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