In message , Nick Gorham
writes
Andy Evans wrote:
Well, my pal chis found (runs the service for Beard Audio) had a 211 amp for
years, and just switched to 845, which he prefers. It's not so hard
to drive -
these days there are SRPP and mu-follower circuits, good interstages - all
sorts of solutions, even pentode drivers are re-emerging (EL84, C3M
etc) Andy.

Yes, I wasn't trying to dismiss the 845, just point out that it had its
own set of problems. At the moment I am exploring what can be done with
211's, but I am not able to make the most of them at the moment as I
don;t trust my op TX's above about 800v, so I can;t make good use of
the 211 yet. The GM70 in the same circuit was suprisingly good. And if
you can work around he high fillament supply problems, I think it is a
good half way house between the 845 and 211, mu of 8, and works well at
about 700v 70ma to give about 15w of single ended output.

I have decided though to concentrate on the 211 first and then look at
what the gm70 can do, currently using a DC coupled 6em7 driver, cloke
loaded and cap coupled to the driver valve.

Final plans involve interstage, I am planning on trying the interstage
that Ae produce, 50H, with a design current of 60ma. And its 98euro for
a pair (double C-Core as well). I will see what the 211 OP TX I am
going to get from them is like and if its ok, the interstage is next

Currently uising 40khz heating from modified lighting transformers..

The GM70 should work with a 211 type TX, but it rearly needs a much
higher current. But in a blind testing at Paul Barkers, the copper
plate GM70 was clearly better, and this is better used at a lower
dissapation that the C plate's 120w.

I thought about the Sowter bifilar TX, but the big problem I can see
with this is being bifaler, it can't be reverse connected, so you can't
take advantage of flux cancilation when grid current flows in A2.


I hadn't thought about using the grid current to offset the flux
produced by the anode current of the driver! You're right, a bifilar
interstage has to be connected in the correct phase. You can do the old
GEC trick though to extend the HF (and used nowadays in the SEPIC dc-dc
convertor topology) of using a capacitor together with the transformer
to short-circuit the leakage inductance. (Only works if the transformer
is *exactly* 1:1 of course.
--
Chris Morriss