John Stewart wrote:

Patrick Turner wrote:

Marcin Slawicz wrote:

Uzytkownik "Patrick Turner" napisal w wiadomosci
...
*** Snip ***

Williamson had a lot of very bright ideas, many of which were ignored
by the makers of so many compromised amps after 1950.

Patrick Turner.


Thanks Patrick for your great explanation. I was familiar with White's
article and really resisted some statements I found there. There is quite a
number of misguiding articles regarding the Williamson amplifier on the web,
and because of my nearly no-experience in tube electronics, I am not always
sure what true and what false is.

At all times in the past, nearly all men with a slightly different way of
building an amp
would routinely say how bad the other guy's amp was.
The same BS goes on today.

I have built just about all types of amplifier, and all are derived from what my
father's
generation thought up.
For example I like the Quad II idea of CFB from the OPT, but I think the way
Quad
implement their idea is very lack lustre, and its a chic amp which has been
dumbed down by accountants.
It still sounds ok though, at a few watts.
I criticise many of the sacred cow brands such as Leak, Quad, Dynaco,
and I don't care whose ego I bruise; if they can't see that these old bits of
junk
**could have** been a lot better pieces of engineering had the makers used a
little more labour
and material instead of buying that new Mercedes for the boss, then they'll
never see anything.

Anyway, I use the Quad II idea in preference to the normal screen taps of UL.
But I use much more CFB, since the amount Quad use doesn't do enough to reduce
Ra-a and thd. Same goes for UL; one still has an output stage with Ro = approx
RL,
but its better than nothing.


If you don't mind, please take a look at my first tube project I am
assembling these days
(http://www.echostar.pl/~slawicz/conc...oncertino8.htm). The site is in
polish and it's not completed yet, but you'll find the schematic there.
The biggest difference between Williamson's amplifier and my one are
(despite the UL output stage) the constant current draw first two stages in
Aikido style described many times by John Broskie in TubeCad Journal
(http://www.tubecad.com/april99/page6.html). I hope it will work properly.
The simulation shows about 14 dB better PS noise and ripple rejection
comparing to typical Williamson front end.
Best regards,
Marcin

There is always a constant current drawn from the PS to any kind of SET
V1 and CPI V2 stages, simply because class A stages do not have a changing
supply power, other than that due to the 2H produced in signal, and in your case
that
will be below 0.1% evan at 2vrms from V1, so the DC voltages at
all electodes won't change much.

The use of the R19 bypassed with C5 and R24 is a step in the right direction
because it makes the set up conditions of
V1 and and V2 easier to arrange so that the anode voltage at V1 is best for
linearity,
and the grid voltage at V2 is best so Ek isn't too high above the heater supply.

Also at very LF, where the williamson margin of stability becomes
much less than at 1 kHz, the R19 and R24 become a voltage divider reducing open
loop gain
by 6 dB below 20 Hz, because C5 graduallly becomes a large and open impedance at
LF.

You could further stabilise the amp by using more LF attenuation in the open
loop response than you have.

Go to

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

There you will see a schematic for 50 watt class AB1 UL channel using KT88/6550
and including an integrated input preamp stage.

It isn't a williamson, but is a derived topology from what Mullard used
back in the 50's, and which has been addopted by 1,001 makers since then.

However, I have a few secret things in there that would NEVER have been done in
1955, such as the gain&phase shift network between the first tube in the power
amp, and the
second, which is 1/2 of the longtail pair.

This network should be of great interest to anyone building their
PP amp especially if the OPT isn't up to the **best** of the Partridge models.
About 99.99% of OPTs sold on this planet are a lot worse than
Partridge OPTs.

There is a network containing 0.47 uF, 0.047 uF, 1M, 220k
which acts to reduce LF gain a maximum of 15 dB, starting at 15 Hz.
By 1.5 Hz, at which many amps would oscillate badly without this network,
the LF gain has become about 12 dB less, and as stability is affected by the
amount
of open loop gain and the amount of FB applied, the amp will have far less NFB
applied
at 1.5 Hz than at say 100Hz, so the amp will be rock stable at *all* LF.
The amount of FB that *can* be applied safely without oscillations
occuring is also dependant on the phase shift of an amp, and stablity
is threatened as phase shift approaches 180 degrees betaween input and output.

LF Phase/Gain networks such as you refer too were not very secret at all in the
50's. For example, they were well understood & used in both the McIntosh MC40 &
MC75.

Cheers, John Stewart

BTW, I have on occasion used LF Phase Correction networks too. You can see an example
of that as it was published in the July 1998 Electronics World over at ABSE under the
heading 6AS7/6080 Amp. A more complete article on the same was published in Glass
Audio Number 2, 1999. The same article also covers a better way to drive low mu tubes.

Cheers, John Stewart