Super Stock Eico HF-85

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.
 


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My HF-85 have evolved into a London double decker. Outer shell later! The whole circuitry is exactly same as everyone's else. The output cap, C35/0.25uf was move inland to between C21 and PC1 #1. Bass and treble pot were ommitted and replaced with calibrated resistors to suit. Bass used 330K+680K and treble used 390K+91K.

Sounds sweet and stupendous!

Finally, my apology for gate-crashing the party!
Can't help it.... too excited!
 
In the original design, 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?
 
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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?

The 81 shielded the phono and the first voltage amp stage. You know this Sam, but for other newer guys reading this, you want to always make sure the shield itself is grounded, either by contact with the tube socket, or I've even seen wire soldered to them.
 
OK...chassis drilled to take the shielded ceramic sockets, and all of the wire pass-through holes have been opened up to 7/16" in order to install rubber grommets. Also drilled one extra hole in the empty area beside the rectifier in order to install a fuse holder inside the chassis. Unfortunately there was no place to put one through the back. Still need to attack the transformer frame with Goo Gone to get rid of the gooped-on Cosmoline that's been there for 50+ years.

The later ST-84 preamp used a "floating potential" heater filament system. I've seen some pages online advocating that system for the HF-85. Discuss.
http://www.audioasylum.com/forums/amp/messages/6247.html
 
Zekk, that's a very different looking way of rebuilding the HF-85, looks cool.
Are you using AC for the tube filaments and are you getting any hum with the way you
laid things out?
 
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.
 
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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.

Well that's silly, I'd think a padded type envelope parcel post would be a few dollars. Maybe check Hoeboyer.:scratch2:
 
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.
 
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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.

Yep, the filament windings use large wire due to current, and they aren't many turns. I've used copper pipe caps for potting enclosures. Maybe something like a cap for 2 or 2.5 inch copper pipe would work. The caps sand and paint easy, or you can leave them shiny copper and just clear coat them. Here are some 300B torrids awaiting micro-crystalline wax.
 
Ha, I mean here they are. I'm not sure how good copper is at shielding, but it has to beat nothing I guess. These are 4" pipe caps.
 

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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.
 
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.

Micro-crystalline wax, it has a nice high melting temp (higher than the old wax they used). I have a bunch of it.
 
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