Super Stock Eico HF-85

I had a couple of ideas tonight: either a cut-up aluminum or stainless water bottle or an oil filter can would work well as a shield. I thought I had a cheap, junky aluminum water bottle around here but I can't find it. Holding the transformer up to my good stainless bottles, I can tell that they'd be a perfect fit. Hopefully that missing bottle turns up.
 
Re-purposing things is fun, especially if you get the stuff free. I do my share of dumpster diving around construction sites and office building re-models. :yes:
 
The Rustoleum "Satin Nickel" paint looked good, but didn't adhere to the chassis very well. I was visiting the family this weekend, so I took a few minutes to set up the blasting equipment and took the surfaces that I had painted down to bare metal. Now with the addition of some primer the paint should adhere fine. I'm going to make decals to mark the inputs and other important points on the back, so it will be primer -> satin nickel -> decals -> satin clear (or maybe flat clear). I might mark the tube lineup, but it's pretty simple. :D

Only tagging the inputs and switched/unswitched outlets on the bottom lable was a really odd decision, especially since that big paper label covered almost all of the vents on the bottom cover. Typical of Eico's "almost got it right" track record.

Last week I picked up a number of aluminum and stainless water bottles and coffee mugs from the thrifts for very short change. Found a stainless coffee mug bottom that will be perfect as a transformer shield once trimmed a bit. I'm also going to build a shield extending from the above-chassis input jack plate to the front wall where the controls are mounted. It shouldn't need a top on it if I make sure the sides and cage are solidly grounded to the main chassis - the purpose will be to simply protect the selector switch from any stray EM junk emanating from the tubes or transformer, the grounded cage and side panel should take care of any environmental crud.

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

Original grounded AC heaters vs. "floating potential" AC heaters vs. DC heaters. Any thoughts? Could an LED (or two) still be used with the "floating potential" setup? I did find a nice, super heavy-duty 100ohm pot when stripping a piece of Motorola test gear last night. Much better than the stock pot in the heater system. Hmm...that's something else to label with a decal. :thmbsp:
 
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This project has sort of bubbled back towards the surface. I'm still trying to decide on the heater circuit between the "floating potential" AC heaters as used in the later ST-84 preamp vs. just going with a DC circuit at 6.3V. Would it be possible to DC rectify the heaters and still go with the floating potential system?
 
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This project has sort of bubbled back towards the surface. I'm still trying to decide on the heater circuit between the "floating potential" AC heaters as used in the later ST-84 preamp vs. just going with a DC circuit at 6.3V. Would it be possible to DC rectify the heaters and still go with the floating potential system?

Yep. Just attach the positive side of the heater DC feed (positive lead of the heater filter cap), to the place where you're getting the 'floating' voltage from, instead of connecting the 'floating' voltage through a hum pot (with DC heaters, you won't need a hum pot).

Regards,
Gordon.
 
Gordon - something like this?




Note - main schematics also updated. Found something minor that had no impact on functionality, but bugged me a bit.
 
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One other suggestion- if there's any concern about too much current on the heater winding after going with DC rectification, you could always go to a solid-state rectifier, like I do with my Dynaco retrofits. That would take an amp off of the heater winding. You would probably have to increase that 2.2K first dropping resistor to a higher value, but that would be the only other change required...

Regards,
Gordon.
 
I thought about that, but I've been able to find precious little information on the current draw between AC and DC. I know Jay has DC heaters on his HF-85 and hasn't smoked it yet...though if it did blow, it would be a good excuse to buy Heyboer's upgraded transformer.

Just looked at the Sam's folder for the HF-85. 2.1A of heater current on a 6.3V secondary rated for 2.2A. They really were living on the edge.
 
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Updated the power supply with a bridge rectifier in place of the 6X4, borrowing liberally from a full-wave schematic Jim McShane sent me for modifying the Citation IV HV supply.



I know the 2.2k dropping resistor will have to change, and I haven't calculated the LED resistor yet.

I'll probably cut the rectifier mounting tab off of the chassis and weld a plug in there. No point in having open wounds if they aren't necessary, and it will give me room to move the PT a bit so I have more room to put a shield can over it. I also plan to build a shield which will run across the chassis between the tubes and the selector switch, and attach to the input jack assembly that floats above the chassis. With a shield over the PT, that should effectively block any stray EM garbage from causing noise in the unit.
 
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Sam -- Your schematic drawings look great, but I would invite you to check the diagram that you have used on at least this drawing for a bridge rectifier. The four diodes within each bridge are shown as incorrectly connected for full wave bridge operation. In fact they are configured as a balanced modulator, as you would often see in a MPX adaptor for recovering the 38 kHz sub-carrier information.

For full wave bridge rectifier operation, two diodes should connect to the AC power with their cathodes (straight line) joined to represent the + output of the bridge, and then two more diodes also connect to the AC power, with their anodes (back side of the triangle) joined to represent the - output of the bridge.

Dave
 
Crap, you're right. I honestly don't remember where I got that from, it was copied from another (and obviously incorrect) schematic.

Edit: found it - I copied it from the Sams schematic for the H/K Citation IV. The fact that I didn't even notice the error for a couple of years is disturbing. Just fixed it on these files and the original H/K files I stole it from. The really annoying thing is that I'm pretty sure Jim McShane warned me about that schematic error when I was rebuilding the CIV several years back.

Updated the power supply schematics throughout the thread with the proper depiction of a bridge rectifier.
 
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Sam, you really only need two diodes, in place of the 6X4, for B+. Cathodes common connected to the 2.2K resistor, and one anode per transformer plate lead. Center tap of transformer remains grounded.

In essence, just delete the two diodes that are directly grounded (BTW, if you fire it up the way you have it shown, the two diodes to ground will produce a direct short to ground through the center tap!).

Regards,
Gordon.
 
I feel a *facepalm* coming on for not noticing the dead short into the transformer there...

Anyway, schematic updated with two solid state diodes replacing the two diodes contained in the original rectifier.

Edit: re-did the 6.3V heater supply with the upgraded filtering capacitance discussed in the 400C power supply redesign thread. With the rectifier tab gone and the chassis welded shut, there should be plenty of room for the caps.
 
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all I know is that I replaced the stock RIAA parts on the input sw with new parts mounted on a small perf bd right at each phono tube. Wether it improved anything or not I'm not sure.
All I know is a friend says it sounds fantastic and better that his marantz 7. by far.
 
Since I can't edit the original post anymore, here are the three versions of the schematic, with a slight correction to the phono loading switch, thanks to dcgillespie.





 
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