Eico HF-87 refurb

[mountainhick]

I am plunging into an amp refurb head first. Ever since I was a young teen I wanted to build a Dynaco, Heathkit or other amp kit, but never got around to it. I got what I think is a good enough deal on this Eico HF-87 and have a copy of the kit assembly manual to work from. I’d like to hear opinions of how deep to go in terms of refurbing/rebuilding this amp.

I am confident that my assembly skills are just fine for the project. My deficiency is in circuitry know how and I am looking forward to learning the basics of tube amp circuitry in the process of this project. This will be learn by doing amp building 101 for me.

The amp works. There is hum at start up, but once warmed up the hum decreases significantly. The trim pots work smoothly and the amp gets very loud, so I am hoping the transformers are good. Apparently the surgistor is still working.

However, the sound is very very flabby, not tight and detailed at all.

The guy I bought it from says he is an amp tech and all he had done was the bare minimum to get sound out of it and try to reduce hum. He tried a few different tubes until he got the least hum, de-oxed the trim pots and re-soldered a few dry joints.

Looking under the hood, the actual build quality does not terrible, but there are still a number of suspicious looking joints. If it’s not really necessary to redo the whole works, I would be happy with a fully working amp with less than pretty build quality. But, I am wondering if I should just go ahead and rebuild pretty much down to new wire. It's not pretty in there but it's not horrible.

Here is a link to download a pdf of the manual complete with schematic and parts list:
Categorized Schematics and Service Manuals for free download


So, opinions please, should I gut the thing entirely and start anew, or work on replacing crucial parts piecemeal?

Also, does anyone know where I can get tubes tested in the Denver/Boulder Colorado area?

[Eli Duttman]

The 1st thing to do is look at the schematic, which I've uploaded. The signal topology is known as a Mullard circuit and we'll get back to that soon enough.

The 1st thing to tackle is the power supply (PSU). It is a matter of routine to replace all electrolytic filter caps. in the PSUs of "vintage" amps. 'Lytics literally dry out with time and flabby bass is the least of the trouble that they can cause. The HF-87 employs a Greinacher ("full wave") doubler PSU. Replace the 300 μF. doubler stack parts with 820 μF./250 WVDC parts from Jim McShane. Jim is a goldmine for parts and advice in restorations of this kind. The OEM diodes are noisy and should be replaced.

On the signal front, the 4X 0.1 μF./600 WVDC waxed paper coupling caps. should be replaced by electrically equivalent 716P series Aluminum foil and polypropylene film Orange Drops.

Contact Jim and you'll get all sorts of good hints for putting another quality vintage unit back to work doing what it should, playing music well.

[mountainhick]

Originally Posted by Eli Duttman

The 1st thing to do is look at the schematic, which I've uploaded. The signal topology is known as a Mullard circuit and we'll get back to that soon enough.

The 1st thing to tackle is the power supply (PSU). It is a matter of routine to replace all electrolytic filter caps. in the PSUs of "vintage" amps. 'Lytics literally dry out with time and flabby bass is the least of the trouble that they can cause. The HF-87 employs a Greinacher ("full wave") doubler PSU. Replace the 300 μF. doubler stack parts with 820 μF./250 WVDC parts from Jim McShane. Jim is a goldmine for parts and advice in restorations of this kind. The OEM diodes are noisy and should be replaced.

On the signal front, the 4X 0.1 μF./600 WVDC waxed paper coupling caps. should be replaced by electrically equivalent 716P series Aluminum foil and polypropylene film Orange Drops.

Contact Jim and you'll get all sorts of good hints for putting another quality vintage unit back to work doing what it should, playing music well.

Thanks,

I just sent an email to Jim.

-Is there any reason to keep any of the old caps?

-Why change the value of the 300 μF to 820 μF?

-What type of diode is not noisy?

[Jim McShane]

Originally Posted by mountainhick

Thanks,

I just sent an email to Jim.

-Is there any reason to keep any of the old caps?

-Why change the value of the 300 μF to 820 μF?

-What type of diode is not noisy?

Hi Ben,

I got your email, I'll reply in detail.

But for the forum let me just say this amp responds very well to increasing the capacitance in the voltage doubler, that's why Eli made the suggestion.

There are some excellent diodes out there that are much less noist then the old types. I use some in my Citation rebuilds that work quite well, install easily and are inexpensive to boot!!

And no, the old caps all gotta go!

I'll get to your email in a few minutes.

[mountainhick]

Originally Posted by Jim McShane

Hi Ben,

I got your email, I'll reply in detail.

But for the forum let me just say this amp responds very well to increasing the capacitance in the voltage doubler, that's why Eli made the suggestion.

There are some excellent diodes out there that are much less noist then the old types. I use some in my Citation rebuilds that work quite well, install easily and are inexpensive to boot!!

And no, the old caps all gotta go!

I'll get to your email in a few minutes.

Excellent, thanks. Now I need to start generating a list of parts to buy. Hereare the capacitors and rectifiers copied from the original parts list with recommendations (in blue) so far::

C1,2 elec 25mfd 6v
C3,4 molded .25mfd 400v
C5,6,7,8 molded .1mfd 600v 10% (Sprague orange drops)
C9, 10 elec 50mfd 50v
C11, 12 disc750 mmfd 1000v 10%
C13, 14 disc 225mmfd 10%
C15 elec 40-20mfd 500v
C16,17 elec, 300mfd 300v (Change to 820 mfd)

C18 molded .03mfd 600v
C19 elec 80mfd 500v

CR1,2 rectifier silicon 750ma 600PIV

[Eli Duttman]

Dude,

There are tweaks beyond improving the power supply. Jim and I will be delighted to discuss those, should you be interested. The fundamentals of the HF-87 are very sound, starting with a nice trafo set. That which is good to begin with can be improved on.

[DrRick]

Has anyone got an extra output transformer for one of these? I bought one missing an OPT about 20 years ago and it has been sitting on the shelf ever since for want of an OPT.

[SY]

Originally Posted by Eli Duttman

Dude,

There are tweaks beyond improving the power supply. Jim and I will be delighted to discuss those, should you be interested. The fundamentals of the HF-87 are very sound, starting with a nice trafo set. That which is good to begin with can be improved on.

I've rebuilt a few of these and have another one sitting in the queue. Although it's a reasonably nice amp and the OPTs are impressively large... they're just not that great. Take a look at the original schematic and see what they did on the output stage to make up for a VERY unbalanced primary. Given the large difference in the DCR from side to side and the large cap across one of the halves, I might guess that the sectioning of the primary was minimal.

If the OP wants to keep close to the original circuit, then I'd suggest:
1. Power supply upgrade- bigger and better caps all the way around, replacement of the old diodes with high speed units (e.g., UF series), regulator for driver stage.
2. CCS to replace 18k tail resistor.
3. Matched plate resistors in phase splitter/driver stage.
4. Conversion to fixed bias- there's a handy-dandy extra winding (50V) on the power transformer for this- presumably, this was a feature in the prototypes, but was dropped for production.
4. Output tubes- EL34s work fine, EL37s work better (but are unobtanium), 7027/super 6BG6/6L6GC also work better (lower distortion, slightly more midrange power).

[mountainhick]

Originally Posted by SY

I've rebuilt a few of these and have another one sitting in the queue. Although it's a reasonably nice amp and the OPTs are impressively large... they're just not that great. Take a look at the original schematic and see what they did on the output stage to make up for a VERY unbalanced primary. Given the large difference in the DCR from side to side and the large cap across one of the halves, I might guess that the sectioning of the primary was minimal.

If the OP wants to keep close to the original circuit, then I'd suggest:
1. Power supply upgrade- bigger and better caps all the way around, replacement of the old diodes with high speed units (e.g., UF series), regulator for driver stage.
2. CCS to replace 18k tail resistor.
3. Matched plate resistors in phase splitter/driver stage.
4. Conversion to fixed bias- there's a handy-dandy extra winding (50V) on the power transformer for this- presumably, this was a feature in the prototypes, but was dropped for production.
4. Output tubes- EL34s work fine, EL37s work better (but are unobtanium), 7027/super 6BG6/6L6GC also work better (lower distortion, slightly more midrange power).

Thanks, I am considering tweaks discussed with Jim, and I think we are already doing several of these. Please let me know your thoughts:

1. Power supply upgrade- bigger and better caps all the way around, replacement of the old diodes with high speed units (e.g., UF series), regulator for driver stage.

Yes
C15 dual section JJ 100/100 500v
C16,17 560/400 clamp mount
C19 dual section JJ 100/100 uf 500 volt cap with both sections in parallel for a total of 200 uf

Also replacing all caps and improving values and types throughout

CR1,2 rectifier: 1000 PIV 3 amp UF5408 Vishay fast diode


2. CCS to replace 18k tail resistor.

I don't know what this means, please elaborate


3. Matched plate resistors in phase splitter/driver stage.

I believe you are referring to matching R28 for V5 and V7 on the other side (V4 and V6) correct? We are also changing value to 250 ohm for modern line voltage

We are also tossing the resistor connected from the EL34 to the 12ax7 heater and rewiring the 12ax7 heater from the regular heater supply. (I really don't get why they did that odd connection originally!)


4. Conversion to fixed bias- there's a handy-dandy extra winding (50V) on the power transformer for this- presumably, this was a feature in the prototypes, but was dropped for production.

Have not considered this yet. Please elaborate what needs to be done here... One practical problem here, Attaching to those leads will be difficult. They are cut quite short.


4. Output tubes- EL34s work fine, EL37s work better (but are unobtanium), 7027/super 6BG6/6L6GC also work better (lower distortion, slightly more midrange power).

I'll go with the EL34s for now. If I feel the need to change after listening for a good while, I'll consider this.


PLEASE PLEASE PLEASE, when referring to part numbers, use this schematic which is from the assembly manual I posted in the OP and matches the parts list I am working from. The part numbers differ from the schematic that Eli posted.

[mountainhick]

Please disregard the CCS issue. I did a little research. At this time I do not wish to deal with a CCS mod... perhaps at a later date.