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