[patrick-turner]

While installing newly potted Hammond SEA1627 OPTs to a pair of Jadis
300B mono amps, my mind came onto the idea of using local Single Ended
CFB without needing a specially wound SE OPT which has two primary
windings, one for anode to B+ and other for cathode to 0V. The SEA1627
Hammond has a 40% UL tap so that many pentoads and tetrodes can be set
up for 40% UL, or if you like, 60% UL if you connect the single anode
winding "around the other way."

The Jadis has a pair of 300B in SE parallel, and so the UL tap on
Hammond OPT is useless unless in future years the owner wishes to
switch from 300B to KT120, so PO max could be increased from 17W to
30Watts, using the usual UL tap going to screens and the primary
having B+ at +430Vdc.

But what if the UL tap was GROUNDED directly to 0V and ALL the primary
placed at oV potential?

One may wonder how the Fk is that done? The simple answer is to make
the B+ supply NON GROUNDED and with its negative sitting on one end of
the primary which is 60% of P turns away from the UL tap and have the
OP tubes with their cathode bias networks all sitting on the other end
of primary which is 40% of P turns away from the UL tap.

Meanwhile the anode goes to the positive +430Vdc of the B+ supply.
The screens may then be taken to a second B+ supply of say +350Vdc,
and would optimally be SHUNT regulated.

The result is that the pentodes or tetroads have internal signal
voltage relationships equal to the normal application of 40%
Ultralinear taps, but 40% of the Vak is feedback voltage in series
with the Vgk input signal.

Rather than weigh down Google groups with a huge boring talk about the
analysis of the result, I give you the result of my calcs indicating
the grounded 40%UL tap and floating B+ PSU will reduce THD at near
full PO to 1.5%, and Ra 400 ohms, and Vg1-0V drive voltage is just
under 100Vac where RLa = 2k0 for a single KT120, at least, based on a
model have for 6550EH and KT88EH.

The problems you will cite are that one needs TWO B+ supplies, and one
for OP tubes floats and is prone to noise via stray C on PT. One
should use a PT where heaters are wound on as a layer between mains
and HT, and heaters are at 0V, so they act as an electrostatic shield.
Then one would be wise to have an ES using a turn of foil (without a
join to avoid a shorted turn) after the OP tube HT winding.

The other bother is the drive voltage of 100Vrms. Be brave men, and
don't be pussy wussies, and use an EL84 in triode with say 4k7 plus
60H choke in series as the feed to +400Vdc with Ia at 14mA. That
should give you Ea at about 322Vdc, OK for trioded EL84, and swing
will exceed 120Vrms with 2H 1.5%, which should cancel with 2H of OP
stage - rather nicely, you shoud find.

40% is rather an extreme amount of CFB. 20% is IDEAL, but in this
silly world hardly anything made for one thing is useful for something
else but nevertheless one can use an SE UL OPT in the manner I have
pointed out and if the OPT had UL taps at less than 40% then you are
quite fortunate.

Of course you can use the similar arrangement for Push Pull with two
OP tubes and where you'd need to ground the primary CT, and have TWO
floating B+ supplies. Consider the OPT with say 40% UL taps, and from
one end
of primary to other we would have A1, S1, CT, S2, A2 connections for
standard UL use for V1 and V2.

V1 has anode to B+1, then B-1 to A2. V1 cathode is to S1. The other
tube, V2 is set up similarly on the other side of PP circuit. Now what
could you call this kind of set up? Partial Circlotron.

Suppose you have the tubes set up for PP and minimal class A. Consider
the class B operation. In normal class B the single tube turning on
sees RLa = 1/4 RLa-a. But in the case above for PC, the single tube
turning on is effectively connected to 70% of the whole primary turns
so its class B load becomes 1/2 RLa-a, and this gives the tube a big
relief from finding itself connected to a load way too low which has
been determined by a fuctard bean counter, and instead the load is
twice the ohms. I leave you all to consider the loads actually seen by
tubes in class A and AB conditions and maybe you find the PC
connection effectively raises the tube loads when compared to plain
old UL which has brainwashed so many people into thinking there's no
other way.

BTW, today I tested the Jadis amp after installing the potted Hammond
OPT and tried some music. WOW, all I can say is that some bass heavy
jazz from Arts FM radio in Canberra sounded just fabulous, and I
wanted to EAT THE BASS NOTES. Highs are beatifully clear, without any
sign of smear, and the amp just dissapeared to leave the musos in my
shed on not-too-good old speakers with not-too-good old EH300B and
EH6SN7. The owner has some old RCA 6SN7, and a quad of Emission Labs
300Bs. Should sound breathtaking on his much better speakers. Used
with mild global NFB, and decent design and better OPT than Jadis
provided, the 300Bs seem to make music from heaven.

Patrick Turner.