[Alex Pogossov]

"patrick-turner" wrote in message
...
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.

Floating supply sounds not too far fetched, given that similar approach has
been entertained in a "cyclotrone" topology.

In your case, a good thing is that the floating supply (for plate only) can
be unregulated and unfiltered -- just a bridge and 220uF or so. Another
smaller power supply, well filtered though, will be powering the screen of
the KT88 and the rest of the circuit.

Also I think that stray capacitance noise will not be so much of a
problem -- because it leaks only in 60% of the OPT, which has 36% of
impedance. Besides, GNFB will kill the residue of noise if any.


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.

There are plenty of brave men, but not many 60H chokes in junk boxes. What
if make a 500...600Vdc supply (doubler) and use an enormous glazed wire
wound resistor as EL84 plate load?