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patrick-turner patrick-turner is offline
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Default Local CFB with grounded ultralinear OPT tap, Partial Circlotron.

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.
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Alex Pogossov Alex Pogossov is offline
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Default Local CFB with grounded ultralinear OPT tap, Partial Circlotron.


"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?


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patrick-turner patrick-turner is offline
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Default Local CFB with grounded ultralinear OPT tap, Partial Circlotron.

On 11 Nov, 11:32, "Alex Pogossov" wrote:
"patrick-turner" wrote in message

SNIP,

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


But we NEVER see a single ended Circlotron! That's what I am sort of
proposing. Turnotron? maybe, I don't give a F what ppl call it, as
long as it works.

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.



Yes, the filtering in the anode supply doesn't need to be perfect
because anode resistance is so high when pentode/tetrode is used, and
the local NFB works to oppose any PSU hum in load, ie, across the
whole OPT which has a tube across it acting as a far lower Ra than it
really has, and with RL shunting it, so if Ra'// RLa = say 500r, and
internal = 10k, then hum of 50mV appears as only 2.5mV which is
transformed to 0.135mV at OPT sec if OPT has 18:1 TR.
In the case of Jadis, hum at C2 470uF at OPT primary = 40mV. OPT
primary load Rla is 1k8 in series with Ra of 2 x 300B = 400r so hum
across OPT = 33mV.
OPT TR = 18 : 1, so 1.8mV appears at output if there is no GNFB. I
have not tested the noise performance yet of the Reformed Jadis, but I
know what to expect to find! I may yet be forced to remove the almost
useless Jadis filter chokes and replace with something with at least
1H, to reduce hum by say 20 dB.
In a good SET amp with no GNFB, hum 0.5mV is travelling well, with
input grounded. To see if hum is entering the signal path one may
ground the OP tube caps with say 2uF, and any hum in OP stage may
become visible on the CRO. Then you move te 2uF cap further towards
the input to the driver stage gris, and see if hum increases, and it
is does, voila, a source of hum is identified. If its F is 50Hz, Ah,
then you have maybe heater to cathode leakage in driver tube. At the
input tube, you need to ground input with shorting RCA plug, and also
short the 100r GNFB resistor between V1 cathode bias network and 0V.
Eventually, the source of the crap that some tube amps serve up to you
will be found, and then corrections can be made to 0V rails and
ground, ie, one does whatever it takes to get the noise low!!!!
My customer here has very sensitive speakers, and hum exceeding 1mV is
not acceptable.

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.


One may like to think like that, but all too often when we measure
stuff we find that what we hoped to expect was BS, and we have to
effect a cure with a dive into parts bin, a good think, and further
unforseen necessary a soldering.

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?-


I've done just this on a number of amps, easy to make HV at fairly low
current of say 14mAdc. If one has a 320VAc winding then you get 432Vdc
which can be R&C filtered for Eg2 with shunt reg, then also have a cap
and diode to make about +700Vdc. If Ea of EL84 is +330Vdc, then RLdc =
370V/14 = 26k0, and liberates 5.2W of heat, and if tube shorts, gives
10W, so the 26k could be say 4x4k7 + 1x6k8 = 25.6k, and each R is 5W
wire wound. The following grid bias R can be connected to end of
grounded OPT primary so its ac load ohms are thus raised by
bootstrapping. So if Rg were 2 x 47k = 23.5k, and boostrapping
increases this to say 90k.
Total AC load on EL84 becomes 26k//90k = 20k, and gain = 20 x 20 /
22.5 = 18.2, not to bad at all, and peak Ia swing at 140Vpk = 7mApk,
ie, 1/2 Ia, also doable, with low THD. But the choke plus say 4k7 to a
screen supply of +400Vdc is BETTER. Then the 25 dissapears, leaving
only 90k loading of Rg.
Gain approaches µ, distortion seems to vanish, and music is better.
The 4k7 series R has a gain shelving function so that gain slightly
reduces where high voltage swings are unlikely to appear at F below
20Hz and above 20kHz. Chokes of say 60H may have shunt C = 300pF, and
the 60H = Ra = 2k5 at around 7Hz. Therefore the use of some GNFB works
just fine even though it seems the choke has introduced another damn
pair of R&C and L&C time constants and phase shifts into the open loop
character. BUT, also JUST AS GOOD is to settle for a B+ rail of say 2
x Ea = 660Vdc, and the use a pir of 2SK1924 in series and arranged as
a as a constant current source. Its easy to get them to act just like
5MO shunted by about 70pF. The use of TWO mosfets in series means that
if Ea is shorted, you have only 1/2 the rain voltage across from drain
to source. MJE340, MJE350, KSE340, KSE350 are also usable bjts. They
can be arranged to sort of work like a choke by offering a lowish load
of say 20k at F below 2Hz, thus stabilising Ea a bit. There are
several ways of doing this stuff. Another is to use the 26k RLdc, but
then bootstrap the top 10k portion to the cathode with elcaps, leaving
the 16k maybe raised to effective 64k.
Patrick Turner.
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patrick-turner patrick-turner is offline
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Default Local CFB with grounded ultralinear OPT tap, Partial Circlotron.

On 11 Nov, 11:32, "Alex Pogossov" wrote:
SNIP

Alex typrd....

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.


I've just done more careful measuring of the noise in the Reformed
Jardis amps.

I did mention I thought the Jardis Chokes had L = only 80mH, roughly.
That was based on measuring 100Hz Vr hum across C2 470uF and across
choke. The 100Hz attenuation factor of the LC then is VC/VL and this
was initially 0.6V/0.05V = 1/12, and XL is about 12 x XC = 12 x 3.4r
at 100Hz = 41r then dividing by 628 gives 0.064H, ie, 64mH, even after
I set the air gap closer to get the minimum Vr with 0.15Adc flow. But
the trap everyone is prone to is that the Vac across a second C in a
CLC filter usually has lots of noise content below 100Hz, hence just
measuring across the C2 tells you a lie. So, I use a 10:1 resistance
divider with 3 x 100k and 1 x 33k so that Vdc is reduced fy factor of
1/10 to reduce the VDC measuring to +43Vdc in this case, and then I
have a 0.22uF across the 300k, so that you have a filter reduces DC
and LF but gives no attenuation at 100Hz, as it is -3dB at around
24Hz. This means nearly all the LF noise on B+ rail dissapears, and by
switching the CRO to max sensitivity and usung the scale I have taped
alongside the screen and calibarted in mV rms, I can read low voltages
even with a few mV of trace wobble due to LF noise.
I found 100Hz noise = 4mV, so attenuation factor of the CLC was better
than I thought at 0.6/0.004 = 150, so XL - 150 x 3.4 = 510r, so L =
0.81H, and that's about right considering Rw = 15r, and the size of
the choke, and the way it responded to correct air gap sizing.
Then I found another Jardis caused source of noise. Jardis put the IEC
chassis plug facing down into chassis and beside the PT. There is a 3
connection double cap on IEC plud with wires to A, E and N and they's
have no effect at LF. Then they had a 50mm green-yellow lead to one of
the 6mm bolts holding PT to stainless steel chassis. Now I had to stop
this amp humming by placing 22r between chassis and 0V rail. But today
I measure 0.47Vac across the 22r, and from PT bolt to chassis nearby,
so some serious I flow is generated within very short distances of
iron material, but hey, I could expect that no? bolts are subject to
stray fields from the large PT. So I reach for junk wire box, find
green-yellow wire, connect Earth terminal to a bolt holding a clamp
for 470uF cap. I get that done and remeasure. Bingo!, only 3mV across
22r, although yeah, the PT bolt is still having some voltage induced
in it. The 0V is connected to chassis via 22r at 300mm away at amp
input end, I find all is well. My ancient olf Fluke hand held meter
probably has dirt inside, and it gives a 2mV reading on Vac even with
leads twised together and propds shorted and away from magnetic
fields. Anyway, I can't measure noise at Vo terminals with meter, even
with no GNFB, But my best probe for my CRO with short earth lead to 0V
and short lead from amp 0V to CRO 0V shows 0.5mV without NFB, and
artifacts are various H of mains. This increase a fair bit without
shorting input to 0V with RCA shorting plug. OK, 120k input R does
make a bit of noise. With GNFB, the noise becomes almost a dead flat
line but I estimate 0.2mV. Earlier I noticed some trace wobble with a
regular timing, which telle me someone is switching something on/off
on mains thus making B+ bounce a bit with main F being probably 8Hz,
which is the Fo for 0.8H plus 470uF. Not much I can do about that, but
hey, a coupla mV of noise below 10Hz doesn't matter. So. I am happy
with result with regard for noise.
OLG response is 20Hz to 16kHz, no GNFB, -3dB.
With 9dB GNFB, CLG response with input CR filter shunted = 6Hz to
56kHz, 5 ohm load, 0.5dB peak at 8Hz, before steep roll off below 5Hz.
With HPF connected, LF pole is 8Hz, -8dB but dead flat from 10Hz
upwards. The shelving means GNFB is reduced to 4.5dB, ( halved ) at
15Hz and 20kKz with lessening FB applied outside this band. Sounds
great, no weird behaviours, all looks well. waether fine, level
horizon, will land for dinner soon.
Then to spend a night on working up ther page for the Net about the
mods, so that if anyone wants a good SET recipe, they can have it.
Patrick Turner.
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