheet metal vibrate. It's probably not an issue in guitar amps, but in hi-fi work I've ended up stuffing them with foam rubber to try and keep them quiet.

[tubeswell]

I built a stand alone reverb unit with an RT initially that had no endbells and I couldn't figure out where a lot of the hum was coming from on the wet mix side - until I changed the OT to one with endbells, which made it a lot quieter. In hindsight I guess that because it was a reverb transformer (and therefore is bound to be more sensitive), that the potential hum sources from EMF got amplified more. Whereas if it was just a OT, you wouldn't notice as much (and indeed I haven't noticed much hum from OTs on low-powered amps that don't have end-bells). But in my humble experience, endbells definitely can make a difference with an RT.

[Bruce / Mission Amps]

The ones with endbells look neater.

One annoying thing I discovered is that PTs with endbells can buzz louder under heavy load. The leakage flux makes the thin sheet metal vibrate. It's probably not an issue in guitar amps, but in hi-fi work I've ended up stuffing them with foam rubber to try and keep them quiet.

Yes, I've heard that too but I haven't found that problem as much as the actual bobbin vibrating. But it could be that's because 60%-75% of my business is vintage vacuum tube amp refurb and repair....
In a tea cup on my bench, I have a handful of small oak wedges (cut paper thin to about 3/16" thick at the other end and about 3/8" to 1/2" wide). I have used these quite a few times by tapping them into the space between the bobbin and the lamination finger.
I wonder if the foam is also, or actually, dampening that bobbin vibration noise.

[R.G.]

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