Many have wondered how the EICO would sound without the tone control compromise in the circuit. Not only does it add the power-on capability, it also adds a host of pots and connections, including the passive PEC units, the first implementation of the IC. This mod below, outlined by Jean-Francois Lessard, does bypass ONE HALF of the 12AU7 stage but the tone controls are out of circuit. This is the only way to keep the 12AU7 pre in the circuit, although there is no real way to bypass the controls and keep the half of the 12AU7. You need to be the judge whether you think the mod beneficial. Surprisingly enough, Jeff said the tone controls in the circuit, even at flat, helped with more mediore recordings; in other words, the damn thing might just sound better with the tone circuit left intact. - kh

Note: All photos at right click to FULL SIZE 1000px - they also dont really correspond with the TEXT.  Jeff  first bypassed the entire 12AU7 but later nixed this and kept on half.   Sadly, the pics are for the total bypass keeping only the 12AX7.  Use the TEXT BELOW if any at all. See ADDENDUM..



MOD DIRECTIONS

Dont undertake ANY mods unless you know what you are doing. You are dealing with HIGH VOLTAGE. OF course, if you go, it just proves that Natural Selection is alive and well.

Unplug the Eico. Wait 24 hours for the caps to discharge

1. For each channel, unsolder the wire (at the tube socket) that goes to pin 7 of the 12AU7 to the center lug of the volume pots (the wiper lug). The wires are probably green if the color is the same than on mine.

2. Put a heat shrinkable tubing on the unsoldered part of the wire. The volume pot to the grid of the first section of the 12AU7 is now removed.

3. For each channel, unsolder the wires (at the tube sockets) that goes from one lug of the the treble pot to the pin 2 of the 12AU7 (if it is like mine, one side is a green wire and one side is a grey wire).

4. You see those wires coming out from a hole in the bottom chassis near the tone control pots. Put a heat shrinkable tubing on the unsoldered part of the wire

5. For each channel, unsolder the coax wire that goes from a 1000 ohm resistor near both bass and treble control pot to pin 7 of the 12AX7 (it should be black coax wires comming out from the same two holes of the previous wires). Put a heat shrinkable tubing on the unsoldered part of the wires. The unsoldering is now done.

6. Now for each solder a piece of wire from the center lug of the volume pot to pin 2 of the 12AU7.


ADDENDUM:  I dont have a single piece of gear with tone controls.  Indeed, all my preamps now only have attenuation and switching.  I dont even use balance anymore.  Except my vintage....it's tone control fever there.  Seems the vintage gear is more the sum of its parts, especially the EICO HF-81.  Jeff has his tone controls back in as well - kh


return to EICO HF-81 Mods Page

If you have any questions or comments concerning the EICO-HF-81, please drop me an email:

eico@i-vol.com