Transformers covers ?

[godi]

Hi every one, I need some inputs from tube amplifiers gurus.

I made a pair of mono bloc 300B Hifi amplifiers, but as a newbe and inexperimented builder I think I made a big mistake, I trow away all the steel transformers covers and repace them with nice wooden custom made covers, they sure look good, but I also lost the shelding effect of these covers. Both amp are ''buzing'', I think that my wire dressing is fine I follow a proven layout but at this point I think that I only need to put back the transformers shields to make the amps quiter.

I'm thinking to remove all the wooden covers and stick some copper self stick material or alluminum tape under the covers and add some good ground to each of the covers.

My question is, do I need ''stell'' to make a good shield or would the aluminium or copper could do the job of shielding ? This subjet will concern guitar amps and the Hifi amps as well.

The schematic I used is from Angela Instruments and is otherwise known as the J.E. Labs 300B.

[img813]http://www.jeangodbout.com/300b/IMG_5008op.jpg[/img]

Thank for your inputs, Jean

[Structo]

Those really look great!

You can get copper foil shielding tape from Stew Mac that has a conductive adhesive.

I use this to shield guitar cavities.

I like to put a dab of solder between overlaps to ensure conductivity.

http://www.stewmac.com/shop/Electronics ... _Tape.html

If you can find a thin copper tape locally you can solder it together then solder a lead to it and ground that.

I would love to see an inside chassis view.

[davent]

Hello,

In Canada (and the US) you can get copper tape from Lee Valley Tools, sold as Slug-Repellant tape.

http://www.leevalley.com/en/garden/page ... at=2,51555 (Stew Macs shipping/duty to Canada can be a killer.)

As far as adhesive goes i use plain copper foil with 3M's Super 77 spray adhesive and there is no conductivety issue between overlapping sheets of foil although i do add a bit of solder at strategic points as added insurance. So the Lee Valley's tape adhesive may work as well.

In the case of shielding here, is it not an issue of magnetic fields interacting and the need for a shield of a magnetic material (such as the steel endcaps) between the transformers to help curb the demons?

dave

[godi]

Well, seems that my diagnosis is probably the right one. There a lot of work involve in removing the covers (I may have to desolder some transformers to be able to remove the wood covers, so I want to do it right on the first take. I already have in stock some copper self stick tape from Stew Mac and aluminium tape from Home Depot.

As you see on the picture, all transformers are completly isolated from chassis ground with rubber and ther even some clear ''plastic or PVC'' tubes over the screws, right now ther absolutly no ground to the transformers.

Thank you very much for all your inputs, Jean

[Lonely Raven]

That is truly beautiful!

I would also recommend (electrically) insulating the copper foil, just in case.

[FUCHSAUDIO]

How about shielding paint ? Amazing project btw...really nice.

[coolidge]

Isn't steel (ferrous metal) better at shielding transformer magnetic fields than non-ferrous metals like copper?

[Structo]

I was thinking about this.

I don't think the metal covers shield that much.

Think about how we have to orient transformers to each other with the core 90 degrees to each other because of the flux lines that they generate.

In looking at your amp, the output transformers are sitting in the back with the PT at the front left.

I'll bet it's more of a lead dress problem, particularly the heater wires proximity.

Do you have a scope to nail down the source of the hum?

[Jana]

One thing that concerns me is the isolation of the transformers from the chassis. Are the transformer laminations/mounting brackets electrically grounded to the chassis? They should be.

[FUCHSAUDIO]

Unless you actually have a hum-field issue (a tube picking up the transformer etc), you may be fine. Not sure why they use copper straps on transformer windings over steel, but there must be a reason....

[overtone]

Exactly what Jana said was going through my mind too.

There are a number of simple things I would check before going into those log-cabin transformers, for example: did you try a shield over the preamp valve already?

[Cliff Schecht]

Unless you actually have a hum-field issue (a tube picking up the transformer etc), you may be fine. Not sure why they use copper straps on transformer windings over steel, but there must be a reason....

It helps keep flux from leaking from the core, hence the name flux band.

[Structo]

One thing that concerns me is the isolation of the transformers from the chassis. Are the transformer laminations/mounting brackets electrically grounded to the chassis? They should be.

Yes I don't think the transformers need to be isolated with the rubber washers at all.

If you were to do anything like that I would isolate or dampen the preamp tube socket to prevent microphonics.

So I would try to just take out the rubber bumpers for now and see how it behaves.

[azatplayer]

Maybe you could try wrapping some thin metal around the trannies first, place some tape along the bottom edge to not scratch that pretty chassis. I guess make sure its making contact on one side of the trannie too.

Not sure about power transformers, but theres an abundance of OT's and chokes that run fine without bells.

Very beautiful by the way, always taken by what the hifi crowd do.

[rp]

I have to ask how you made the bells or had them made. Hand carved? CNC?