Eico HF-81 tube integrated

zhagerott

Member
I purchased this HF-81 a few months back on the bay. The seller had done some work on this one, mainly changing the coupling and power supply caps. He also changed a few out of spec resistors, but left most of the ceramic caps as they tested fine? I am a believer in keeping vintage amps as close to stock as possible, circuit/component wise, to retain their original sound and to keep them operating in their original design paramters. I did replace more caps and resistors, and bypassed the funky focus (balance) control which induces gain and actually raises the volume of the channel it's moved to. I also bypassed the function switch which inverts left/right channel, mono selection etc. I did this mainly to get a bit more detail from the preamp stage as it is a bit veiled. Future changes may include changing resistors in the signal path for better quality mainly to help with transparency. Except for the pair of Eico branded Mullard 12au7's, all tubes tested at 100 on my mercury tester.

Cosmetically, I polished and cleared the brass faceplate, repainted the panels argent silver rather than the original military grey, and painted the cage gold to accent the faceplate. I also repainted the brown knobs a satin black.

The great thing is it sounds even better than it looks. It is a very musical and involving piece of equipment. Big toe tapping factors going on here. Nice bass, never boomy, liquid smooth but detailed mids with nice layering and separation, and nice highs. The biggest suprise was the tube phono stage using the mullard 12ax7 tubes. Very nice!!! Great dynamics, detail, and especially vocal timbre is excellent. Attack and decay with drums is very quick fast and punchy as is the timbre of piano and acoustic guitar. For an all in one unit these are a great value in my opinion.
 
Yup HF-81's can sound very noce and that looks to be a cosmetic gem nice paint job !

I can't figure out your first paragraph though :naughty: You say you don't believe changing the amp from stock unless you absolutely have too and then say your going to change this and that to change the sound LOL !! So it sounds to me like you do believe in changing them from stock. Oh and get those ceramic disc out of there if you want it to be more transparent. Unless the original resistors are way out of spec and have taking on tons of moisture from poor storage conditions the caps will do leaps and bounds more then changing resistors for transparency. Auricaps work incredibly well in the HF-81.

Craig
 
Hi Craig,

Yes they are a great sounding unit. There seems to be a synergy between the amp and preamp sections on these. I tried my homebrew foreplay clone and just used the amp portion of the HF-81 and it sounded nice, and detailed, but it seemed to be missing a bit of the "magic" this unit has when using the amp/pre together.

As far as changing the stock design, I mainly meant if at all possible, not changing values on components unless voltages are out of spec, and trying to keep the same style caps etc. I know alot of people do complete rebuilds changing all resistors, ceramic caps to boutique parts. I'm sure they sound completely different than stock, cleaner and "modern", but I'm not really after that on this one. The changes I'm making hopefully will retain the original tone, and just help the transparency a bit. Actually it does sound just fine the way it is, just very musical and fun to listen to. Fun stuff!:thmbsp:
 
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Hello,
I also have an HF-81 I am working on. As I am new to this I need all the help I can get. How do you go about bypassing the focus and function selector switches? Thanks.
Jim
 
Hi Jim,

On the focus control, most guys just solder a jumper wire across each channel on the focus pot. As far as the selector switch, you can check where the wires for the left and right channel go from the function switch to the selector switch. You can just jump directly from the selector switch to the volume or level pot. This will bypass both the focus and function switch. I had trouble locating replacment pots for the focus and level with shafts that are long enough. The main reason I bypassed mine was the focus pot was bad on mine, and the selector switch was worn. I'm also looking to replace my volume pot as it drifts quite a bit through it's range, but again, can't find a dual gang 250k pot with long enough shafts. Actually with the output of modern cd/dvd players, I wonder if a guy could get by with a 100k audio taper pot? Would be worth a try anyway.........Btw, the shafts on the original pots are much smaller in diameter, but the 1\4" modern shafts could be machined to accept the original eico knobs.

You can also use this as a stand alone amplifier. You can run a seperate preamp into the tape out jacks on the eico. Just be sure to have your selector switch to aux 1 or 2, and the volume all the way down. If the selector is set to mic/tape or the phono stage there may be some increased noise in the background as these are all higher gain. Best of luck on your project, they are a great unit. :)

Zane
 
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