Looking for suggestions for 'low end' EICO HF-81 restoration

mhardy6647

Lunatic Member
Howdy:

I have a working but tired EICO HF-81 stereo PP EL84 integrated amp that I'm feeling like (finally) sprucing up. There are several on-line resources for upgrading this amp (e.g., Kelly "mobile homeless" Holsten and Roger Stevens), but they tend to aim a little higher than I would. Attached photo 'borrowed' from Kelly's site: http://home.earthlink.net/~eico_hf81/index.htm

I am thinking of changing the two P/S caps under the deck and the coupling caps. Looking for suggestions beyond that. Stevens' site suggested changing cathode bias resistors to 'lower the temp'. The amp does run dang hot (an EICO tradition, as I recall) so that's tempting. Anyone here bypass or otherwise 'improve' the FOCUS (balance) control? I still don't get how that's supposed to work.

Also, the tubes in this particular amp are a hodgepodge. Since the HF-81 uses a mess of 12AX7's (and some 12AU7's, too, IIRC), I was wondering if anyone had any opinions about any current production 12AX7's that were "OK"... not looking to rival classic tubes, just something that'd work and not be embarassing, or awful, to listen to.

Thanks as always for your attention.
 
You can perform a google search & find many sites regarding modifications. I do not have experience regarding the Eico HF-81, however the amplifier should respond to the following modifications.

First, measure the resistors for proper value as the old carbon resistors can drift way out of specification. Replace as necessary. Either NOS carbon or Vishey metal film are ok. No metal oxide types please. Vishey are ok, but the old carbon types do add a vintage sound that many prefer.

The Eico has a 165 ohm cathode resistor for each channel. Some audiophiles replace the 165 ohm with 180 ohm. Remove the bathtub type 165 ohm resistor in each channel cathode circuit and install a Dale 200 ohm 5 watt resistor. Install a 100uF Blackgate bypass capacitor across that resistor. Next, install a 10K ten-turn resistor in series with a 2K resistor across (parallel) the new cathode resistor in each channel for bias adjustment.

I experimented with EL84 and at 330 volts B+, a bias of -14.0 volts performs well without causing too high a plate current. Your EICO may be higher voltage, hopefully not much higher for best tube life.

Next, remove the coupling caps from amplifier stage to stage & install AuriCap. I tried Sprague 'Orange Drop' 716P, but the caps do have some smearing in the midrange. Some call it a vintage sound, I call it distortion. I have also tried expensive new & vintage oil/foil types, but never liked the sonics. However, circuits respond differently.

Install new preamp tube cathode bypass capacitors where used in the EICO. I would use Blackgate, but a Panasonic low ESR type are ok. Some others advice on the EICO sites may be helpful as well.

I also suggest new power supply capacitors. After all, old electrolytic caps are on the verge of failure and I guess your caps have not been replaced.

Tubes play a large part in the sound. I tried about every tube manufactured as I inventory & sell 1000s of tubes. I like Mullard 12AX7 & Tungsram 12AU7 in my circuits. The Tungsram ECC82 is the only 12AU7 I ever cared for. Problem is every circuit responds differently to a tube from different manufactures.
I do know this, I never did like any new manufacture preamp tubes. They will instantly trash my systems performance.

I tried EL84 (6BQ5) Amperex early '60s, Mullard late '50s 'D' getter & '60s ring type getter, Telefunken, RCA, Sylvania black & grey plate, Raytheon, and others. The tube that outperformed all were GE EL84 oval grey plate with code of 188-5. Usually I have experienced poor performance with GE including most preamp tube, so- go figure.

This is a beginning of the mods for best quality sonics, but you have to start somewhere. I am not sure of the HF-81 audio transformers, but suspect they are good quality.

I also must add, others post many mods & I am sure the changes do increase performance to a great extent. I simply prefer to experiment for best sonics. After all, how does one know what is best?

Be careful, high voltage can quickly stop a heart from beating. If you do not understand & respect high voltage, send the amp out to an expert and work some overtime to pay for their skills.
 
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Eico tricks

Something easy to do on that amp is use the record out jacks to make dedicated power amp only inputs. IIRC the jacks are kinda in the middle on the back, and you have a fairly straight shot to the grid of the power section driver tube.

In other words, if you have a good preamp, you can bypass all the tone controls etc. and go straight to the power amp section. It's easily reversible, non-damaging. And you can pull some unused phono stage tubes to ease the burden on the hot-running power transformer.

I did this for a while with one, and was much happier with it that way.
 
Sounds like a pretty good idea, Mr. Tubino. It does trouble me how hot the power transformer runs. Pretty typical of EICO gear, IME. My venerable HF-52 mono integrated's power transformer (which was huge) was almost too hot to touch when it was up to speed.

Indeed, I remember that one of my father's HF12's (essentially half of an HF-81) had a power transformer burn up one afternoon. No fuse in the HF12!
 
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