I thought I had an old Mu metal shield from a tube scope around here somewhere. In this case, as the steel chassis seems to shield it fine from below, a similar chunk of steel from a deceased Eico receiver should do the trick.
would the "exact way" be grounding the shield at both ends?
makes more sense to me.
Creates ground loops. More noise than if you only ground one end, IME.
Regards,
Gordon.
In the original issue, the low-level amplification tubes were shielded, the other three 12AX7s were not. I have five matching Chinese ceramic 9-pin shielded sockets with the spring-insert shields to keep the tubes firmly planted in place. Would there be any benefit to shielding all five 12AX7s? I know the Fisher 400C/CX/CX-2 and the Heathkit SP-2 stuck shields on almost everything, while the Scott 130 put shields on four of seven tubes. Were they going overboard or was Eico being cheap?
Something I thought of as possible source of above-chassis noise is the unshielded transformer. Edcor has some end bells that should fit reasonably well.
Something I thought of as possible source of above-chassis noise is the unshielded transformer. Edcor has some end bells that should fit reasonably well.
Edit...and they want over $15 to ship a pair of little bits of steel that cost less than $4. I asked if they had any other shipping options...nope.
Eico fixed that problem with the ST-84. I don't know why anyone would skip shielding transformers on any reasonably hi-fi piece of gear, though the console manufacturers loved to leave their OPTs open. I found a Chinese place selling on eBay that makes potted-type covers for transformers, but they're all too big. The smallest listed is 70mmx70mmx70mm. I sent an email asking if by any chance they make a 60mmx60mmx50mm version. If they do, all it will need is four little holes for the mounting screws. Easier than drilling the transformer for screw holes. I should look around in my transformer boxes, too. Might be something in there I can scavenge.
Something else I've thought of - the side panels and cage are painted, which means they're insulated from each other and the chassis. I'm going to insert star washers between the chassis and the side panels when painting them, and put star washers under the cage screws when installing it. That way the whole chassis is functioning as a shield from any environmental noise. All that steel is there, might as well use it.
I thought I might have a bad transformer, the heater winding came back with a surprisingly low resistance. Checked the Fisher 400C and Dynaco PAS3 transformers...all really, really low on the heater winding. Guess it's OK.
What do you use to pot the transformer into the copper cap? I might hit the hardware stores tonight and see if anyone has one with a 2.5" inner diameter.
While Mu metal may be the ultimate shield, it seems like pretty much anything will stop stray electrical crud if it has a bit of heft and is grounded properly.