From: paul@gaitlab1.uwaterloo.ca (paul j guy)
Newsgroups: rec.audio.high-end
Subject: Re: WANTED : Heath Kit W6M Modification
Date: Wed, 6 Jan 1993 00:58:16 GMT
Organization: University of Waterloo, Waterloo, Ontario, Canada
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu

In article <1ic5eeINNmpq@uwm.edu> "Donald S. Chiu" (AED-EWD) <dchiu@PICA.ARMY.MIL> writes:
>Recently I acquired a pair of Heath Kit W6M amps.  They are in good
>sounding condition.  Could someone tell me how and where I can improve
>the sound of these amps.  I had no background on electronics so any
>modifications will be done by outside shops.  Any suggestions are welcome.
>Thanks in advance.
>
   Can't say that I know the W6M's, I am currently working over a W4M and
a W5M. Tweaking any of these old amps is going to be done purely for
the love of old tube amps, the hours you can put in are very long. Unless
you find a well defined sheet of modifications, any garden variety
electronics shop will throw its arms up in despair when you ask to
have the sound 'improved'. 'Refurbished' is more likely, then new tubes
are going to put in, old electrolytics replaced, line cords replaced and
the unit brought up to current safety spec's.
  If better sound is wanted (and better sound is a very subjective thing)
you would be best to find some electronics type who is experienced in
audio and old tube equipment, and whose tastes in sound are similiar
to yours. In my case, bass is not that important, but a smooth
and non-strident treble is very important, since I listen to a lot
of chamber music from CD's. As a consequence, transient response,
roll-off, and IM distortion are the main things I concentrate on 
improving.
  I have so far spent 30-40 hours changing the circuitry. The changes
include: converting to triode output
         biasing with a fixed - supply (this effectively adds 50v to plates)
         replacing plate load resistors
         rebiasing driver stages
         changing coupling capacitors
         changing feedback network, and compensation
         putting in an inner differentiating feedback loop(s)
         new line cord, 110v wiring
         better power supply filtering
  I was very surprised at the measured quality of the output transformer.
The 3db points were around 7Hz and 50 kHz. Were it not for that, I would
have probably pitched the amp.
  At any rate, I guess the point I'm trying to make, is that it could be
a long and expensive process, unless you find someone in your area who
actually likes this sort of thing. There is no fixed way to go, no real
circuit mod's that I know of, and even if there are, you must be
careful of the parts quality, and the final adjustments and trimming.
  I hate to say it, for the cost, you might be further ahead buying a
new tube amplifier (expensive as they are) or a cheaper 'recommended'
solid state amplifier. Definitely the case for preamps (IMHO). If
I was living nearby I could easily be tempted to do the mod's for you,
so your best bet is to find some similiar nut case who likes doing
tweaks in his (or her) spare time.

-Paul


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