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EICO HF-81: What if we could....
The Super HF-81
 
By Jean-Francois Lessard & Kelly Holsten



Preliminary doodling of proposed layout
of Super HF-81 sans preamp by Jean-Francois Lessard

Just what if we took the excellent little EICO HF-81 circuit and decided to duplicate all the relevant parts including the all important output transformers, and make a Super HF-81 with exposed tubes on a wood chassis with copper/aluminum top plate? What if you made an HF-81 but sans all the extra switching, tone controls, and other superfluous pieces of the equation that serves to cloud rather than help?  Would it reach yet another plateau?

The thought has been twirling around within my skull for a good five years or so but nothing much has come from it. I even bought a second unit to use the needed parts to get this amp out of the nebulous, head-scratching stage. The second unit is still sitting here, occasionally put to use, yet I don't have the heart nor will to tear into the beast. Obviously, the heart and soul of this project HAS to be the output iron and circuit for the HF-81 including many of the foibles.

But what should be included? What should be removed?  Do you leave the tone controls in?  How does one keep them out without compromising the circuit?  How much switching capability?  Phono?   How about those damn PECs?  




SUPER BASTARD EICO UPDATE 11/2005
Built by Jean-Francois Lessard
and last seen in Zurich, Switzerland..


Lately, I've been debating whether it would be even worth the trouble if you have a stock EICO HF-81, at least after debating with Jean-Francois for far too many emails to admit.  This is almost surely the feeling shared by my compatriot in Quebec as Dr. Jeff finally broke down and built what he thought was going to be a better mousetrap (if really more an EICO HF-86 which is essentially the AMP section of the 81).

See the beauty below now in use in Zurich....


This amp used a volume pot in front of the amp with ONE set of inputs.   It's was the paragon of simplicity, more or less.  Problem is that it didn't match the sonics of even a stock up-to-snuff EICO HF-81, at least not in the all important soul department, something the original HF-81 seems to nail in spades, all the while providing an amazingly transparent look into the performance while avoiding becoming a tool for "audiophiles" to "listen to their systems.".

Ole Mr. Jeff decided to make several changes which could have affected the outcome, more or less:

1. He didn't use EICO output transformers
2. He opted for a CHOKE LOADED circuit
3. He used more PS capacitance
4. He opted to ditch the dual EZ-81 rectifier approach (also in HF86)
5. He opted to ditch the preamp stage of the HF-81 (effectively an HF-86)
6. He didn't use EICO output transformers
7. He was a clown.

Ok.  In all honesty,  I think Jeff has given up making an HF-81 clone and was just going for a nice EL-84 amp.  And that's what he got.  But he is the first to admit, it was no EICO HF-81.

The jury is out on how an exact duplicate of the circuit employing the same output iron might  sound.  So far, no one has mailed me any pics or description of such a unit.  The last correspondence I received from Dr. Lessard is repeated verbatim, below:

Don't F*** with the Eico. Only do the following:

• Remove the mono/stereo switch.
• Bypass the mediocre balance
• Replace the power transformer if it leaked its wax.
• Replace the two main PSU caps
• Possibly replace the Black Cat caps.
   They are probably leaking DC like crazy.
• Replace the electrolytics on the EL84 cathode resistor.
• Replace carbon composition resistors that have drifted out of specs                 (probably 10 to 20% of the resistors values are totally off).
• Replace the 165 ohms cathode resistors on the EL84 to 175-180 ohms
   if you plan to use the new crop of EL84. Don't bother if you use NOS.

Close the POS and forget about the tone control. 
As is, it's better than 90% of the stuff out there.

- Dr. Jeffus

As for the power transformers, they are notoriously problematic as they are being taxed.  Heyboer, a GREAT company to deal with, makes a direct replacement PT that's excellent.  They have also supposedly done copies of the original EICO output transformers but the jury is out on how they compare exactly.

Anything new on this subject will be added here.

- kh



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