Audio Asylum Thread Printer Get a view of an entire thread on one page

For Sale Ads FAQ / News / Events

**EICO HF-85 Report**

BACKGROUND

In my previous message:

http://groups.yahoo.com/group/eico/message/351 ,

I reported that it is better to have 'floating potential' (FP) than
not in the specific pre-amp circuit design of EICO HF-85. Instilled
by the circuit of a nicely sounding EICO ST-84, I planned to adopt a
similar 6.3V AC heater supply in my EICO HF-85 pre-amp..

MODIFICATION

Procedures

a) Locate the 100 ohm pot. Loosen the pot from chassis and remove the
earthing ring so that the centre tap of the pot will NOT be grounded
when the pot is put back to chassis

b) Add insultor to centre tap of the pot to ensure that it will not
accidentally touch the chassis. Re-assemble this pot to the chassis
carefully. Make sure that none of the 3 leads of the pot touches the
chassis

c) Remove R59 (220K) from point 'X' (128V)

d) Use a wire to connect point 'X' to Pin 2 of V6

e) Solder a 120K resistor to Pins 2 and 5 of V6

f) Solder a 100K resistor to Pin 5 of V6 and the other end of this
resistor to the Ground (i.e. the -ve of 'can' capacitor)

g) Solder a 10uF/100V electrolytic capacitor, positive lead to Pin 5
of V6, negative lead to V6 socket base (i.e. chassis)

h) Use a wire to connect Pin 5 of V6 to the centre tap of the 100 ohm
pot

i) Check all soldered points and tweaks. An Ohm-meter is very useful

j) After you turn on the amp for a few minutes (when the rectifier
tube 6X4 is working), measure the Pins 4, 5 & 9 of each 12AX7, i.e.
the heater pins. They should have around +58V 'floating potential'.

k) The voltages of various points are shown in the 2 circuit diagrams
(photos). Compare yours with those in the diagrams

Note: Why it is called 'floating potential'? It is because the 6.3V
AC heater supply is never allowed to ground. The photos show Steps
(a) and (b) and the end-product of the modification. Good luck to
you! (DISCLAIMER: I am not responsible for any of your loss! :)

AUDITION

I use the same audition set up as in the previous occasion for ST-84,
i.e. source from CD - Tsai Ching's vocal and piano and Yo-Yo Ma's
cello with John Williams, power amps EICO HF-60 monoblocs.

I had Mr Ma (NOT the Yo-Yo (grin)) together with me to audition my
modified EICO HF-85 and A-B to EICO ST-84 for about 3 hours this
Sunday afternoon. The findings are:

EICO ST-84

We re-affirm the nice sounding of ST-84 - smooth mid to high ranges
with amusing details and warmth - high definition in short. Bass is
tigh and tireless - plentiful of 'air'. Ma added that ST-84 is one
of the most 'tubie' sounding pre-amps. I found its mid range is more
pleasing than many much more expensive vintage pre-amps.

Modified HF-85

Ma had heard of my HF-85 before so he can tell of the difference
after the modification. We both witnessed the 'coarse' or 'driness'
in the mid to high ranges disappeared! Female vocal is sweet,
sweet. What disappeared also is a 'screened' sound stage, which was
distastely present before the modification. Believe it or not - it
could be the FP magic. I am now willing to use my HF-85 regularly,
side by side with my Jadis JP-200 cloned pre-amp.

In short, we both found the modified HF-85 sounded very close to ST-
84, except a little more background noise/hum than ST-84, which is as
quiet as a solid state amp. The congested chassis of HF-85 is
perhaps to blame.

Our members may now wish to try and try. Welcome to report your own
findings.

BTW, would you call it Richard's modification or FP modification to
HF-85? :)

Richard
(http://groups.yahoo.com/group/eico/)

Product Weakness:	'Coarse' sound when stock
Product Strengths:	Inexpensive EICO HF-85 can be easily modified to give nice sound of EICO ST-84, rare and more expensive

---

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

---

You can not post to an archived thread.

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2025, Audio Asylum, all rights reserved ]


---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---