[flipper]

On Mon, 1 Oct 2012 09:52:20 -0700 (PDT), Analogdino
wrote:

From Flipper: "The UL taps should be enough to tell you it isn't a 'P.A.' amp. It's the most powerful of their 'W', so called, 'Williamson' amp line except the W7-M is obviously not a Williamson circuit while the others, W3-M. W3A-M, and W5-M, are."
Me: Agree. I identified it loosely as a PA amp due to the 250 ohm o/p option but the UL format implies it was meant to be Hi-Fi . Not "Williamson", of course, as no driver stage. Odd that they used the "W" designation... IMO, the W4 and W5 are clearly higher quality amplifiers (I don't know the W3 but did refurbish a W4-AM.) Your points on the 6L6 sub are taken... but see below! Also, 3,500 ohms P-P is a given... I'm not about to but a new OPT!

First let me correct myself; the W-6M is the 'most powerful'. That
raises the point, though, that it depends on how you define
'Williamson'. Heathkit refers to them as Williamson 'type' and I get
the impression their emphasis is on the 'basic design requirements',
as Williamson described it. Which, I think, explains the previously
mentioned "rating" question. Williamson says 'negligible distortion'
up to the 'maximum rated output' so that's where Heathkit 'rated' that
one even though it could put out more continuous Watts. 'Rating' lower
that 'maximum' may seem like a 'cheat' but, remember, Williamson specs
"15-20 watts" so Heathkit could argue that theirs not only does more
'Williamson' Watts (the 25 Watt rating) but can do over TWICE the
power before clipping and it's arguably better to have a 'teensy' bit
of distortion at 45 Watts vs the waveform chopped off at 20.

I tend to agree with your interpretation that a 'Williamson' has the
'drivers' but if we go by the actual 'Williamson' design then none of
the Heathkits are true 'Williamsons' because the original uses triode
outputs (KT66 wired as triode) and the Heathkits are UL.

Williamson explains why (he 'added') the 'driver' but that doesn't
necessarily apply if you have more gain in the outputs, needing less
drive, and sufficient overall gain. He simply says a pentode front end
doesn't have enough gain (not surprising with triode outputs), not
that there's anything else 'wrong' with using one, and I suspect
Heathkit would have argued they're using 'better' tubes (EL34, for
one) and UL to achieve the 'same thing' consistent with the 'type'.

Btw, Williamson also offers the alternative of using a self balancing
paraphase, which I'm sure would horrify some people, and explains that
this eliminates the need for the (additional) drivers, so even
Williamson doesn't 'lock' the 'type' (assuming there is such a thing)
to one topology.

The W-6M goes another step and operates Class AB2 using cathode
followers, which isn't mentioned by Williamson.

I didn't think you'd change OPTs. The point was really to use the
'right' tubes because the load impedance isn't correct for the 6L6
operating Class AB at that B+

You mentioned just 'living' with the lower Po but distortion will be
higher too, because it's too low an impedance for the 6L6, although
you could use a lower speaker impedance tap to raise it. The low end
would suffer somewhat but probably not enough to notice.