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  #1   Report Post  
bill ramsay
 
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Default low voltage high current heater supplies

dear all

I am playing with some 813's at the moment. I am trying to get the
heater supply sorted out.

the 813 needs 10v at 5 amps, the do glow v. bright when lit up.
pretty neat.

I have a breadboard amp going at the moment which someone else has
given me.

The current heater supply uses a 20 odd volt ac transformer, bridge
rectiifer, feeding a 2n3055 based regulator, thing is putting out
about 50watts in heat. as it is stereo there is 100w in heat being
generated.

whenever i model a 5 amp 10V heater supply in PSUD i get some really
whacky readings, ie.

28V transformer to feed 100,000u cap, with a 0.5ohm resistor, then
1000u cap into the tube. ie. 2 ohm load,

or am i missing something here, 28V ac seems a long way away from 10v
dc, or is this an artifact of the high current pulsing away and
charging the caps?

I wanted to build a dc supply that does not generate so much heat in
itself.

any thoughts, ideas etc. especially the printable ones would be
greatly appreciated.

kind regards

bill ramsay.


  #2   Report Post  
Duncan Munro
 
Posts: n/a
Default

On Mon, 24 May 2004 23:34:35 +1200, bill ramsay wrote:

28V transformer to feed 100,000u cap, with a 0.5ohm resistor, then
1000u cap into the tube. ie. 2 ohm load,

or am i missing something here, 28V ac seems a long way away from 10v
dc, or is this an artifact of the high current pulsing away and
charging the caps?


Bill, what is the source impedance of the transformer? It needs to be very
low for what you are proposing here. If you can mail me the psu file, I'll
have a look at it for you.

--
Duncan Munro
http://www.duncanamps.com/
  #3   Report Post  
Yves
 
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"Duncan Munro" a écrit dans le message de news:
...
On Mon, 24 May 2004 23:34:35 +1200, bill ramsay wrote:

28V transformer to feed 100,000u cap, with a 0.5ohm resistor, then
1000u cap into the tube. ie. 2 ohm load,

or am i missing something here, 28V ac seems a long way away from 10v
dc, or is this an artifact of the high current pulsing away and
charging the caps?


Bill, what is the source impedance of the transformer? It needs to be

very
low for what you are proposing here. If you can mail me the psu file,

I'll
have a look at it for you.


Hi !
PSUD II is a great tool !! Thanks Mr Munro !
Simulating with a (probable) 10.5v and 0.15 Ohms internal impedance tranny,
result is right for one tube (5A) using a 50,000µF cap.
Yes, it remains some 3V pp ac component, but does this really matters ?
It 'is already 10 times lower than using AC :)

Yves.

--
Duncan Munro
http://www.duncanamps.com/



  #4   Report Post  
Patrick Turner
 
Posts: n/a
Default



bill ramsay wrote:

dear all

I am playing with some 813's at the moment. I am trying to get the
heater supply sorted out.

the 813 needs 10v at 5 amps, the do glow v. bright when lit up.
pretty neat.

I have a breadboard amp going at the moment which someone else has
given me.

The current heater supply uses a 20 odd volt ac transformer, bridge
rectiifer, feeding a 2n3055 based regulator, thing is putting out
about 50watts in heat. as it is stereo there is 100w in heat being
generated.

whenever i model a 5 amp 10V heater supply in PSUD i get some really
whacky readings, ie.

28V transformer to feed 100,000u cap, with a 0.5ohm resistor, then
1000u cap into the tube. ie. 2 ohm load,

or am i missing something here, 28V ac seems a long way away from 10v
dc, or is this an artifact of the high current pulsing away and
charging the caps?

I wanted to build a dc supply that does not generate so much heat in
itself.

any thoughts, ideas etc. especially the printable ones would be
greatly appreciated.

kind regards

bill ramsay.


To get enough heater power for 4 x 813 for a stereo PP amp,
you would need 4 separate 10v x 5A power supplies.
I guess many folks would use centre tapped winding for each tube,
to allow for normal AC operation, but let's assume you insist on DC.

A separate PT rated fo about 350 VA would be suitable, and a toroidal
rated at 500VA would be nice.
You could try gettting a supplier to give you a toroidal tranny
with no secondaries wound on, and then do your own secs,
because the turns required for each 10v may only be between 15 and 20
turns
of say 1.5mm thick wire.

To reduce transformwer noise to a minimum, I would use a choke input
supply
for each tube, so the tranny AV would have to be about 12vrms to make
+10DV.
10v at 5A is an RL = 2 ohms, so the choke value needs to be at least
2 / 940 = 2.1 mH, which would be easy to make using about 2.0 mm dia
wound around a stack of iron with a core area of about 25mm x 20 mm,
with a very generous gap.
In fact, if you made a choke with 20 mH, it would ber a lot lot better.
The choke size quickly goes up with the inductance needed,
and the DCR, which needs to be minimized because Pd = I x I x R.

The capacitance would preferably need to be as high as you can afford,
but no need for ultra high ripple current ability, because the choke
input
avoids the high charge currents to any caps,
so I would use multiple paralleled 10 x 4,700 uF caps normally used for
SS amp rails supplies,
say to make to make 47,000 uF.
The AV input to the choke after the diodes is in the vicinity of 8vrms at
twice the
mains F, ( just guessing ) and the LC filter with 20 mH and 47,000 uF has
an impedance ratio
of 12.5 ohms to 0.034 ohms at 100 Hz, ( and better with 60 Hz mains ),
and so the 8v of hum is reduced to 8 x 0.034 / 12.5 = 0.021 vrms, which
imho will not modulate the cathode signal or be heard.
It would even be possible to make the chokes using a normal transformer
bobbin
with a centre hole say 1"H x 1.5"T and with a winding area of 2" x 0.75"
and use 1.6mm dia wire, and use some cut up old transformer
cores for a solenoid type of core, which won't saturate, but which would
give you about 4 times the inductance for the same turns using an air
core.
Such chokes are prone to hum, and should be well varnished and potted.

The choke in my class A SS amp which has a CLC type of 3 amp supply
has a 1.25" stack of 1" tongue lams, and its about 15 mH at 3 amps,
and I use 9,400uF off the high current bridge rectifiers, and 45,000 uF
after the choke,
and the ripple is much lower than a choke supply, just a few mV.
No need to waste power using SS regulator pass elements.

With a choke in a choke input supply, you will never get the theoretical
maximum voltage of 0.63 x the peak value of the AV input voltage from the
transformer
because of the DCR of the choke, and only 0.5 ohms means a V drop of
2.5 volts with 5 Amps.
And it means 12.5 watts of heat liberated in the copper winding.
So it is imperative to get the DCR of the choke to a low value,
preferably
less than 0.25 ohms.

Get the supply done for one tube, and then make all the rest the same.
The turns required for 12vrms on the power tranny can be trialled using
some thin wire to establish the voltage, so start with 20 turns, and
measure the
voltage, and remove turns until you drop to a few volts above where you
need
to be, say at 15v. Then use some thick wire, wound on tight,
taped down well onto the core.
Removing more turns is easy.
If you get 11v at 5 amps for a single filament supply,
then with 4 such supplies expect the voltage to drop to 10v each.

For a regulated supply, you would need a couple of low voltage high
current
BJT in parallel as a pass element, with a drop across the BJT
of say 4 volts, and so the supply from a cap input filter has to be 14v.
The AVrms would have to be about 12vrms, about the same as for a choke
input filter.
12v gives a theoretical max of
(1.414 x 12v) = 17DV, but there is a drop across two silicon diodes of
the bridge,
and then the loss due to the ripple characteristic, and one is lucky
to get 14DV from 12vrms of AV.
But using 9,400 uF caps for C1 will give a ripple voltage of
1.2 vrms at 5 amps DC.
To limit the effects and noise of the quite high peak charging currents
instead of resorting to
a choke input, the use of series R between the diodes and C1 is
recommended,
and a value where R = 5 x ZC is recommended, so at 100 Hz, Z9,400uF =
0.17 ohms,
so the R could be 0.82 ohms, and rated at 20 watts.
This would however raise the voltage needed by the cap input filter to
give the same
DV output.
The ripple voltage will not change, but the charge period will increase,
thus reducing the peak charge current occuring when no series R is used.


A CRC filter is also possible, and if C1 is 9,400 uF, then R should be at
least
10 times the reactance of C1, and the C2's reactance at least 1/10 of R,
so if ZC = 0.17 ohms, then R could be 1.7 ohms, rated at 100 watts,
then another 9,400 uF, which would reduce the ripple from 1.2vrms to 0.12
vrms,
which is a poor result compared to a choke, and the V drop across 1.7
ohms
would be 8.5 v, which liberates 42.5 watts in the R!
And this is a real lousy result.
R cannot be much less than 10 times C1's reactance value, because
otherwise the
diodes begin to see the C2 as their load as well.
If we said to hell with peak charge currents, and used
100,000 uF caps straight off the rectifiers, then the ripple would be
0.11 vrms.
( I have such a supply in an SS amp with two mighty 100,000 x 75 volt
Sprague computer grade
caps each 220mm long, and 75mm dia, with about a 20 amp ripple current
rating.
No chokes, and the noise induced into the amp is quite negligible because
of careful
layout of wiring, and the use of 50 dB of global NFB. the DCR of the
tranny windings and mains
wiring in the house walls provides the charge limiting DCR. )

Since you only want 10v, the caps will be higher C value for the size,
and 100,000 uF at 16v at high ripple won't be too big.

If a CRC suply is needed, then R needs to be about 0.22 ohms to
get a 10 times , -20 dB drop in ripple at C1, if C2 is also 100,000 uF,
from 0.11vrms
to 0.011 vrms.
It dissipates only 5.5 watts, and the V drop is only 1 v.
But a choke of 1mH would have a Z of 0.628 ohms at 100 Hz,
and C2 of 100,000 uF has 0.016 ohms, so the attenuation factor
in the LC filter is 0.025, 25 times better than the R.

Somewhere there is an efficient compromise between C used and choke
size, and what you are prepared to pay for.
The choke input filter or CLC would be more reliable than 4 x solid state
regulated
power supplies, imho.

Patrick Turner.







  #5   Report Post  
Adam Stouffer
 
Posts: n/a
Default

bill ramsay wrote:
dear all

I am playing with some 813's at the moment. I am trying to get the
heater supply sorted out.

the 813 needs 10v at 5 amps, the do glow v. bright when lit up.
pretty neat.


I would buy two 5v switching supplies and wire them in series. Both of
these surplus should cost less than the heatsink for a TO-3 transistor.


Adam


  #6   Report Post  
Fabio Berutti
 
Posts: n/a
Default

AFAIK all valve producers gave at least a 5% allowance for heater
voltage/current. If Your mains supply is reasonably stable I'd say that a
simple CRC or CLC rectifier is OK; being the tubes used in fixed (grid) bias
(I suppose, being a 100+W tube, it is used for POWER) it is feasible to feed
2 of them from the same tranny & rectifier and reduce a bit the pounds of
steel to be used (I already had my spine repaired by a surgeon and it was no
fun at all, I now set the threshold for my projects to = 50 pounds). The
voltage can be adjusted by trial and errors simply varying a bit the R in
the CRC filter. I did it for a phono preamp (OK, 1.5 amp all together..)
and it works. Plain and easy.
IMHO I'd use plain AC filaments in such a beast. I read a WE (or maybe it
was from RCA) paper stating that large power tubes were better used with AC
filaments, because in this way filament temperature tends to average (being
either positive or negative 50 times per second), while with DC filaments,
in fact, a part of the "cathode" always works with a hotter bias and the
other with a lower one, - the filament temperature (and electron emission)
is not symmetric - the valve does not work properly. This paper showed a
graph of the filament temperature against it length in a DHT, and well, a
difference of only a few volts in a tube having a bias of about 100V was
enough to have either one half too cold or one half too hot. I'm sorry I
don't remember where I found it.

Ciao

Fabio


"bill ramsay" ha scritto nel messaggio
...
dear all

I am playing with some 813's at the moment. I am trying to get the
heater supply sorted out.

the 813 needs 10v at 5 amps, the do glow v. bright when lit up.
pretty neat.

I have a breadboard amp going at the moment which someone else has
given me.

The current heater supply uses a 20 odd volt ac transformer, bridge
rectiifer, feeding a 2n3055 based regulator, thing is putting out
about 50watts in heat. as it is stereo there is 100w in heat being
generated.

whenever i model a 5 amp 10V heater supply in PSUD i get some really
whacky readings, ie.

28V transformer to feed 100,000u cap, with a 0.5ohm resistor, then
1000u cap into the tube. ie. 2 ohm load,

or am i missing something here, 28V ac seems a long way away from 10v
dc, or is this an artifact of the high current pulsing away and
charging the caps?

I wanted to build a dc supply that does not generate so much heat in
itself.

any thoughts, ideas etc. especially the printable ones would be
greatly appreciated.

kind regards

bill ramsay.




  #7   Report Post  
bill ramsay
 
Posts: n/a
Default

thanks for all your replies.

I think that i will give up on the dc feeding heaters on the 813 for
the meantime. The current demands are too high for passive managment
and the heat output excessive for active.

The amp I am looking at is a Stereo SE 813 device.

Soooo, I have found a supplier who can provide me with a 160 VA
toroid, with two 12v, 80VA windings.

The 813's, need 10v at 5A, So i am going to 'burn off' some of the
volts over some dropper resistors. I will put one in each leg. so to
drop the one volt per leg i will get some 0.2ohm resistors. The same
supplier has 0.2 and 0.47 ohm resistors in 5 watt packages, i will
parallel up two 0.47 beasties per leg, giving a drop of 2.35v, net
result on the heater, 9.65. Heat dissipation is in the order of 5w,
with two 5 watt resistors in parallel, I have 100% coverage, so that
is ok.

I will then use an old fashioned heater pot to null out any hum.

Also using fixed bias so i don't have to elevate the cathode/heater.

Interestingly, I have done a search on 813 amps, and, Pete Millet,
www.pmillett.addr.com\default.html uses a switched mode psu device
from teleco service to create the dc required. can't get them here in
NZ, so I am back to my dropping resistors.

I will post some pictures in the next couple of weeks on progress. I
am also putting together a web page, I have some old and interesting
articles on transformer design, how hanna curves are actually made,
interestting ultra low impedance drivers etc. I have been promising
myself some time to do this, now that it is coming into winter here,
maybe the time is ripe.

kind regards

bill


  #8   Report Post  
bill ramsay
 
Posts: n/a
Default

On Mon, 24 May 2004 21:18:11 GMT, "Fabio Berutti"
wrote:

AFAIK all valve producers gave at least a 5% allowance for heater
voltage/current. If Your mains supply is reasonably stable I'd say that a
simple CRC or CLC rectifier is OK; being the tubes used in fixed (grid) bias
(I suppose, being a 100+W tube, it is used for POWER) it is feasible to feed
2 of them from the same tranny & rectifier and reduce a bit the pounds of
steel to be used (I already had my spine repaired by a surgeon and it was no
fun at all, I now set the threshold for my projects to = 50 pounds). The
voltage can be adjusted by trial and errors simply varying a bit the R in
the CRC filter. I did it for a phono preamp (OK, 1.5 amp all together..)
and it works. Plain and easy.
IMHO I'd use plain AC filaments in such a beast. I read a WE (or maybe it
was from RCA) paper stating that large power tubes were better used with AC
filaments, because in this way filament temperature tends to average (being
either positive or negative 50 times per second), while with DC filaments,
in fact, a part of the "cathode" always works with a hotter bias and the
other with a lower one, - the filament temperature (and electron emission)
is not symmetric - the valve does not work properly. This paper showed a
graph of the filament temperature against it length in a DHT, and well, a
difference of only a few volts in a tube having a bias of about 100V was
enough to have either one half too cold or one half too hot. I'm sorry I
don't remember where I found it.

Ciao

Fabio

Fabio

thats a good point, ie. with the cathode/filament temperature, in
Pete Millets site, he mentions that he reverses the dc heater supply
every time the unit is switched on. this is to even out wear on the
filamentary cathode, don't know if this has any merit or not, it is
something to do with one side of the cathode being more negative than
the other so more electrons are emitted. sounds plausible, but I
don't know if the distribution of electron emission is too much to be
worried about, anyway see my later post, I am going back to AC with
hum pots.

[the 813 uses a filamentary cathode, ie. direct heated].


kind regards

bill.


"bill ramsay" ha scritto nel messaggio
.. .
dear all

I am playing with some 813's at the moment. I am trying to get the
heater supply sorted out.

the 813 needs 10v at 5 amps, the do glow v. bright when lit up.
pretty neat.

I have a breadboard amp going at the moment which someone else has
given me.

The current heater supply uses a 20 odd volt ac transformer, bridge
rectiifer, feeding a 2n3055 based regulator, thing is putting out
about 50watts in heat. as it is stereo there is 100w in heat being
generated.

whenever i model a 5 amp 10V heater supply in PSUD i get some really
whacky readings, ie.

28V transformer to feed 100,000u cap, with a 0.5ohm resistor, then
1000u cap into the tube. ie. 2 ohm load,

or am i missing something here, 28V ac seems a long way away from 10v
dc, or is this an artifact of the high current pulsing away and
charging the caps?

I wanted to build a dc supply that does not generate so much heat in
itself.

any thoughts, ideas etc. especially the printable ones would be
greatly appreciated.

kind regards

bill ramsay.




  #9   Report Post  
Patrick Turner
 
Posts: n/a
Default



bill ramsay wrote:

thanks for all your replies.

I think that i will give up on the dc feeding heaters on the 813 for
the meantime. The current demands are too high for passive managment
and the heat output excessive for active.

The amp I am looking at is a Stereo SE 813 device.

Soooo, I have found a supplier who can provide me with a 160 VA
toroid, with two 12v, 80VA windings.

The 813's, need 10v at 5A, So i am going to 'burn off' some of the
volts over some dropper resistors. I will put one in each leg. so to
drop the one volt per leg i will get some 0.2ohm resistors. The same
supplier has 0.2 and 0.47 ohm resistors in 5 watt packages, i will
parallel up two 0.47 beasties per leg, giving a drop of 2.35v, net
result on the heater, 9.65. Heat dissipation is in the order of 5w,
with two 5 watt resistors in parallel, I have 100% coverage, so that
is ok.

I will then use an old fashioned heater pot to null out any hum.

Also using fixed bias so i don't have to elevate the cathode/heater.

Interestingly, I have done a search on 813 amps, and, Pete Millet,
www.pmillett.addr.com\default.html uses a switched mode psu device
from teleco service to create the dc required. can't get them here in
NZ, so I am back to my dropping resistors.

I will post some pictures in the next couple of weeks on progress. I
am also putting together a web page, I have some old and interesting
articles on transformer design, how hanna curves are actually made,
interestting ultra low impedance drivers etc. I have been promising
myself some time to do this, now that it is coming into winter here,
maybe the time is ripe.

kind regards

bill


Why not have all 4 tubes running off the same
heater AC supply, simply grounded via a CT,
ie, with one winding of 5v-0-5v at 20 amps?

The tubes can have fixed bias applied to each tube, with a
10 ohm R in the plate circuit to indicate the anode current.
The screens can be run for UL or triode, or from a common supply for
straight beam tetrode.

I fixed a 300B amp last week with a DC supply to the filaments,
and the tube needs 5v at 1.2A, and its hard to see how one end of the
cathode being
5 volts more or less positive than the other would cause a major
difference in emission and
wear, since the plate current is only 65 mA.
The cathode filament appears to glow no brighter at either end.
The only problem you might have with AC heating is with hum,
which is supposed to be able to be suppressed by adjusting the balance of
the applied AC.
So you can have the AC supplied from an un-centre tapped winding, then
have
a pair of 10 ohm resistors from each end of the filament taken to a 10 ohm

pot with the wiper taken to ground.
I don't like this idea because you have a lot of DC cathode current flow
through a pot, so you
need to have a very well rated wire wound pot.
But this does allow hum nulling, if there is any from the heater, but I
would have thought
a plain grounded CT winding with fixed bias would do the trick.

As soon as you go for cathode bias, you'd need 4 filament windings,
to allow each tube to find its own bias setting via the cathode voltage.

Cathode bias works well only for circuits using a high bias current,
and a lot of class A% in the AB power characteristic for the rated load.
Then a fair amount of power is lost in the cathode R, but in for a penny
as for a pound, because
already the use of tubes uses lots of power which contributes nothing to
the audio,
such as the 200 watts of filament power for 4 tubes.

If you wound two 10v secondaries onto a 500 VA toroid using 2.5mm dia
wire, you should be able to trim the
wanted filament AV to being very close to 10vrms,
and no dropping resistors would be needed.
That would give you heater power for the two channels.
A 500VA toroid is easier to wind the thick wire secondaries onto.
And yet it still isn't a huge tranny. Unlike many toroids which are noisy
when using a rectifier producing lots of DC, the use of AC should give you
quiet running.


Patrick Turner.


  #10   Report Post  
bill ramsay
 
Posts: n/a
Default

On Tue, 25 May 2004 10:50:16 +1000, Patrick Turner
wrote:



bill ramsay wrote:

thanks for all your replies.

I think that i will give up on the dc feeding heaters on the 813 for
the meantime. The current demands are too high for passive managment
and the heat output excessive for active.

The amp I am looking at is a Stereo SE 813 device.

Soooo, I have found a supplier who can provide me with a 160 VA
toroid, with two 12v, 80VA windings.

The 813's, need 10v at 5A, So i am going to 'burn off' some of the
volts over some dropper resistors. I will put one in each leg. so to
drop the one volt per leg i will get some 0.2ohm resistors. The same
supplier has 0.2 and 0.47 ohm resistors in 5 watt packages, i will
parallel up two 0.47 beasties per leg, giving a drop of 2.35v, net
result on the heater, 9.65. Heat dissipation is in the order of 5w,
with two 5 watt resistors in parallel, I have 100% coverage, so that
is ok.

I will then use an old fashioned heater pot to null out any hum.

Also using fixed bias so i don't have to elevate the cathode/heater.

Interestingly, I have done a search on 813 amps, and, Pete Millet,
www.pmillett.addr.com\default.html uses a switched mode psu device
from teleco service to create the dc required. can't get them here in
NZ, so I am back to my dropping resistors.

I will post some pictures in the next couple of weeks on progress. I
am also putting together a web page, I have some old and interesting
articles on transformer design, how hanna curves are actually made,
interestting ultra low impedance drivers etc. I have been promising
myself some time to do this, now that it is coming into winter here,
maybe the time is ripe.

kind regards

bill


Why not have all 4 tubes running off the same
heater AC supply, simply grounded via a CT,
ie, with one winding of 5v-0-5v at 20 amps?

The tubes can have fixed bias applied to each tube, with a
10 ohm R in the plate circuit to indicate the anode current.
The screens can be run for UL or triode, or from a common supply for
straight beam tetrode.

I fixed a 300B amp last week with a DC supply to the filaments,
and the tube needs 5v at 1.2A, and its hard to see how one end of the
cathode being
5 volts more or less positive than the other would cause a major
difference in emission and
wear, since the plate current is only 65 mA.
The cathode filament appears to glow no brighter at either end.
The only problem you might have with AC heating is with hum,
which is supposed to be able to be suppressed by adjusting the balance of
the applied AC.
So you can have the AC supplied from an un-centre tapped winding, then
have
a pair of 10 ohm resistors from each end of the filament taken to a 10 ohm

pot with the wiper taken to ground.
I don't like this idea because you have a lot of DC cathode current flow
through a pot, so you
need to have a very well rated wire wound pot.
But this does allow hum nulling, if there is any from the heater, but I
would have thought
a plain grounded CT winding with fixed bias would do the trick.

As soon as you go for cathode bias, you'd need 4 filament windings,
to allow each tube to find its own bias setting via the cathode voltage.

Cathode bias works well only for circuits using a high bias current,
and a lot of class A% in the AB power characteristic for the rated load.
Then a fair amount of power is lost in the cathode R, but in for a penny
as for a pound, because
already the use of tubes uses lots of power which contributes nothing to
the audio,
such as the 200 watts of filament power for 4 tubes.

If you wound two 10v secondaries onto a 500 VA toroid using 2.5mm dia
wire, you should be able to trim the
wanted filament AV to being very close to 10vrms,
and no dropping resistors would be needed.
That would give you heater power for the two channels.
A 500VA toroid is easier to wind the thick wire secondaries onto.
And yet it still isn't a huge tranny. Unlike many toroids which are noisy
when using a rectifier producing lots of DC, the use of AC should give you
quiet running.


Patrick Turner.



Hi Patrick

As I said elsewhere, I am using two 813's in SE mode for stereo. To
use four in PP, generating over 200 Watts would need a very
substantial output transformer, and a hoist. so I am passing on that
opportunity!!!

I am going for fixed biasing, the transformer that i have found is a
toroid with two 12v windings on it, both rated at 80VA so there is
plenty of 'headroom' as it were, ie. not running at full tilt. so
with the dropping resistors i will get the volts and current that i
need.

I can get my hands on some 100ohm wirewound 3watt pots. so using one
of these across the filament, with earth on the wiper I should be
able to null out any hum.

If I work on the principle that the balance point is near the middle,
this means that 50 ohms worth of the pot will get something like 45 ma
(operating point is close to 900v at 90ma or so) per half. this is a
dissipation of just over a tenth of a watt, so with two halves, round
about a quarter of a watt. So I am well in, I could use a carbon
linear but i feel safer with the wirewound beastie.

Of course you are correct on using on large transformer, i have been
lucky here to find what i need.

As for the dc potential on the cathode, while it sounds plausible, I
cannot see that it is significant.

Kind regards

bill ramsay


  #11   Report Post  
Patrick Turner
 
Posts: n/a
Default



bill ramsay wrote:

On Tue, 25 May 2004 10:50:16 +1000, Patrick Turner
wrote:



bill ramsay wrote:

thanks for all your replies.

I think that i will give up on the dc feeding heaters on the 813 for
the meantime. The current demands are too high for passive managment
and the heat output excessive for active.

The amp I am looking at is a Stereo SE 813 device.

Soooo, I have found a supplier who can provide me with a 160 VA
toroid, with two 12v, 80VA windings.

The 813's, need 10v at 5A, So i am going to 'burn off' some of the
volts over some dropper resistors. I will put one in each leg. so to
drop the one volt per leg i will get some 0.2ohm resistors. The same
supplier has 0.2 and 0.47 ohm resistors in 5 watt packages, i will
parallel up two 0.47 beasties per leg, giving a drop of 2.35v, net
result on the heater, 9.65. Heat dissipation is in the order of 5w,
with two 5 watt resistors in parallel, I have 100% coverage, so that
is ok.

I will then use an old fashioned heater pot to null out any hum.

Also using fixed bias so i don't have to elevate the cathode/heater.

Interestingly, I have done a search on 813 amps, and, Pete Millet,
www.pmillett.addr.com\default.html uses a switched mode psu device
from teleco service to create the dc required. can't get them here in
NZ, so I am back to my dropping resistors.

I will post some pictures in the next couple of weeks on progress. I
am also putting together a web page, I have some old and interesting
articles on transformer design, how hanna curves are actually made,
interestting ultra low impedance drivers etc. I have been promising
myself some time to do this, now that it is coming into winter here,
maybe the time is ripe.

kind regards

bill


Why not have all 4 tubes running off the same
heater AC supply, simply grounded via a CT,
ie, with one winding of 5v-0-5v at 20 amps?

The tubes can have fixed bias applied to each tube, with a
10 ohm R in the plate circuit to indicate the anode current.
The screens can be run for UL or triode, or from a common supply for
straight beam tetrode.

I fixed a 300B amp last week with a DC supply to the filaments,
and the tube needs 5v at 1.2A, and its hard to see how one end of the
cathode being
5 volts more or less positive than the other would cause a major
difference in emission and
wear, since the plate current is only 65 mA.
The cathode filament appears to glow no brighter at either end.
The only problem you might have with AC heating is with hum,
which is supposed to be able to be suppressed by adjusting the balance of
the applied AC.
So you can have the AC supplied from an un-centre tapped winding, then
have
a pair of 10 ohm resistors from each end of the filament taken to a 10 ohm

pot with the wiper taken to ground.
I don't like this idea because you have a lot of DC cathode current flow
through a pot, so you
need to have a very well rated wire wound pot.
But this does allow hum nulling, if there is any from the heater, but I
would have thought
a plain grounded CT winding with fixed bias would do the trick.

As soon as you go for cathode bias, you'd need 4 filament windings,
to allow each tube to find its own bias setting via the cathode voltage.

Cathode bias works well only for circuits using a high bias current,
and a lot of class A% in the AB power characteristic for the rated load.
Then a fair amount of power is lost in the cathode R, but in for a penny
as for a pound, because
already the use of tubes uses lots of power which contributes nothing to
the audio,
such as the 200 watts of filament power for 4 tubes.

If you wound two 10v secondaries onto a 500 VA toroid using 2.5mm dia
wire, you should be able to trim the
wanted filament AV to being very close to 10vrms,
and no dropping resistors would be needed.
That would give you heater power for the two channels.
A 500VA toroid is easier to wind the thick wire secondaries onto.
And yet it still isn't a huge tranny. Unlike many toroids which are noisy
when using a rectifier producing lots of DC, the use of AC should give you
quiet running.


Patrick Turner.


Hi Patrick

As I said elsewhere, I am using two 813's in SE mode for stereo. To
use four in PP, generating over 200 Watts would need a very
substantial output transformer, and a hoist. so I am passing on that
opportunity!!!


Ah, I misread your use of a pair of tubes in SE, one for each channel.
That still means 100 watts for the heaters, in two separate supplies.



I am going for fixed biasing, the transformer that i have found is a
toroid with two 12v windings on it, both rated at 80VA so there is
plenty of 'headroom' as it were, ie. not running at full tilt. so
with the dropping resistors i will get the volts and current that i
need.

I can get my hands on some 100ohm wirewound 3watt pots. so using one
of these across the filament, with earth on the wiper I should be
able to null out any hum.


Speaker level adjust pots may do, but maybe you have to go to Farnell
for something nice...



If I work on the principle that the balance point is near the middle,
this means that 50 ohms worth of the pot will get something like 45 ma
(operating point is close to 900v at 90ma or so) per half. this is a
dissipation of just over a tenth of a watt, so with two halves, round
about a quarter of a watt. So I am well in, I could use a carbon
linear but i feel safer with the wirewound beastie.


True, a 50 ohm pot would do.
But *definately* not a carbon type.
10vrms across the pot gives 2W, plus the DC diss
of 0.1 watts from the cathode current.



Of course you are correct on using on large transformer, i have been
lucky here to find what i need.

As for the dc potential on the cathode, while it sounds plausible, I
cannot see that it is significant.


I'd try AC heating first.

With 813 in SE, perhaps you could run
a cathode feedback winding for local FB, using between 12% and 20%
of the primary turns in a winding devoted to the cathode.
I suspect the tetrode class A performance probably won't be very linear.

Ordinary UL would be difficult because of the screen rating, so better to take
the screen
to the lowest fixed voltage without compromising the anode voltage swing.
This means you may have 900v anode supply, but
perhaps only 350v screen supply, and this means the grid bias voltage needed is
lower.
For SE, I quite like cathode bias which sets itself reliably, and in use it
does not drift.

With my SE amps using 4 x 6CA7 in parallel, I have 4 separate RC, one for each
tube's cathode circuit,
consisting of 240 ohms and 1,000 uF bypass cap.
These four RC networks are joined commonly to the top of the feedback winding
from the OPT.

In your case, there is nothing to stop you connecting the CT of the filament
winding
to the top of an RC network for biasing which can be taken to a FB winding at
the bottom,
or just ground the bottom of the RC network if pure terode is tried.

The FB winding will reduce the gain of the 813 considerably, so more drive is
required,
but the benefit is that the tube with CFB acts like triode, ie,
thd will be dramatically reduced, and its spectral content shoulod be better
than in pure tetrode, and the Ro should be a couple of ohms at the OPT sec.

The cathode FB does not only work to provide series voltage negative feedback
in series with the grid voltage.
The distortion of the tube is not only fed back inverted in phase to the grid,
but also to the screen, since there is a signal difference between screen and
cathode with
CFB, unless the screen is bypassed with a large cap to the cathode
instead of to ground.

Basically, using CFB the way I am humbly suggesting is similar to
having 1/2 a Quad II circuit.

Where the screen is bypassed to the cathode, then the cathode circuit
sees the impedance of the dropping resistor from the B+ to the
screen supply, so the value of R dropper shouldn't be too small.
Anything over a kohm would be OK.

Where the screen is fully bypassed to the cathode, the tube is operating in
pure beam
tetrode mode.
Where CFB is used in this case, the spectra of harmonics produced is similar to

plain beam tetrode without any CFB, although its reduced by the CFB,
to an amount about equal to the amount of gain reduction.
The Ro due to CFB is usually slightly more with a bypassed screen to cathode.
But the simplest way, and the best way, imho, is to use the screen
bypassed to 0V.

You should also find that the driver tube may have to work a little harder with
CFB,
but then you also find that the 2H of the driver will more likely
tend to cancel the 2H of the output tube, which is a bonus, imho.

I guess you may be aiming to use about 8k for a load, and get a swing of nearly
800v peak,
which would give you just on 40 watts.
Grid bias seems to be only about -15v required for 90mA at 900v.
Since Gm is about 4 mA/V, then the tetrode gain into
8 k would be about 32 times, so for 565vrms of plate signal,
some 17.65 volts at the grid is required, and this exceeds the
negative bias voltage, so only class A2 seems possible for full power.
If there was 12.5% CFB, ( 1/8 of the total primary turns, or three layers of 24
layers of wire
in the primary, ) then the CFB voltage would be 70v, and plate voltage 494v,
and the grid input would be about 90 vrms, which sounds like a lot,
until you work out a decent way to make 90vrms at low thd.
The anode resistance of the pure tetrode 813 looks very high from the valve
curves,
because the plate resistance line is nearly horizontal at 90 mA of bias
current,
and my guess the Ra = approximately 40 kOhms.
Because Gm = about 4 mA/V, the tube U is around 160 at the bias point
proposed.
If the screen was bypassed directly to the cathode, you'd
have 18vrms needed between G1 and K to make the tube produce the
565 A to K signal voltage, and there would be the 70v at the K.
The fraction of the output fed back in series with the G1 signal
is 12.5%, or 1/8, ie, Beta, B, is 0.125.

From this, knowing U is 160, and knowing Ra = 40,000, we can work
out the effective Ra after feedback is applied.
Ra' = Ra / ( 1 + [U x B] ) = 40,000 / ( 1 + [ 160 x 0.125 ] ) = 40,000 / 21
= 1.9k.

If the OPT has a Z ratio of 8k to 5 ohms, or 1,600 to 1,
then the effective Ra' appears at the secondary as
1,900 / 1600 ohms, plus the secondary winding resistance,
so expect to see Ro at the sec = 1.5 ohms.
Bypassing G2 to 0V instead of to the K will give a figure slightly
higher than the pure tetrode with FB case, but methinks the spectra
with bypassing to 0V is better.
Even 10% of CFB would be very beneficial .

The effectively applied amount of CFB varies with load value,
since thre gain of a tetrode varies almost directly as the load value
so that where the load is halved, so is the gain,
but with the CFB, the variations in applied grid voltage are
very much reduced for different load values.
So the driver does not have to produce sucha wide range of voltages
for different loads, and perhaps there is less imd produced in the driver as a
result if global FB is also applied to the amp to reduce Ro
below 0.5 ohms.
I found that in my amps with 6CA7, only about 8 dB
of global FB was needed in addition to the CFB.

In my case the 4 paralleled 6AC7 tubes
gave me a composite tube where Gm = about 40 mA/V,
Ra = 3k, U = 130, so with
B = 0.125, Ra' = 173 ohms, and since the opt has a Z ratio
of 246 :1, Ro = about 0.7 + Rw = 1ohm, approx,
for where G2 was bypassed to the K, but
I have bypassed G2 to 0V, and Ro is about 1.6 ohms.
With global FB, it is about 0.6 ohms.

In my amps the OPT has a primary load of 1,200 ohms,
so the leakage inductance needs to be 6.6 times lower than a tranny
with a load of 8k, for the same HF response.
This is easy to achieve with interleaving, and there are only 1,904 turns on
the primary
on a 60 stack of 44 tongue material.
The maximum signal voltage across the P winding from A to K is about 220vrms.
With an 813, there will be about 565 vrms, so
the P turns would have to be 4,889, if the core was the same size.
In other words, 2.56 times the turns I am using.
With the same winding interleaving geometry, this means the LL
will be 6.6 times that inmy tranny.
There will also be 6.6 times the primary inductance, which is a good thing,
because the relationship between P inductance to load
should be maintained so ZLp = RL at 20 Hz.
In my case, RL is 1.2k, and I have over 10H of Lp.
But with 813, and RL = 8k, you'd need 66H to get the same LF pole I am getting.

I would use a 75 stack of 44 tongue for the OPT for tubes like the 813,
or 845, GM70, 211, etc.
This allows the turns to be reduced, and thus the wire to be thicker,
and the winding losses to be lower, since getting copper losses below
10% isn't all that easy with SE amps.

I am not advising anyone on what they must do, or ought to do,
but just trying to explain some of the issues involved, and leaving options
open.

For example, an SE amp with 813 in pure tetrode could be set up
with nothing but global FB, and to equal the above figures using local CFB
and a small amount of global, some 20 dB of global FB has to be used
to get Ro down from 25 ohms without any FB whatsover
to about 0.5 ohms.
If this is done, the thd for the 20 dB of GFB will be lower than the combined
FB method
because the driver stage is also fully enclosed by the FB loop,
and the drive voltage applied to the output tube is much lower.

Whether it sounds as well is a moot point.

Patrick Turner.





Kind regards

bill ramsay


  #12   Report Post  
bill ramsay
 
Posts: n/a
Default



I'd try AC heating first.

With 813 in SE, perhaps you could run
a cathode feedback winding for local FB, using between 12% and 20%
of the primary turns in a winding devoted to the cathode.
I suspect the tetrode class A performance probably won't be very linear.

I had thought about that after reading your earlier article/post on
that matter.

I have found someone in Auckland who can wind transformers for me. I
have a pair of SE 6k trannies with an Ultralinear tap on it.



Ordinary UL would be difficult because of the screen rating, so better to take
the screen
to the lowest fixed voltage without compromising the anode voltage swing.
This means you may have 900v anode supply, but
perhaps only 350v screen supply, and this means the grid bias voltage needed is
lower.


I have been looking at the specs for the 813 and the max screen
voltage is 750V [ this is from the AM modulator /power amp ratings].
so using the UL should not be such a problem.

The curves in pentode mode the curves look ugly, and if listening to
the breadboard model that i have at the moment is anything to go by,
they are really ugly to listen to as well.

So i think that I will be following pete Milletts lead on this and
going with triode strapping. If you go to diyaudio.com and do a
search on 813 in the tube section you will come across a set of triode
curves which are absolutely gorgeous. Mind you the power goes down,
but who cares, 21W vs 30 odd, it is stil too loud.


For SE, I quite like cathode bias which sets itself reliably, and in use it
does not drift.


I quite like the idea of cathode bias as well, However, I was not
keen on having too high a voltage hanging around the hum pots etc.

I have doodled using two pots, one across the filament to null the ac
ripple, then taking the wiper to ground through a resistor and
through another 100 ohm pot so i could vary the bias. I will strap
the wiper to the top of the pot so if it ever comes off, the cathode
will lift higher, driving the tube out of conduction. Time will tell,
it is much simpler and this method ages well as it were. You lose
valuable volts tho. I have plenty of time to try both.


With my SE amps using 4 x 6CA7 in parallel, I have 4 separate RC, one for each
tube's cathode circuit,
consisting of 240 ohms and 1,000 uF bypass cap.
These four RC networks are joined commonly to the top of the feedback winding
from the OPT.

In your case, there is nothing to stop you connecting the CT of the filament
winding
to the top of an RC network for biasing which can be taken to a FB winding at
the bottom,
or just ground the bottom of the RC network if pure terode is tried.

The FB winding will reduce the gain of the 813 considerably, so more drive is
required,
but the benefit is that the tube with CFB acts like triode, ie,
thd will be dramatically reduced, and its spectral content shoulod be better
than in pure tetrode, and the Ro should be a couple of ohms at the OPT sec.

The cathode FB does not only work to provide series voltage negative feedback
in series with the grid voltage.
The distortion of the tube is not only fed back inverted in phase to the grid,
but also to the screen, since there is a signal difference between screen and
cathode with
CFB, unless the screen is bypassed with a large cap to the cathode
instead of to ground.

Basically, using CFB the way I am humbly suggesting is similar to
having 1/2 a Quad II circuit.

Where the screen is bypassed to the cathode, then the cathode circuit
sees the impedance of the dropping resistor from the B+ to the
screen supply, so the value of R dropper shouldn't be too small.
Anything over a kohm would be OK.

Where the screen is fully bypassed to the cathode, the tube is operating in
pure beam
tetrode mode.
Where CFB is used in this case, the spectra of harmonics produced is similar to

plain beam tetrode without any CFB, although its reduced by the CFB,
to an amount about equal to the amount of gain reduction.
The Ro due to CFB is usually slightly more with a bypassed screen to cathode.
But the simplest way, and the best way, imho, is to use the screen
bypassed to 0V.

You should also find that the driver tube may have to work a little harder with
CFB,
but then you also find that the 2H of the driver will more likely
tend to cancel the 2H of the output tube, which is a bonus, imho.


I guess you may be aiming to use about 8k for a load, and get a swing of nearly
800v peak,
which would give you just on 40 watts.
Grid bias seems to be only about -15v required for 90mA at 900v.
Since Gm is about 4 mA/V, then the tetrode gain into
8 k would be about 32 times, so for 565vrms of plate signal,
some 17.65 volts at the grid is required, and this exceeds the
negative bias voltage, so only class A2 seems possible for full power.


Youi know i thought that the grid bias on the breadboard that i have
is all wrong, tis currently sitting at about -90v, which means that
the signal does not have far to go when the tube goes out of
conduction. in fact when the signal level goes too high you can see
something is wrong, it distorts terribly, and the ammeter in the
cathode, [voltmeter across a sense resistor] starts to wobble. ie.
no longer class A.


If there was 12.5% CFB, ( 1/8 of the total primary turns, or three layers of 24
layers of wire
in the primary, ) then the CFB voltage would be 70v, and plate voltage 494v,
and the grid input would be about 90 vrms, which sounds like a lot,
until you work out a decent way to make 90vrms at low thd.


Now it is funny you should say that, I am/have also played with some
IXYS high voltage constant current reglators, theser are trickty
things, once used one as a load of a SE 807 with a parafeed PP
oputput tranny. Wasn't very loud, only a watt or two but it proved
the point. Too high plate voltages for such measly power tho. the CC
reg that i am working with can withstand 900v, conduct 250ma [these
are at the extremes you realise] but they can only dissipate 20W. and
that does not bear close scrutiny either, you would need a heatsink
the size of the queen mary. possibly water cooled. anyway, i am
looking to use the good oly 6sn7 with a ccs plate load, according to
pete milletts site, it is good for 90 volts rms, with distortion
obviously, but mainly 2H.



The anode resistance of the pure tetrode 813 looks very high from the valve
curves,
because the plate resistance line is nearly horizontal at 90 mA of bias
current,
and my guess the Ra = approximately 40 kOhms.
Because Gm = about 4 mA/V, the tube U is around 160 at the bias point
proposed.
If the screen was bypassed directly to the cathode, you'd
have 18vrms needed between G1 and K to make the tube produce the
565 A to K signal voltage, and there would be the 70v at the K.
The fraction of the output fed back in series with the G1 signal
is 12.5%, or 1/8, ie, Beta, B, is 0.125.


yup, not nice, so i will be sticking with the triode curves.


From this, knowing U is 160, and knowing Ra = 40,000, we can work
out the effective Ra after feedback is applied.
Ra' = Ra / ( 1 + [U x B] ) = 40,000 / ( 1 + [ 160 x 0.125 ] ) = 40,000 / 21
= 1.9k.

If the OPT has a Z ratio of 8k to 5 ohms, or 1,600 to 1,
then the effective Ra' appears at the secondary as
1,900 / 1600 ohms, plus the secondary winding resistance,
so expect to see Ro at the sec = 1.5 ohms.
Bypassing G2 to 0V instead of to the K will give a figure slightly
higher than the pure tetrode with FB case, but methinks the spectra
with bypassing to 0V is better.
Even 10% of CFB would be very beneficial .

The effectively applied amount of CFB varies with load value,
since thre gain of a tetrode varies almost directly as the load value
so that where the load is halved, so is the gain,
but with the CFB, the variations in applied grid voltage are
very much reduced for different load values.
So the driver does not have to produce sucha wide range of voltages
for different loads, and perhaps there is less imd produced in the driver as a
result if global FB is also applied to the amp to reduce Ro
below 0.5 ohms.
I found that in my amps with 6CA7, only about 8 dB
of global FB was needed in addition to the CFB.

In my case the 4 paralleled 6AC7 tubes
gave me a composite tube where Gm = about 40 mA/V,
Ra = 3k, U = 130, so with
B = 0.125, Ra' = 173 ohms, and since the opt has a Z ratio
of 246 :1, Ro = about 0.7 + Rw = 1ohm, approx,
for where G2 was bypassed to the K, but
I have bypassed G2 to 0V, and Ro is about 1.6 ohms.
With global FB, it is about 0.6 ohms.

In my amps the OPT has a primary load of 1,200 ohms,
so the leakage inductance needs to be 6.6 times lower than a tranny
with a load of 8k, for the same HF response.
This is easy to achieve with interleaving, and there are only 1,904 turns on
the primary
on a 60 stack of 44 tongue material.
The maximum signal voltage across the P winding from A to K is about 220vrms.
With an 813, there will be about 565 vrms, so
the P turns would have to be 4,889, if the core was the same size.
In other words, 2.56 times the turns I am using.
With the same winding interleaving geometry, this means the LL
will be 6.6 times that inmy tranny.
There will also be 6.6 times the primary inductance, which is a good thing,
because the relationship between P inductance to load
should be maintained so ZLp = RL at 20 Hz.
In my case, RL is 1.2k, and I have over 10H of Lp.
But with 813, and RL = 8k, you'd need 66H to get the same LF pole I am getting.

I would use a 75 stack of 44 tongue for the OPT for tubes like the 813,
or 845, GM70, 211, etc.
This allows the turns to be reduced, and thus the wire to be thicker,
and the winding losses to be lower, since getting copper losses below
10% isn't all that easy with SE amps.


Thanks for this, I have been playing with transformer designs as well,
but nearly always gave up as i did not have the confidence to build
one. Now that I have found someone and seen there work [only last
night on the breadboard] i will probably do something about it. I am
not confident on the OP{ tranny that i have at the moment now that I
understand the design process that went into it. [goes something like
this, VA rating of the core for equivalent power tranny used, windings
something like an old design, no-one knows anything about the maths,
only the construction]. So my part in this is to get the maths right,
then the design and get someone else to build it. Once i have my
breadboard rebuilt, if the transformer sucks, i will bounce my
design ideas off you, ok?


I am not advising anyone on what they must do, or ought to do,
but just trying to explain some of the issues involved, and leaving options
open.

For example, an SE amp with 813 in pure tetrode could be set up
with nothing but global FB, and to equal the above figures using local CFB
and a small amount of global, some 20 dB of global FB has to be used
to get Ro down from 25 ohms without any FB whatsover
to about 0.5 ohms.
If this is done, the thd for the 20 dB of GFB will be lower than the combined
FB method
because the driver stage is also fully enclosed by the FB loop,
and the drive voltage applied to the output tube is much lower.


I had been looking to use the 811A [the real one], and global feedback
would most certainly have been necessary, the curves are pretty
cramped. the second deal killer was the need for input power as the
811 operates in A2. the upside is that it only needs about 20Vrms to
get full signal out of it, approx 16W, so it is not that bad. again
the transformer was peculiar, approx 4k, i would have a CFB in that
to help with the distortion taming.

thanks for the post.

will keep you posted on progress.

This breadboard i have at the moment belongs to another chap who has
built one for himself, I am going to construct one from scratch
myself this weekend, using his op transformers.

I will try and take pictures.

kind regards

bill





Whether it sounds as well is a moot point.

Patrick Turner.





Kind regards

bill ramsay


  #13   Report Post  
Patrick Turner
 
Posts: n/a
Default



bill ramsay wrote:


I'd try AC heating first.

With 813 in SE, perhaps you could run
a cathode feedback winding for local FB, using between 12% and 20%
of the primary turns in a winding devoted to the cathode.
I suspect the tetrode class A performance probably won't be very linear.

I had thought about that after reading your earlier article/post on
that matter.

I have found someone in Auckland who can wind transformers for me. I
have a pair of SE 6k trannies with an Ultralinear tap on it.


6k to what? if it was 6k to 8 ohms, then if there was a dip to 4 ohms in the
speaker,
the amp wouldn't like that much, but you can't much reduce Ea and increase Ia
to suit the loading because the 813 needs about 900v for Ea;
it seems from the curves that it gets more non linear as the B+ is reduced.

Is the person a commercial winder?
What is his name if you don't mind me enquiring?
One reasom is that I get a few private emails from NZ ppl wanting
laminations and gear for DIY trannies, and perhaps someone to wind something for
them...



Ordinary UL would be difficult because of the screen rating, so better to take
the screen
to the lowest fixed voltage without compromising the anode voltage swing.
This means you may have 900v anode supply, but
perhaps only 350v screen supply, and this means the grid bias voltage needed is
lower.


I have been looking at the specs for the 813 and the max screen
voltage is 750V [ this is from the AM modulator /power amp ratings].
so using the UL should not be such a problem.


Whoa there, the max G2 rating may well be 750v, but this means
that the grid bias would have to be much greater, to compensate for the
rise in Eg2, which raises Ia, if nothing is done to
reduce Eg1, to keep the bias current low as you want.
for DC conditions, UL is just like triode connecting,
since Ea = EG2.
But it may work out better, because the more negative G1, the less grid current
you have to provide.
A guy used a pair of PP 813 in triode in the London Audio Club,
and his project is described on their website. He only got around 55 watts.

The curves in pentode mode the curves look ugly, and if listening to
the breadboard model that i have at the moment is anything to go by,
they are really ugly to listen to as well.


UL may surprise you.
With my 13E1, the tube was bleedin awful in tetrode,
but 66% taps for UL gives me 25 watts from 72 watts of plate and screen dissipation.

Only 15 watts in triode, so the slight swing to UL gives me
10 "free" watts, with no increase in the thd, which is just like a triode's.
So if you experiment with UL, and you only have a 40% tap,
try reversing the tranny around to get 60% taps; it may work better than 40%.


So i think that I will be following pete Milletts lead on this and
going with triode strapping. If you go to diyaudio.com and do a
search on 813 in the tube section you will come across a set of triode
curves which are absolutely gorgeous. Mind you the power goes down,
but who cares, 21W vs 30 odd, it is stil too loud.

For SE, I quite like cathode bias which sets itself reliably, and in use it
does not drift.


I quite like the idea of cathode bias as well, However, I was not
keen on having too high a voltage hanging around the hum pots etc.


This does not matter. The cathode could be up at 100volts,
but the whole heater winding and adjust pots don't see any extra stressfull
voltages.



I have doodled using two pots, one across the filament to null the ac
ripple, then taking the wiper to ground through a resistor and
through another 100 ohm pot so i could vary the bias. I will strap
the wiper to the top of the pot so if it ever comes off, the cathode
will lift higher, driving the tube out of conduction. Time will tell,
it is much simpler and this method ages well as it were. You lose
valuable volts tho. I have plenty of time to try both.


IN triode or UL, maybe you will find the cathode voltage is so high that
you can halve it by having 50v of fixed bias as well as the cathode bias,
which will still self regulate fairly well.

I don't know what Ra an 813 has in triode, probably down at 1k to 2k,
and U is probably quite low, maybe only 7, but with a cathode R of 500 ohms,
the effective Ra as far as DC is concerned is about 5k.
So variations in B+ won't give the dramatic swings in idle currents
which regulated fixed bias gives.
Put simply, If Rk is 500 ohms, it should self regulate the Ia fairly well.




With my SE amps using 4 x 6CA7 in parallel, I have 4 separate RC, one for each
tube's cathode circuit,
consisting of 240 ohms and 1,000 uF bypass cap.
These four RC networks are joined commonly to the top of the feedback winding
from the OPT.

In your case, there is nothing to stop you connecting the CT of the filament
winding
to the top of an RC network for biasing which can be taken to a FB winding at
the bottom,
or just ground the bottom of the RC network if pure terode is tried.

The FB winding will reduce the gain of the 813 considerably, so more drive is
required,
but the benefit is that the tube with CFB acts like triode, ie,
thd will be dramatically reduced, and its spectral content shoulod be better
than in pure tetrode, and the Ro should be a couple of ohms at the OPT sec.

The cathode FB does not only work to provide series voltage negative feedback
in series with the grid voltage.
The distortion of the tube is not only fed back inverted in phase to the grid,
but also to the screen, since there is a signal difference between screen and
cathode with
CFB, unless the screen is bypassed with a large cap to the cathode
instead of to ground.

Basically, using CFB the way I am humbly suggesting is similar to
having 1/2 a Quad II circuit.

Where the screen is bypassed to the cathode, then the cathode circuit
sees the impedance of the dropping resistor from the B+ to the
screen supply, so the value of R dropper shouldn't be too small.
Anything over a kohm would be OK.

Where the screen is fully bypassed to the cathode, the tube is operating in
pure beam
tetrode mode.
Where CFB is used in this case, the spectra of harmonics produced is similar to

plain beam tetrode without any CFB, although its reduced by the CFB,
to an amount about equal to the amount of gain reduction.
The Ro due to CFB is usually slightly more with a bypassed screen to cathode.
But the simplest way, and the best way, imho, is to use the screen
bypassed to 0V.

You should also find that the driver tube may have to work a little harder with
CFB,
but then you also find that the 2H of the driver will more likely
tend to cancel the 2H of the output tube, which is a bonus, imho.


I guess you may be aiming to use about 8k for a load, and get a swing of nearly
800v peak,
which would give you just on 40 watts.
Grid bias seems to be only about -15v required for 90mA at 900v.
Since Gm is about 4 mA/V, then the tetrode gain into
8 k would be about 32 times, so for 565vrms of plate signal,
some 17.65 volts at the grid is required, and this exceeds the
negative bias voltage, so only class A2 seems possible for full power.


Youi know i thought that the grid bias on the breadboard that i have
is all wrong, tis currently sitting at about -90v, which means that
the signal does not have far to go when the tube goes out of
conduction. in fact when the signal level goes too high you can see
something is wrong, it distorts terribly, and the ammeter in the
cathode, [voltmeter across a sense resistor] starts to wobble. ie.
no longer class A.


Maybe you got a high EG2 if you are in pure tetrode.
The curves I have show only -15v for G1 where EG2 is at +300v
The whole of the plate resistance line when Eg1 = 0V is under
the horizontal 150 mA plate current line.



If there was 12.5% CFB, ( 1/8 of the total primary turns, or three layers of 24
layers of wire
in the primary, ) then the CFB voltage would be 70v, and plate voltage 494v,
and the grid input would be about 90 vrms, which sounds like a lot,
until you work out a decent way to make 90vrms at low thd.


Now it is funny you should say that, I am/have also played with some
IXYS high voltage constant current reglators, theser are trickty
things, once used one as a load of a SE 807 with a parafeed PP
oputput tranny. Wasn't very loud, only a watt or two but it proved
the point. Too high plate voltages for such measly power tho. the CC
reg that i am working with can withstand 900v, conduct 250ma [these
are at the extremes you realise] but they can only dissipate 20W. and
that does not bear close scrutiny either, you would need a heatsink
the size of the queen mary. possibly water cooled. anyway, i am
looking to use the good oly 6sn7 with a ccs plate load, according to
pete milletts site, it is good for 90 volts rms, with distortion
obviously, but mainly 2H.


I don't know what driver you have, but a guy in Melbourne took my advice
for a driver for an 845 which has a 6BX7.
We decided he ought to have at least 20H, so after I gave him a general recipe,
he winds a choke and got a lot more than that,
and then placed the choke between the B+ and the existing DC supply resistor.
He then got a much wider voltage swing and lower thd, and a much better sound,
from waht he said later.
The 845 has its grid still cap coupled to the 6BX7 plates,
but because there effectively is much power being wasted in the DC load R,
he could reduce the grid bias R to make sure the grid voltage stays put.
The AC load the 6BX7 is the impedance of the 8k DC R and L in series, which is
always at least 8k at DC and at HF, but at 1 kHz, the R + L
might be 300 k, a negligible load which approaches a CCS, leaving the 100k grid bias
R
as the main load driven, which allows for a more horizontal load line,
and a wider V swing.
Some folks like a 2A3 as a driver, choke loaded.

In my latest SE amps, even with CFB, I only need a max of 38vrms
drive voltage, and the paralleled 12BH7 copes well,
and sees a combined AC/DC load of 44k, plenty high enough.

But in the inpur stage which is 12AU7, I have a CCS using a single
MJE350 at 4 mA, and the AC/DC load is the 220k following cap coupled
bias R.
The thd of the AU7 was halved when I went to using a CCS instead of the
75k DC supply R.

The 2H of the input tube tends to add to that of the output tube, so its thd should
be low.


The anode resistance of the pure tetrode 813 looks very high from the valve
curves,
because the plate resistance line is nearly horizontal at 90 mA of bias
current,
and my guess the Ra = approximately 40 kOhms.
Because Gm = about 4 mA/V, the tube U is around 160 at the bias point
proposed.
If the screen was bypassed directly to the cathode, you'd
have 18vrms needed between G1 and K to make the tube produce the
565 A to K signal voltage, and there would be the 70v at the K.
The fraction of the output fed back in series with the G1 signal
is 12.5%, or 1/8, ie, Beta, B, is 0.125.


yup, not nice, so i will be sticking with the triode curves.

From this, knowing U is 160, and knowing Ra = 40,000, we can work
out the effective Ra after feedback is applied.
Ra' = Ra / ( 1 + [U x B] ) = 40,000 / ( 1 + [ 160 x 0.125 ] ) = 40,000 / 21
= 1.9k.

If the OPT has a Z ratio of 8k to 5 ohms, or 1,600 to 1,
then the effective Ra' appears at the secondary as
1,900 / 1600 ohms, plus the secondary winding resistance,
so expect to see Ro at the sec = 1.5 ohms.
Bypassing G2 to 0V instead of to the K will give a figure slightly
higher than the pure tetrode with FB case, but methinks the spectra
with bypassing to 0V is better.
Even 10% of CFB would be very beneficial .

The effectively applied amount of CFB varies with load value,
since thre gain of a tetrode varies almost directly as the load value
so that where the load is halved, so is the gain,
but with the CFB, the variations in applied grid voltage are
very much reduced for different load values.
So the driver does not have to produce sucha wide range of voltages
for different loads, and perhaps there is less imd produced in the driver as a
result if global FB is also applied to the amp to reduce Ro
below 0.5 ohms.
I found that in my amps with 6CA7, only about 8 dB
of global FB was needed in addition to the CFB.

In my case the 4 paralleled 6AC7 tubes
gave me a composite tube where Gm = about 40 mA/V,
Ra = 3k, U = 130, so with
B = 0.125, Ra' = 173 ohms, and since the opt has a Z ratio
of 246 :1, Ro = about 0.7 + Rw = 1ohm, approx,
for where G2 was bypassed to the K, but
I have bypassed G2 to 0V, and Ro is about 1.6 ohms.
With global FB, it is about 0.6 ohms.

In my amps the OPT has a primary load of 1,200 ohms,
so the leakage inductance needs to be 6.6 times lower than a tranny
with a load of 8k, for the same HF response.
This is easy to achieve with interleaving, and there are only 1,904 turns on
the primary
on a 60 stack of 44 tongue material.
The maximum signal voltage across the P winding from A to K is about 220vrms.
With an 813, there will be about 565 vrms, so
the P turns would have to be 4,889, if the core was the same size.
In other words, 2.56 times the turns I am using.
With the same winding interleaving geometry, this means the LL
will be 6.6 times that inmy tranny.
There will also be 6.6 times the primary inductance, which is a good thing,
because the relationship between P inductance to load
should be maintained so ZLp = RL at 20 Hz.
In my case, RL is 1.2k, and I have over 10H of Lp.
But with 813, and RL = 8k, you'd need 66H to get the same LF pole I am getting.

I would use a 75 stack of 44 tongue for the OPT for tubes like the 813,
or 845, GM70, 211, etc.
This allows the turns to be reduced, and thus the wire to be thicker,
and the winding losses to be lower, since getting copper losses below
10% isn't all that easy with SE amps.


Thanks for this, I have been playing with transformer designs as well,
but nearly always gave up as i did not have the confidence to build
one. Now that I have found someone and seen there work [only last
night on the breadboard] i will probably do something about it. I am
not confident on the OP{ tranny that i have at the moment now that I
understand the design process that went into it. [goes something like
this, VA rating of the core for equivalent power tranny used, windings
something like an old design, no-one knows anything about the maths,
only the construction]. So my part in this is to get the maths right,
then the design and get someone else to build it. Once i have my
breadboard rebuilt, if the transformer sucks, i will bounce my
design ideas off you, ok?


Unless you have the maths right, its guesswork, which nearly
always leads to regret and compromises.
But the design of an SE tranny is not difficult.
The HF performance follows the same rules as a PP design.
Above 2 kHz, the iron cores could be pulled out of tube audio amps, and the
performance should not be impaired!
When Iron is used for the LF part of audio in SE amps, the
trick is to consider the OPT as one would a choke with a large AV across it,
and gapping needs doing with care to get the maximum inductance,
but without saturation as F is rolled down below 20 Hz.
T achieve that I try to make ZLp = RL at 20 Hz.
This means at the full 1 kHz power, the tube will see a load
which is partially reactive at 20 Hz, in fact it will be
0.707 times RL, and the early signs of SE clipping will be seen on a CRO.
Triodes don't lose much gain at the lower than full power levels when this Lp:RL
ratio is used.
Tetrodes do lose gain due to RL being shunted by L, but then at LF, the tetrode amp
often sees a high value load because of higher speaker impedances at LF,
so the driving source resistance powering the OPT at LF is very high,
so the thd is very high, and gain, before F is low enough to allow L shunting,
which means NFB is essential to control the bass response and thd..

The turns for the size of core can usually be worked on the basis of
the B being no greater than 0.3 Tesla at 50 Hz.
Losses should be less tha 10%.
There is a bit of a guide at my website in the section on output transformers.


No reason why the process cannot be part of the discussions here
so anyone else can comment, or learn something.


I am not advising anyone on what they must do, or ought to do,
but just trying to explain some of the issues involved, and leaving options
open.

For example, an SE amp with 813 in pure tetrode could be set up
with nothing but global FB, and to equal the above figures using local CFB
and a small amount of global, some 20 dB of global FB has to be used
to get Ro down from 25 ohms without any FB whatsover
to about 0.5 ohms.
If this is done, the thd for the 20 dB of GFB will be lower than the combined
FB method
because the driver stage is also fully enclosed by the FB loop,
and the drive voltage applied to the output tube is much lower.


I had been looking to use the 811A [the real one], and global feedback
would most certainly have been necessary, the curves are pretty
cramped. the second deal killer was the need for input power as the
811 operates in A2. the upside is that it only needs about 20Vrms to
get full signal out of it, approx 16W, so it is not that bad. again
the transformer was peculiar, approx 4k, i would have a CFB in that
to help with the distortion taming.

thanks for the post.

will keep you posted on progress.

This breadboard i have at the moment belongs to another chap who has
built one for himself, I am going to construct one from scratch
myself this weekend, using his op transformers.

I will try and take pictures.

kind regards

bill


Just beware of the 900 volts, its quite deadly if care isn't taken....

Patrick Turner.




Whether it sounds as well is a moot point.

Patrick Turner.





Kind regards

bill ramsay


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