Eico HF-81 rebuild(s)

[Dmannnnn]

I had been waiting what seemed like forever for parts to arrive for the lafayette 224 I was going to tackle as my first restoration. Sure enough an HF-81 caught my eye on the auction site and I bit. Here are the auction pics.

A little rough, but still intact and no major problems. A previous owner drilled a hole in the faceplate for a power switch, Ouch! I will obviously need to keep a switch there, but for what I don't know yet. It appears to have the complete original tube set of eico branded mullards. I powered it up when it arrived and it seemed to be running OK but not sounding great and not too loud. The pots are all filthy and I'm still not sure if any of them can be salvaged.

Anyone who has ever worked on a one hundred+ year old house will know that once you start digging it isn't good, you keep finding more and more problems. Well I installed a new power cord, started to pull old caps and soon realized that this amp needed serious help. It is obviously kit-built and very poorly at that. After pulling bad wire after bad wire and having a second tube socket lug break off, I just pulled it all out, every bit. So it will be a ground up restoration.

Wouldn't you know it, two days later while searching for information on the amp I found one listed across the country on craigslist. I contacted the seller and made a deal. I figured I could do the work simultaneously and possibly end up with a pair of mono HF-81's! Here is a pic of the second unit.

When it arrived I realized the second unit was a very early model (serial no. 2216) and had a couple differences. The mode switch only has three positions (stereo, reverse, and mono) and on the horizontal inputs there is a fourth unused jack for the right channel. Those seem to be the only differences. It was also beautifully assembled. It came with a purchase receipt (April fool's day 1960), a full size schematic, an assembly manual complete with 3 separate pages of addenda, a front panel template for console mounting, and an unused chassis sticker with the input schema. This one should just be a simple cleaning and recap.

Sorry for the longish intro. I will be documenting the rebuild of the first unit in this thread. Any input is appreciated as this is the first time I have done this.

 

[Dmannnnn]

Here is a picture of the stripped chassis. That grime and dust really gets baked on!

I went from scotch-brite to steel-wool to power tools. I finally ended up using a random orbital sander on the chassis and it actually turned out pretty well. I stared with 220 and went to 400 grit. You can see the scuff where my last sheet shredded before I could finish. It actually looks like a very fine orange peel finish and almost looks lacquered. Which brings up this question. Since I sanded it, I assume the chassis will be much more prone to corrosion. Is there something I should use to treat it and prevent rust while not attracting dust? Would it be OK to give the top of the chassis a little bit of lacquer?

You can see in the picture above where I enlarged the capacitor opening to hold the new JJ multi-section can.

Please ignore the dates imprinted on the pictures. Maybe I can figure out how to turn that off, or maybe even set it for the correct date!

 

[gogofast]

WOW!

that's a nice cleaning job. i've always admired people who gut everything out and then rebuild. i never had guts to do that. i've done the hf-81 a few times and they really sound nice with no noise. i'm looking forward to your progress.

thanks for posting.

 

[mhardy6647]

mine is also the early version.
yours is going to end up much prettier

 

[djnagle]

Beautiful job ! I always liked the looks of those bits.

 

[Dmannnnn]

WOW!

that's a nice cleaning job. i've always admired people who gut everything out and then rebuild. i never had guts to do that. i've done the hf-81 a few times and they really sound nice with no noise. i'm looking forward to your progress.

thanks for posting.

I probably wouldn't have gone so far if I didn't have the full assembly manual. The guy who built the nicer unit even scribbled-in the addenda updates to the margins of the manual.

mine is also the early version.
yours is going to end up much prettier

I don't know, yours looks to be in pretty good shape for a 50 year old amp. I've seen examples that get the knobs and faceplate buffed out and they are stunning. These are fairly plain looking amps, but I've always been a fan of understated design. On the other hand, I also like far-out designs that are done well, like djnagle's 'rat-rod channels burning-man' amp (I love those chunky welds, It is a very manly execution :ntwrthy.

I have already stripped the enclosure panels and have a satin brown paint that is a somewhat close match to the original (but a bit darker). I am honestly considering painting the enclosure in a mettalic paint, something that will really set-off the brass faceplate, like an electric-blue or candy apple red. It's iffy since I don't have any professional spray gear or a paint booth, but canned spraypaints have really come a long way.

 

[gogofast]

mine is also the early version.
yours is going to end up much prettier

yours is one of the few i've seen without ugly home drilled on/off switch in the front panel. what were those people thinking? they couldn't put it in the back?

 

[Dmannnnn]

I think I have the color picked out. I first tried a satin finish brown that was just a shade darker than original, but when I tested it on a panel, the result looked like a large piece of milk-chocolate. Appetizing but not very attractive. I seriously stood in the paint aisle at home depot for 15 minutes contemplating the candy-apple red and electric blue. I was sure I could make that amp look like a piece of shiny candy, but it would take some body filler and a lot more elbow grease to go along with an endless search for perfection while shooting the paint. I've been down that road before and I know that it will just cause frustration without a dedicated clean-room to shoot in.

So, I opted for a rustoleum mettalic flake brown. It seems to be a satin finish with shiny bits. I tested it on the face panel and the results were awesome. It provides a little texture to hide imperfections and it really looks nice. I only shot one coat and it should only get better as I do more.

While waiting for the paint to dry, I held my breath and put the power sander to the nicer faceplate. I held my breath for fear of screwing it up, not the fumes. The results were better than expected. It almost looks like new. There are scratches from the 220 grit paper (finest I could find for the sander) but they are so tight and evenly spaced that it looks like a very fine texture on the brass. I think that if I can rig up another sheet of 400 grit for the sander it will be as good as I could ask for. A mirror finish is hard to get and harder to maintain, so I will not even try.

Here is the faceplate.

You can see the paint color on this one. The flash and angle make the sanding marks much more prominent than they really are.

 

[Tristen]

Pretty :3

 

[Dmannnnn]

The input selector and mode switches have had all their components removed and been cleaned and lubricated. Here is a before of the selector.

And an after.

I disassembled the selector and cleaned all the contacts on each wafer. the oxidation was really bad on most of them. Here is a before and after of the wafers. You can see where the contacts keep a clean stripe, but the oxidation was so bad and thick it probably could have caused problems. I used a q-tip with lemon juice and baking soda to polish them.

The preamp section had three low-level inputs: Mic, Phono, and Tape, only the phono will be kept. I'm hoping the construction of the selector switch will allow me to use one set of inputs for different cartridge loadings without having to change jacks.

The next step will be to desolder and clean all the lugs on each tube socket, then I should be able to start the assembly.

 

[BinaryMike]

Impressive work on those switch wafers. Now my Dynaco SCA-35 project is going to take even longer...

 

[Dmannnnn]

Impressive work on those switch wafers. Now my Dynaco SCA-35 project is going to take even longer...

Yeah, but it's not too bad and you might as well do it while you're in there.

Correction: I used vinegar and baking soda, not lemon juice, although either should work well. Make sure to flush away all the excess baking soda to keep the switch from grinding. Afterwards I used a toothbrush and 91% rubbing alcohol to remove any flux residue.

 

[Dmannnnn]

All the tube sockets have been desoldered and cleaned. Again using a toothbrush with alcohol to remove any flux residue. Both the phono preamp tubes and tone tubes are being replaced with new micalex sockets w/shields. The originals did not hold up so well and the shields are completely oxidized.

I also lost one of the black sockets to a broken lug, so I will also replace two more with unshielded micalex. I'm wondering which sockets would be better to replace, the last pair of 12ax7 tubes, or the rectifer tubes?

I now have about 95% of the needed resistors to complete the project. I won a 3lb. grab bag of carbon comp resistors for a pretty good price (There were no other bidders). I can't believe some sellers are getting a dollar a piece for these. Once I learned the color coding, finding the right pieces was pretty easy. I just grab a big handful and put it on a piece of cardboard. l look for the specific two-color band combination and they jump out at you pretty quick. Then it's just a matter of glancing at the third band to see if it is the correct resistor.

There must be 400-500 resistors in there.

95% of them are still within a 10% tolerance and probably 80% are still within 5%. They are marked as 5% resistors.

Question: :scratch2:
Should I use all carbon comp or are there places where a 1% metal film should be used? Here is the schematic.

 

[jwagner]

Wow, impressive restoration. Since you've gone this far, I'd put in all new sockets, 1% metal films throughout, and probably some reasonable polyprop coupling caps. (and of course anything electrolytic gets replaced too). That will of course change the sound of the amp, and a lot of people with HF-81's like the old softer sound better. If you decide not to go all met films, I'd at least put them in the phono section to reduce the noise. I don't remember the grounding scheme on the HF-81, but it might be worth considering running a bus bar instead of grounding through the chassis.
Good luck,
Jim

 

[Dmannnnn]

Thanks, Jim.

The original grounding scheme is to ground it anywhere and everywhere. As soon as I get the chassis put together I will try to work out a new scheme, either a bus or a star-ground. I think the bus might make for a neater wiring job.

As far as resistors go, I'd like to make it quiet and reliable, but I also want to keep some of that vintage character. I think the grid resistors will stay carbon comp but I'm not sure about all the others. I don't know electronics so when I look at the schematic I just see the basic outline - Each tube has a resistor separating it from the power supply and a resistor separating it from ground. I don't know how the type of either of these resistors will affect the amp; which needs to be more accurate or which will affect the sound more.

Any opinions are appreciated. (i.e. use metal film on the cathode and carbon comp on the anode). Anyone reading this, let me know how you would do it.

The phono amp will be open to experimentation. I have Dayton 1% polypro, mallory 150's, orange drops, and Russian k40 and k42's to choose from. I'd like to see what kind of differences I can hear. I will use the 1% metal film's here if I have them.

All the tube sockets and RCA's are on the chassis now. Tomorrow it will be the transformers and terminal strips. Hopefully it will be ready for filament and bus wiring tomorrow night.

 

[BinaryMike]

You will get plenty of vintage sound character from the tubes and transformers. Carbon comp resistors have only one possible place in my equipment: as parasitic oscillation suppressors (stoppers) at tube grids or anodes. That's because they have less inductance relative to carbon and metal film types. In every other respect, they are inferior.

Consider stepping up to 1W or bigger film resistors, even where 1/2W was used in the original design, to maximize the linearity advantage. This applies primarily to resistors that see a lot of voltage from end to end, but it can make a difference in feedback networks, too.

1% tolerance resistors can materially improve stereo channel balance, and that's a worthy goal considering the small additional expense.

High-end metal film types are probably justified at R5 and R6, where they could yield reduced hiss noise. A few manufacturers even used wirewound resistors in this position.

 

[mhardy6647]

FWIW: My vote would be against metal film resistors, at least in the signal path. Modern carbon films sound good and are cheap and reliable. Metal films have their place, but the signal path of an HP-81 (again IMNSHO) isn't it.

Do ditch the ceramic coupling caps, though

 

[BinaryMike]

One more thing... Consider moving the feedback tap from the 16R output terminal to the 8R terminal (or 4R if you will be using 4-ohm speakers). Amplifier performance is better optimized when the feedback tap is connected to the load. You will need to adjust feedback divider resistance to equalize gain and feedback, but that's a simple matter. You *might* find that you like the sound better with reduced feedback, which happens if you make no change in the feedback network.

 

[Redboy]

Do ditch the ceramic coupling caps, though

Even that's contentious for this particular amp. There are some passionate (and vocal) individuals out there who swear that the ceramic caps are part of the magic.

This is a great thread so far! I'm looking forward to future installments, with pictures (I love pictures!).

:thmbsp:
Nate

 

[mhardy6647]

Even that's contentious for this particular amp. There are some passionate (and vocal) individuals out there who swear that the ceramic caps are part of the magic.

This is a great thread so far! I'm looking forward to future installments, with pictures (I love pictures!).

:thmbsp:
Nate

Yeah, I know; I am simply reporting based on my own HF-81 and my own ears.

The current restoration in this thread is gorgeous so far!