Re: Alternative OPT current balance method



Ian Iveson wrote:

John Byrns wrote:

I am toying with a mew amplifier design incorporating
several unusual
design features, and the output transformer I am planning
on using will
not tolerate unbalanced DC in the primary.

Patrick says it's not new, I notice, but he doesn't cite a
particular precedant. Anyway, what are the unusual design
features? Why are you planning to use an intolerant OPT? It
seems perverse to deliberately create a problem to fit a
solution.

The circuit John has presented has been invented probably before 1960
and I saw it cited in some ancient magazine, probably an old copy of
Wireless World.
However, maybe it is different in detail to old ideas of preventing dc
imbalance with series tube PP amps.

If one really wants to keep out the the Idc and use series tubes, then
you can have a series pair with an OPT which is cap coupled to one end
of a single primary, with the other end taken to 0V. No dc in the
winding at all.


If there is a good alternative reason for your intolerant
transformer plan, perhaps you could say what it is?

Just about all PP OPT set up conventionally are intolerant of dc
imbalance because they have ungapped cores and with a high max µ, so the
slightest dc imbalance causes serious increases in THD/IMD. Again if
nobody objects to weight and size and cost then there you can have choke
feed to a pair of output tubes using a choke with CT and then cap couple
the OPT which has its CT grounded. This allows normal drive, but then
the choke suffers the imbalance if it occurs. But then if the coke is
gapped, and had a massive sive and lots of turns then it may withstand
far more dc imbalance than any normal OPT.
Or one might use TWO individual air gapped chokes, but you can't get the
large wanted inductance to prevent inductance shunting the load.


Several solid state current servos designed to solve this
problem have
been posted here recently, however I would like to avoid
polluting my
design with a solid state current servo.

Why? Is this pollution of your idea of purity of design, or
of the signal? Also, there may be other alternatives.
Perhaps routine manual bias resetting? Just how intolerant
are you planning this transformer to be?

After a bit of head scratching I came up with the
following scheme which
I hope I will be able to integrate into my overall
amplifier design.

http://fmamradios.com/stuff/CurrentBalance.gif

What's that odd resistor for? Don't it need adjusting for
Vak balance? What's the penalty for Vak being out of balance
as the valves drift? Are there any other problems that might
arise from asymmetrical grid resistances?

The only good reason for planning transformer intolerance is
the hope of achieving better low frequency performance
AFAIK. That is, you sacrifice flexibility by maximising
primary inductance. I wonder therefore how good your design
is at maintaining perfect full power AC balance at LF?
Particularly if it's running in AB. If LF AC balance isn't
perfect, then surely your intolerant transformer will
complain?

Indeed.

The OPT performance between dc and say 20Hz is a grey area which needs
careful consideration.

Where one does have two series tubes with say an 800V supply, one can
cap couple a single winding OPT as I stated above.

Its an easy OPT to wind compared to a conventional PP OPT with CT
because it has half the P turns. The load is a lot lower than a normal
PP RLa-a load.

In the late 1950 Philips made a range of amps using 2 x EL86 in series
with a supply = +400Vdc, and the anode cathode junction was at +200V and
there was no OPT. The speaker had an 800 ohm voice coil impedance and
was driven by an electro cap off the a-k join.

But there still was a kind of small transformer to get the screen
voltage of the top tube to follow its cathode voltage while being held
at +400V. A choke would have worked fine though with cap bypass coupling
to the cathode. The bottom tube just has normal fixed screen supply and
at 1/2 the supply voltage for both tubes, ie, +200V.

While working in class A the load of 800 is shared between the two
output tubes and each tube sees 1,600 ohms. If one runs such an amp only
in class A then with 12W Pda in each EL85, each tube puts 5W into 1,600
ohms or 10W into the 800 ohms.

If you wanted to use the EL86 in a normal PP amp under the same Ea/Ia
conditions then the OPT would have a primary load of 3k2 a-a, ie, with
twice the turns of the single winding OPT.

To get drive meant a special bootstrapped circuit was used with a 12AX7.

But where the top tube has to have a large grid signal delivered, one
may have a 1:1 IST with two windings. One winding is driven with a CF
tube which is driven by a gain triode to make say 30Vrms. The other
winding has one end connected to the a-k join of the two power tubes and
thus the drive to the top tube is always equal to the bottom tube and
there is symetrical drive conditions.
NFB is applied the normal way, but with some care about gain/phase shift
compensation networks because you have an IST in the signal path.

To avoid having huge drive voltage and be able have an OPT with much
lower RL, and also have no screen voltage to worry about then the 6AS7
or 6C33C come to mind as excellent canditates. But one could run EL34 or
KT88 with Ea at 250V OK if one doesn't mind having a screen drive choke.
4 x KT88 would give 40W PP class A and load would be 560 ohms. The cap
to couple it can be 100uF, and Lp would need to be at least 20H.
The LF pole for resonance is 3.6Hz, and the load to damp the resonance
is about 600ohms or less, so when loaded the OPT won't have a peaked LF
response. Such a peak can make NFB difficult to apply. The higher the
LP, the lower is the Fo, and one could also have the coupling cap value
a lot higher, say 470uF, but then onmce you move Fo down to say 0.36Hz,
then one is i the grey area with the cap charging and discharging an the
core of the OPT is subject to slow ac changes and the resulting
temporary saturation effects.

As I mentioned in a previous post, If one doesn't like series PP tubes,
then one may as well bite the bullet and have a large PP OPT set up
conventionally, and then just air gap the PP core.

Oh, and all series output tube connections require a biased heater
supply for one of the pair of tubes.

Philips didn't bother though because the EL86 has a high heater-cathode
voltage rating.

Patrick Turner.






A couple of points to note, the two tubes are connected in
series for DC
thereby insuring equal DC currents through the two halves
of the OPT.
For AC (audio) the tubes are connected in the normal
push-pull
configuration. The current through the both tubes is
determined by the
bias applied to the lower tube, this bias is provided by a
cathode bias
resistor in the example schematic, but could fixed bias
could also be
used. The grid of the upper tube is biased at
approximately half of the
B+ supply voltage by two resistors so that the B+ voltage
is split
equally between the two tubes and each tube operates with
normal B+
voltage across it.

This scheme does have several unusual requirements which
my make it
unsuitable for some applications.

1. It requires an output transformer where each half of
the primary has
separate leads, with no common "center tap" lead.

2. It requires twice the normal B+ voltage.

3. It requires a separate heater transformer for the upper
tube to avoid
undue heater to cathode voltage stress.

Comments or thoughts?

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/
.

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