Super Stock Eico HF-85

[Sam Cogley]

Fixed a few errors (including a couple from the original schematic).

The Eico is a bit odd in that every input, both low and high level, has a discrete position on the main selector wafer. Every other tube preamp I've seen with a tape head input feeds the low-level gain stage through a single position on the main wafer. The HF-85 actually feeds the low-level gain stage back through a separate contact strip on the low-level input wafer, then out to the main selector wafer. It's rather less elegant than the way Heathkit and Harmon-Kardon approached the problem, but it makes for a far more versatile unit.

 

[jaymanaa]

I'd do a 47k for sure, with 1% metal films. Some of the old classic carts liked 100k. If you wanted to get fancy you could use a pair of dip switches to combine or separate some tiny loading resistors for a virtually infinite range (within the realm of cart loading that is).

 

[jaymanaa]

Fixed a few errors (including a couple from the original schematic).

The Eico is a bit odd in that every input, both low and high level, has a discrete position on the main selector wafer. Every other tube preamp I've seen with a tape head input feeds the low-level gain stage through a single position on the main wafer. The HF-85 actually feeds the low-level gain stage back through a separate contact strip on the low-level input wafer, then out to the main selector wafer. It's rather less elegant than the way Heathkit and Harmon-Kardon approached the problem, but it makes for a far more versatile unit.

This is what prodded me to move everything under the chassis on the 81. I felt as though I was getting some noise from either all the wiring or having the RIAA components topside. I would definitely suggest using shielded wire, and my best luck was grounding only one end of the shield back close to the RCA inputs. I can't say for sure that it made the stage quieter, because when I decided to change it, I redid the power supply to the phono section, and totally changed the design of the stage to one with a higher parts count and more filtering; 220uf bypassed by film. Don't forget to post up some pics. You've got me thinking about re-re-building mine. It's a sickness, worse than golf, I swear.

 

[Sam Cogley]

Jay, does this look like your power supply? The LED is something I'm going to do to de-stress the PT, even if I keep the heaters AC. One could also switch the HV supply to a bridge rectifier from the old 6X4 to get another heater filament out of the line.

 

[jaymanaa]

Jay, does this look like your power supply? The LED is something I'm going to do to de-stress the PT, even if I keep the heaters AC. One could also switch the HV supply to a bridge diode from the old 6X4 to get another heater filament out of the line.

Close, I'll sketch out my PS and scan it. I worried a bit about the increased current on the fil windings, but knock on wood, I haven't had any issues.

 

[Sam Cogley]

The LED should help with the current draw. Those stupid little bulbs pull a surprising amount of juice.

 

[Sam Cogley]

Thanks for sharing your schematics! They are beautiful. I'm doing a few things to my HF-85 right now as well. One thing I'm curious about is the value of the safety/death cap. Shouldn't it be 0.01uf? I only ask because that was the advice someone gave me when I was working on my AF-4. Also someone had recapped my HF-85 before I bought it and used a 0.1uf Orange Drop there and there is definitely AC on the chassis.

Whoops. You are correct. I'll fix that on the schematics.

 

[GordonW]

'Twere me, I'd be inclined to 86 the tone controls, and convert the last stage to a cathode follower. Buffered output is always the one thing that the HF85 didn't have, that made me think twice about using them...

Regards,
Gordon.

 

[GordonW]

Also, if you keep the heaters AC, you'll want to put a blocking diode in series with the LED. You don't want to feed reverse voltage of more than about 5v into most LEDs. It won't immediately kill them, but it will reduce their lifespan...

Regards,
Gordon.

 

[GordonW]

This is what prodded me to move everything under the chassis on the 81. I felt as though I was getting some noise from either all the wiring or having the RIAA components topside. I would definitely suggest using shielded wire, and my best luck was grounding only one end of the shield back close to the RCA inputs.

I do know this- the last Dyna PAS I redid, I did shielded wiring this exact way. It definitively- without question- made it significantly less prone for cross-talk between the inputs. Worth it, just for that.

Regards,
Gordon.

 

[ferninando]

I do know this- the last Dyna PAS I redid, I did shielded wiring this exact way. It definitively- without question- made it significantly less prone for cross-talk between the inputs. Worth it, just for that.

Regards,
Gordon.

would the "exact way" be grounding the shield at both ends?
makes more sense to me.

 

[earthman]

Grounding both ends changes it from a shield to a shield plus current path, so any difference in potential may result in ground loop noise.

Awesome thread, I recently hauled my 81 out of storage, need to replace the burnt up PT & recap it, but it remains one of my favorite listening experiences and must be revived.

 

[xbart]

Sam, thanks for providing me with your schematics. Your post got me motivated to work on my HF 85. Most of you are way ahead of me in your knowledge of this equipment.

My HF 85 was somewhat torn apart when I found it and part of the circuit had been altered. I originally spent some time, just getting it put back together using the original schematic and getting it working. I'm using your revised schematic now and have gone through and made several of the modifications all ready. So far so good!

I have a question about the filter caps, I originally re-stuffed my can with 22uf, 39uf & 39uf capacitors trying to match the original schematic. If I wanted to just add another cap between the new 47K resistors, would that be ok? Or should the 22uf cap be replaced with a larger value cap. If I can just add one, can a 33uf cap work (that is what I have on hand). Would it matter what order the 22uf and 33uf caps where in?
-Brad

 

[Sam Cogley]

Most of my modifications are based on dcgillespie's notes from various threads. Maybe he can chime in with some ideas. I've actually thought about going with a 22uF in that first position to possibly take a bit of load off of the rather small power transformer. I don't want to send Heyboer $120 for their upgraded part just yet...

 

[xbart]

Most of my modifications are based on dcgillespie's notes from various threads. Maybe he can chime in with some ideas. I've actually thought about going with a 22uF in that first position to possibly take a bit of load off of the rather small power transformer. I don't want to send Heyboer $120 for their upgraded part just yet...

Sam,
After my posting, I came upon this thread where Dave addresses that issue
http://www.diyaudio.com/forums/tubes-valves/96203-hf-85-rebuild-advice-4.html
of not needing a 20uf in the first filter for this preamp. That the 2,2K resistor does the job.
-Brad

 

[jaymanaa]

I do know this- the last Dyna PAS I redid, I did shielded wiring this exact way. It definitively- without question- made it significantly less prone for cross-talk between the inputs. Worth it, just for that.

Regards,
Gordon.

Yep, that's what I found. It can also, in some situations, create an antenna which is not good, but that would be a special case if you lived under power lines or some other weirdness. What I do is leave the shield wires long and tin them but don't hook them up until the pre is powered up and running. Then you can experiment with what works best in your particular situation. Sometimes grounding both ends, sometimes one end (try both ends separately). Whatever is best can then be connected permanently. I normally have at least a dozen jumper wires on stuff when I first power it up, so I can dial in the best locations for grounding. I kind of obsess on hum and buzz, but really, most of it can be dealt with, and sometimes you get the brass ring of dead quiet.:yes:

 

[jaymanaa]

Most of my modifications are based on dcgillespie's notes from various threads. Maybe he can chime in with some ideas. I've actually thought about going with a 22uF in that first position to possibly take a bit of load off of the rather small power transformer. I don't want to send Heyboer $120 for their upgraded part just yet...

The 81 I did up for Jeff already had the Heyboer PT when I got it. FWIW, it runs nice and cool, but I have the outputs biased fairly cool too. I'd rather have expensive output tubes last forever than 3 or 4 more watts. It just doesn't change the actual volume that much. Maybe I shouldn't be referring so much to the 81, but it does use the same circuit as the 85 up front (for all intents). I read some really good articles on this circuit during the course of mine and Jeff's machines. I'll see if I can find some links, and get that PS sketch up here tonight.

 

[Sam Cogley]

Besides using shielded wire (Eico actually did, but it was three wires per shield - not exactly optimal), would building a metal shield over the selector switch help?

 

[Sam Cogley]

'Twere me, I'd be inclined to 86 the tone controls, and convert the last stage to a cathode follower. Buffered output is always the one thing that the HF85 didn't have, that made me think twice about using them...

Regards,
Gordon.

This is a "super stock" HF-85, not a completely different preamp in an Eico case.

Also, if you keep the heaters AC, you'll want to put a blocking diode in series with the LED. You don't want to feed reverse voltage of more than about 5v into most LEDs. It won't immediately kill them, but it will reduce their lifespan...

Regards,
Gordon.

Good info. Thanks! I've also thought about using two LEDs in series/parallel if I keep the heaters AC. I've had good luck doing that with white LEDs for lighting in vintage receivers. Cuts down on any possible flicker quite a bit.

 

[jaymanaa]

Besides using shielded wire (Eico actually did, but it was three wires per shield - not exactly optimal), would building a metal shield over the selector switch help?

I had that very thought Sam. I considered taking an old scope cathode shield because it would likely be mu metal and already have a curve started. Depending on the CT you might find something just right. Then you could bend a tab on each side for a simple mount. At least one of the articles I read blamed some phono noise on the RIAA stuff being mounted on the selector above the chassis.