Well Anatech has reminded me that my original 1959 vintage Eico HF-85 preamp is running on borrowed time. I'm wondering which components I should be replacing and with what? I have heard that component selection is critical on this pre-amp to maintain the original sound..
I have also heard that the original carbon resistors are critical to the sound.. so if that is true (is it?) can I leave those? Should I measure them and see if they are in tolerance?
Which cap types can stay?
As you can tell I am a beginner.. so please try and respond so a beginner can understand.
The current condition of the unit is a very small amount of hum and amout 50mv of DC if I remember right.. it sounds great but lacks in detail a little..
thanks in advance for any advice you can offer.
I have also heard that the original carbon resistors are critical to the sound.. so if that is true (is it?) can I leave those? Should I measure them and see if they are in tolerance?
Which cap types can stay?
As you can tell I am a beginner.. so please try and respond so a beginner can understand.
The current condition of the unit is a very small amount of hum and amout 50mv of DC if I remember right.. it sounds great but lacks in detail a little..
thanks in advance for any advice you can offer.
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I have 2 of these. One thing I recommend to do asap is to replace the power cord with a 3 prong cord. Both zapped me ocasionally 
Also, try using the tape out as pre out, this has less gain but more natural and less distorted sound. In my experience, the coupling caps changed the sound more than the resistors. If you want to maintain the "original" sound, then try retaining as much original components as possible; however, most of the original components are likely drift off the tolerance .....
Also, try using the tape out as pre out, this has less gain but more natural and less distorted sound. In my experience, the coupling caps changed the sound more than the resistors. If you want to maintain the "original" sound, then try retaining as much original components as possible; however, most of the original components are likely drift off the tolerance .....
Yes, strip & rebuild is good advice for something that old. When I got an old 1948 Radio Craftsmen 6V6GT push pull amp last month off Ebay for $115, I had to order the Sams Photofact schematic off the web, and found out that someone had also completely rewired the thing as well and made a few mistakes, probably failed and gave up, because the one metal tube's filament power was not hooked up so that it would light up, and you can't see that by looking. The OPT was new (an old 60's Triad universal), but the power trans and all the tubes were original.
I completely gutted & rewired the underside only, bypassed the 2 metal cans with new electrolytics, and the whole job only took me the better part of a Saturday morning and afternoon. Some of the resistors were original, but on measuring them, several were off by more than 20%, so that alone merited a complete rewiring, besides the caps & messed up circuit. Works fine now with the same original tubes.
I completely gutted & rewired the underside only, bypassed the 2 metal cans with new electrolytics, and the whole job only took me the better part of a Saturday morning and afternoon. Some of the resistors were original, but on measuring them, several were off by more than 20%, so that alone merited a complete rewiring, besides the caps & messed up circuit. Works fine now with the same original tubes.
back in the day - the problem I had with my HF-85 was that volume pot mistracking Left/Right and having severe balance problems. If that's in good working order - I would 'blueprint' the unit if you want to keep the vintage tone.
It's possible to get new carbon comp resistors (or use Kiwame carbon films). Use metal films in the power supply section. Use new oil capacitors (russians or other) to keep that pyramid 'tone'. I would ditch the ceramic caps for something a little more modern though JJ sells a multi-section electrolytic that could be used to replace the original.
The other option would be all metal film resistors and modern film capacitors.
I really liked the sound of my HF-85, especially using the tape-outs. I thought it sounded better than the PAS - but it's been awhile since I've compared the two.
It's possible to get new carbon comp resistors (or use Kiwame carbon films). Use metal films in the power supply section. Use new oil capacitors (russians or other) to keep that pyramid 'tone'. I would ditch the ceramic caps for something a little more modern though
JJ sells a multi-section electrolytic that could be used to replace the original.The other option would be all metal film resistors and modern film capacitors.
I really liked the sound of my HF-85, especially using the tape-outs. I thought it sounded better than the PAS - but it's been awhile since I've compared the two.
kstagger said:back in the day - the problem I had with my HF-85 was that volume pot mistracking Left/Right and having severe balance problems. If that's in good working order - I would 'blueprint' the unit if you want to keep the vintage tone.
It's possible to get new carbon comp resistors (or use Kiwame carbon films). Use metal films in the power supply section. Use new oil capacitors (russians or other) to keep that pyramid 'tone'. I would ditch the ceramic caps for something a little more modern though
The other option would be all metal film resistors and modern film capacitors.
I really liked the sound of my HF-85, especially using the tape-outs. I thought it sounded better than the PAS - but it's been awhile since I've compared the two.
WHich ones are the ceramic? And by oil caps does that mean electrolytic? Sorry warned you I was a beginner.
Those flat ones are ceramic. Oil referred to Paper In Oil (PIO) caps, they are the coupling caps - those 4 black pyramid and 4 paper caps.
Another mod that I recommend to do - the loudness control. In the original form, it can max out most of the amp's with less than 1/4 turn!!! Rewire the loudness will allow you to turn the volumne control higher which provides more balance tracking.
Another mod that I recommend to do - the loudness control. In the original form, it can max out most of the amp's with less than 1/4 turn!!! Rewire the loudness will allow you to turn the volumne control higher which provides more balance tracking.
Here's a well done site where a guy details his rebuild of an HF-81. Not an HF-85, but the parts he replaced should still apply.
http://home.earthlink.net/~eico_hf81/
Hope you find it useful.
http://home.earthlink.net/~eico_hf81/
Hope you find it useful.
BHD said:Here's a well done site where a guy details his rebuild of an HF-81. Not an HF-85, but the parts he replaced should still apply.
http://home.earthlink.net/~eico_hf81/
Hope you find it useful.
This page you sent answers all my questions and MORE! THANKS!!
Hi Joe,
I rebuilt an HF-81 for another person. It sounded wonderful once done.
Some advice. Do not buy paper in oil caps or carbon composition resistors. They are not reliable and are rather over priced. Just use a decent axial plastic type (poly) capacitor and metal oxide resistors. The only "vintage" sound carbon comps will give you is a hiss and possibly light popping in the background.
Take picture and replace both resistors and caps at the same time. If you get rid of the ceramic caps at the same time, you will improve the sound substantially. Try to place the new components in the same physical positions as the old ones.
-Chris
I rebuilt an HF-81 for another person. It sounded wonderful once done.
Some advice. Do not buy paper in oil caps or carbon composition resistors. They are not reliable and are rather over priced. Just use a decent axial plastic type (poly) capacitor and metal oxide resistors. The only "vintage" sound carbon comps will give you is a hiss and possibly light popping in the background.
Take picture and replace both resistors and caps at the same time. If you get rid of the ceramic caps at the same time, you will improve the sound substantially. Try to place the new components in the same physical positions as the old ones.
-Chris
anatech said:Hi Joe,
I just want you to not pay too much, but end up with what you want.
The "magic wire" guys just hate advice like that!
-Chris
I hear ya brother..
If I can get the definition that my SS preamp has back into the hf-85 it will once again be my primary preamp.. it desperately needs new jacks too.. that alone could make a difference as they dont clamp well to the center conductor and they seem sorta corroded.. I have no idea what material was used on those but it seems to be aluminum.. thanks again.
Hi Joe,
They could use spacing out also. I think they were cadmium plated or some such. Nothing wrong with that, it's the 40 years.
I would suggest new jacks, not gold plated. Lose some jacks you don't need so you can get a reasonable space between them.
Another trick I pulled on mine was to remove the hum balance pot and raise the heater line about +30 VDC above ground. I replaced the last resistor in the B+ string with two that gave me a 30 V potential. A 10 uF cap from there to ground completed the mod. Hum dropped to a very low level by doing this.
-Chris
They could use spacing out also. I think they were cadmium plated or some such. Nothing wrong with that, it's the 40 years.
I would suggest new jacks, not gold plated. Lose some jacks you don't need so you can get a reasonable space between them.
Another trick I pulled on mine was to remove the hum balance pot and raise the heater line about +30 VDC above ground. I replaced the last resistor in the B+ string with two that gave me a 30 V potential. A 10 uF cap from there to ground completed the mod. Hum dropped to a very low level by doing this.
-Chris
barchetta said:
Prediction: It will come out sounding the same as it is now. But I guess it will just not burn my house down.
Probably not. Replacing ancient dried up electrolytic PS caps with good cheap sprague atom electrolytics, ceramic coupling caps with decent ones like 716P orange drops and replacing old carbon resistors with values that have drifted all over the place with new high tolerance ones will probably make a HUGE difference in the sound.
You'll see.
BHD said:
Probably not. Replacing ancient dried up electrolytic PS caps with good cheap sprague atom electrolytics, ceramic coupling caps with decent ones like 716P orange drops and replacing old carbon resistors with values that have drifted all over the place with new high tolerance ones will probably make a HUGE difference in the sound.
You'll see.
Hey I have a question about this.. when we talk about resistors that are out of spec, can I determine if they are in spec by simply measuring them with no power applied? Or can they change while under power?
I guess what I am getting at is there are lots of them and I could easily measure then all quickly rather than having to replace them all.. I know many of you would say to me just replace them all.. but call me lazy..
I did order some of the main caps last night.. have to hunker down and order all the disc type.. its exhausting looking all these up and making sure they will fit.
BTW- I have VERY little hum in my old preamp as it sits.. totally original.
Hi BHD,
716P type capacitors are the wrong ones. You can not install them in the same place the originals went because they are radial caps, you need axial types.
Hi tolerance resistors will not help you. Those circuits are designed with 20% parts in mind. Certain resistors should be matched together and not to a value.
Hi Joe,
I normally replace all resistors that have been overheated or changed colour not matter how they read. That may be most of them in some sets. No tolerance band gives you a 20 % tolerance, silver is 10 % and gold is 5 %. It would be very odd to see an original 5 % resistor in there. That was a precision resistor back then. If you have a 20 % resistor reading over 10 % off value, you may want to change it anyway. It's already heading off somewhere.
-Chris
Change all the coupling caps, not just ceramic ones. Actually, ceramic coupling caps are normally safer that the tubular ones. They typically do not get leaky. They just sound awful.
716P type capacitors are the wrong ones. You can not install them in the same place the originals went because they are radial caps, you need axial types.
Hi tolerance resistors will not help you. Those circuits are designed with 20% parts in mind. Certain resistors should be matched together and not to a value.
Hi Joe,
Normally you can just measure them. Leaky caps will throw your reading out, and power supply caps may need to charge to give you an accurate reading. You will see the reading changing with time. There are times when you will have to lift one lead to be sure. Some defective resistors will change their value as they warm up under load.
I normally replace all resistors that have been overheated or changed colour not matter how they read. That may be most of them in some sets. No tolerance band gives you a 20 % tolerance, silver is 10 % and gold is 5 %. It would be very odd to see an original 5 % resistor in there. That was a precision resistor back then. If you have a 20 % resistor reading over 10 % off value, you may want to change it anyway. It's already heading off somewhere.
-Chris
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