[Patrick Turner]

On Apr 2, 7:20*am, Tom Schlangen wrote:
Hi Patrick

Most ppl think the UL connection for EL34 always means 43% screen taps.
This % was always regarded as optimal because triode like THD is
achieved and Rout reduced to give Rout = RL. But this was mainly for
class AB PP amps.

Yeah, and most winders wholeheartily will agree that 43% percent is ideal
because it is just the end of the 4th winding block of a 7-way
interleaved design (means 14-way interleave for PP), hence easily to
manufacture. Same with 50%, 40%, 33% and 25% (depending on number of
interleaves).

So much on "magic UL% numbers", honi soit qui mal y pense (I don't mean
you personally).

Regards,

Tom Schlangen

Well I agree entirely that recipes for UL OPTs had the primary = 14
layers and you could then build up the bobbin from its bottom with 4P
layers, one secondary section, 3P layers, CT, 3P layers, secondary
section, 4P layers.
This gave a PSPSP interleaving pattern. Designs like this were
publicised and analysed in Wireless World in the 1950s. And why such
crap was analysed and considered fit for audiophiles I don't know.

But it turns out that if you draw a graph of Ra vs UL tap %, then
you'll find a rapidly falling Ra until about 43% and then gradual
additional reduction of Ra until you get to 100% UL which is triode
connected.
So the 43% was at the knee of a curve. It worked well considering that
peple wanted some reasonably high gain because most designs
incorporated GNFB of up to 30dB in many cases. Remember the days when
there were wars over who could get the lowest distortion figures like
say 0.03% at 1dB below clipping?

A Williamson triode amp gave 1% at near clipping without GNFB. With
20db GNfB this 1% reduces to 0.1%, and with 30dB it would be 0.03%,
and one has to have a very wide BW OPT to get this and understand how
to apply phase tweaking circuits to make the amp stable
unconditionally. Makers now don't use wide band OPTs and they know
**** all about "critical damping". Past arts have been forgotten.

If you read RDH4 on the comparison of 3 watts from a 2A3 or from a
6F6, the harmonics production of the 6F6 is utterly appalling compared
to the triode. The UL does a lot to reduce the crap. But I like more
than 43%. In class A circuits where the load line is near horizontal
and one does not need to make the Ea min swing down much more than it
does with triode in class A1. But its useful if you can get the extra
swing. And you don't need to move the screen tap very far away from
the anode to get the extra swing, and with say taps at 60% to 75% the
tube behaves very much like a triode with very little 3H and other
higher H. Its not a problem if the gain is low with EL34. Even triode
gain is OK. 6550 also give very near full tetrode power when using 60%
screen taps. Manley Labs have EL34 with 25% UL and Ea = Eg2 = 550V.
I don't know why they bother with only 25%. But then they try to
squeeze many watts as possible; it is good for sales but not so hot
for fidelity or reliability.

Patrick Turner.