[Patrick Turner]

On Oct 7, 10:21*pm, "GRe" wrote:
Has anybody tried or seen a SE topology as shown below?
(Put reader to Courier-New font to view the schematic)
I'm asking because for a fun project I'm trying to get
an idea whether the topology is viable or not.

T1 is a transformer without gap. To cancel the field in
the core, caused by the primary current Ip, a current
Is of opposite direction, with a current value of Ip*N,
is imposed upon the secondary by a constant current sink.
N is the winding ratio, V+ is a regulated voltage.

* * * * * * * Ip -- * *T1 * *Is --
* * * * * *+----------) | (----------- V+
* * * * * *| * * * * *) | (
* * * * V1 | * * * * *) | (
* * * * * --- * * * * ) | (
* * * * * --- * * * * ) | (
* * * * * --- ----o---) | (-----o-----||----+
*Vi ----- - - * * | * * * * * * | * * C1 * *|
* * * * * *O * * *B+ * * * * * *| * * * * * |
* * * * * *| * * * * * * * * * *| * * * * * |
* * * * * *| * * * * * * * * * *| * * * * * |
* * * * * *| * * * * * * * * * *| * * * * * |
* * *+-----o * * * * * * * * * *| * * * * * | /|
* * *| * * | * * * * * * * * *+---+ * * * *+-+ | Spkr
* * *- * *+-+ * * * * * * * * |CCS| Is * * +-+ |
*Cc *- * *| | Rc * * * * * * *+---+ * * * * | \|
* * *| * *+-+ * * * * * * * * * | * * * * * |
* * *| * * | * * * * * * * * * *| * * * * * |
* * *| * * | * * * * * * * * * *| * * * * * |
* * *+-----o--------------------o-----------o
* * * * * * * * * * * * * * * * * * * * * * |
* * * * * * * * * * * * * * * * * * * * * *--- GND

I have a pair of line matching transformers handy with
a 22:1 winding ratio on the end taps, Rsec. = 0.7ohm,
Rprim. = 188ohm and a max. power handling of 10W.
Onset of core saturation is at around 115Vrms at 50Hz.
Primary inductance measures 73H at 100Vrms and 50Hz.

With an 8ohm load total Ra, including loss resistance,
will be around 4K4 which is a good value for an ECL82
operating at a B+ of 200V and a plate current of 35mA.
For this plate current Is needs to be 770mA, then the
power dissipation in the transformer is about 0.65W.
I don't think this amount of power causes a problem.

What could be the pitfalls of this topology?

Rgds,
Gio

Anyone is free to provide cancelled dc magnetization of the core by
means of applying a fixed positive supply voltage to one end of the
OPT sec, then have CCS from live end of sec to 0V and cap couple the
speaker.

But you are putting yourself to a lot of trouble just for 4watts from
ECL82 / 6BM8, although it is a good learning experience.

If I were you and you wished to get a blameless 4 to 6 watts I would
use an EL34 strapped as a triode then driven with paralleled 12AU7 /
12AY7 / 12AT7 or trioded EF80 / 6BX6.

Instead of a high current CCS at the sec I would use a choke feed to
the anode of the power tube, say 40H at 60mAdc, and then cap couple
the primary winding of your OPT using a 60uF motor start cap rated for
450Vdc. The sec is then conventionally set up with one end grounded,
and the live end taken to a speaker. Such a choke should be nearly the
size of the OPT and can have similar dc resistance for the winding as
the OPT primary. The number of choke turns should be about twice the
turns for the OPT primary.

The primary inductance of your OPT may well be also about 40H if you
are lucky. It will be in parallel with the choke inductance.
But more analysis is required on your part about the properties of
your OPT. A typical anode load might be 5k and the response is down
-3dB at bass when total XLp = ( RL in parallel with Ra ) = 5k // 1.3k
= 1k, so if Lp was 20H, the LF pole = 8 Hz.

You could also use a CCS feeding the anode with say 60mA from a supply
rail of twice the idle Ea of the power tube.
So if Ea was 300Vdc, you'd need a rail of +600Vdc so that the Va may
swing UP and DOWN about 200 peak volts.
To make a suitable CCS using solid state does take some doing and
knowledge about how easily solid state devices fail in these
circumstances, and some knowledge about having say two high voltage
rated power mosfets in series for the CCS pass elements and having
adequate diode clamps to prevent any stray back emfs from inductances
from blowing crap out of delicate SS input circuits.

A carefully air gapped choke is far easier to use once you have such a
choke.

The other alternative to a CCS on the secondary side of the OPT is to
have the arrangement you show in your schematic but replace the solid
state CCS with a choke plus RC parallel network. The low voltage V+
would be say +20Vdc. The choke will need to be about 100mH, and air
gapped for up to 1Adc, and R = 19 ohms and adjusted for the wanted Idc
flow and the C =
10,000uF. The +20V rail needs at least 10,000 uF bypassing to 0V and
speaker drive cap could be 4,700uF.
The choke will not be easy to obtain, and may need to wound on a core
about the same size as your OPT and properly gapped and using say
1.5mm wire dia but at least there are not many turns needed.

Of course the end result of using transformers you have and which you
do not say are air gapped makes your exercize rather complicated and
fiddly to adjust just right compared to just buying say a pair of a
ready made SE OPTs from Hammond.

Option 3 may be to use a pair of OPT tubes such as EL86 and in series
and with B+ supply of 400V and then cap couple the OPT to the cathode-
anode junction. Philips produced a 10W amp with RL = about 1k and
pentode connection operation which requires a high L screen feed
choke. The idea was to allow use of a speaker with voice coil load of
about 800 ohms. But of course with time the extremely fragile voice
coil became easily fried, fused, or corroded, and nobody ever mourned
the passing of the Phillips silly idea to try to save the cost of a
proper PP OPT.

Patrick Turner.