4d57e9fc
From: apogosso@tpg.com.au
"patrick-turner" wrote in message
news:c72355b4-ca04-4f40-8751-f5d3cd47dbb2@m4g2000pbd.googlegroups.com...
> 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?
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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