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From: keen@austin.ibm.com (RG Keen)
Subject: Re: Vintage Amplifer Advice
Date: Mon, 7 Feb 1994 17:10:39 GMT

The Heath W5-m is a replay of the Williamson circuit, and 
may well have better stock performance than the ST-70, 
depending on the output transformers. One of the mods
of the ST70 in Glass Audio reduced the ST-70 to 
essentially a Williamson with good results.

R.G.

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From: rons@tv.tv.tek.com (Ron Selberg)
Subject: 6B4 Heath W5's
Date: 27 Apr 1994 15:40:55 GMT

Some ramblings from a DIY tube-a-phile.

I recently finished building these amps and thought I'd share some interesting
findings.
I had two of these amps both with bad pwr transformers and had wanted to try
6B4's in push pull. I was lucky enought to find some military grade pwr
transformers with the correct hv windings as well as three 6v windings plus
the 5v winding.

I stuck reasonably close to the Heath ckt, but used a 5687 for the driver
instead of the 12AU7. The 5687 has alot lower plate resistance.
I also used the Heath scheme for balancing the bias on the output tubes. With
these being cathode biased, it required DC for the 6B4 filaments and seperate
windings for each filament as well. I tried to use AC but it would not work
due to a bad hum problem.

An interesting note. Simply rectifying the 6.3v results in too much voltage.
I had to put two .6ohm resistors in series with each leg of the filament to get
back to 6.3v. 6B4's are hard enough to find without me blowing up the filaments!I used the .6ohm resistors as part of a pi filter with 22kuf caps on either
side and the DC is clean!

I initally used a pair of the "famous" UTC output transformers in the ckt. Since
they were 6K P-P I figured this would work out better than the 8K P-P of the
Peerless units in the Heaths. And those UTC's are supposed to be great, right?

Well, when I measured the amps for freq response, and looked at square waves,
I began to wonder. These amps contain NO GLOBAL FEEDBACK by the way.
The UTC's gave me 7hz to 75khz at 1 watt out. These are the 3dB points. At full
power, 9 watts in this case, the response changed to 10hz and 63khz 3dB down.
This was with quite visible distortion of the sine wave at the low end.
Square waves showed ringing, but looked pretty good. And the amps sounded quite
good. Hmmmm, wonder what those Peerless units look like.

It was an easy swap so I stuck one in. WOW! Freq response was now 3dB down at
97khz! The low end was as good or better than the UTC as well. At full power,
now at 8 watts, there was no change in the response. Still good to 97kHz.
At 15hz full power there was no visible sinewave distortion. And.....
square waves looked darn near perfect. No ringing or overshoot. Even at 10khz.
I have not seen this ever in a non-fedback amp.
And the amps sound very fine. Alot more detailed and airy. I am sure glad I 
gave the Peerles outputs a try. These are good transformers!!! 
For you Heath lovers, these are the larger of te two types used on the amps.

I know these are only 8 watters, but I have a horn system and will use them
to drive the horns.

I'm really pleased, the 6B4's have a neat nostalgic look and really sound good.

Sheesh, seems like I'm beating a path towards single-ended!

Ron