From: paul@gaitlab1.uwaterloo.ca (paul j guy)
Newsgroups: rec.audio.high-end
Subject: Re: EICO HF20 amps, good iron.
Date: Tue, 22 Dec 1992 16:00:06 GMT
Organization: University of Waterloo, Waterloo, Ontario, Canada
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu

In article <1h7761INNntk@uwm.edu> markt@cup.hp.com (Mark Thompson) writes:
>I aquired a pair of EICO HF20s this weekend for a price that i couldn't pass
>up, $5 each.  I seem to recall that these had somewhat good iron.  The 
>amp looks somewhat like the Heath W5M.  There are many similarities between 
>the amps internally as well.  They both use a "Williamson" driver stage, and 
>they both use 6L6GCs for outputs.  The HF20 has a preamp integrated though.  
>This is nice since that gives more headroom in the power supply, not much, 
>if one doesn't use the preamp.
>What i'm wondering is what sound might i expect to get from these transformers
>if i upgraded the dirver stage?
>
   I have played around with one of these, the preamp is not very good. I
bypassed the preamp stage. If you use a lower resistance pot, (I remember
the original one is about 500kohm) you won't get the high frequency
droop from the series resistance. With a 50kohm pot, and bypassed preamp
you will get a very reasonable square wave response. If my memory
serves me well, I got up to 40-50kHz at -3db from full power, which is
quite good for a tube amp.
   Make a point of changing the resistors on the phase splitter stage. The
originals are very noisy, and not well matched. Be sure you use 2 watt or
higher, don't use wire-wound or composition. On the other hand you shouldn't
need to go to bulk foil types ($$$$$). It's not a bad idea to replace 
the unbypassed carbon composition types, that are carrying any significant
current.
   If possible, you should regulate the voltage to the preceding stage
to the phase splitter (all part of the same tube), using a shunt zener
or VR-tube. The reason for this, is to eliminate low frequency feedback
that is coupled from the output through the power supply.
  I made a fixed bias supply from the filament supply to bias the output
tubes (with some bias developed across a SMALLER cathode resistor). This
effectively gave a higher power supply.
  Put at least 100 mfd across the plate supply for the output tubes,
and maybe put a 5-10 mfd polypropylene in parallel to make you feel
good. 
  Replace all the cathode resistor bypass caps with decent modern versions.
  If you want to get real nutty, take the Williamson transformer screen
taps off the screens, and connect the screens to the plates for TRIODE
operation. (tape over the transformer leads). I found the power was not
significantly reduced even though the theory says you should drop to less
than 1/2 power. Since the screens are running almost the same DC potential,
the circuit won't need rebiasing.
  All in all, it's a damn good little amplifier, if you get disillusioned
about them, I'll take them off your hands, at a healthy profit for you.
(if you have an orphan Heath W5M, I can give it a good home.....)

-Paul


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Paul J Guy			work phone:519-885-1211 ext 6371  
paul@gaitlab1.uwaterloo.ca	home:519-744-5016   FAX:519-576-3090
pguy@healthy.uwaterloo.ca	..remember...bullshit baffles brains...