Re: Alternative OPT current balance method

Patrick Turner wrote in message


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.

Tolerance is quite a complicated thing when it comes to
transformers.

The story goes that the more evenly a transformer magnetises
the core, the greater the inductance that can be achieved
for a given amount of iron, the more linear the relationship
between inductance and current, and the less leakage,
particularly if the core has no sharp corners. Because of
the more effective use of the iron, less can be used, making
for a comparitively dainty transformer with lower
capacitance than an EI of the same inductance.

So if you want plenty inductance for bottom end grunt
combined with low capacitance and leakage for a top end
extended enough to use stacks of feedback, then you should
use a toroid, or an "R-core", or C-core, in that order.

The story continues with a downside: the more even the field
distribution, the more sharply the core saturates,
especially if less iron has been used. This is where the
toroid's reputation for DC intolerance comes from, I think.

Perhaps you could make a better toroid? If you were to use
the same core area as you would for an EI, would that make
it less intolerant? If you aimed for the same HF performance
as you would for an EI, you could make a humungous toroid, I
would guess.

Vanderveen, of toroid fame, shows a sketch of the output
resulting from poor AC balance. From memory it looks like
the signal disappears around the voltage zero crossing
points. He doesn't explain how this happens though...it's
just shown to illustrate how to adjust it.

So I have a question. Taking John's transformer as an
example, each half of the primary winding will on its own
have a certain inductance, which will be doubled if they are
both operating in unison, because twice the windings gives
four times the inductance. Consequently, any unbalanced part
of the signal will see half the load, and so result in twice
the current for a given voltage amplitude (and so a zero at
twice the frequency?). Question is, will an intolerant
transformer find this more of a problem at low frequencies
than a tolerant one?

This is part of a more generic question because a
conventional PP output stage is never quite balanced anyway,
obviously particularly when operating in AB.

I was set off on this train of thought by John's assymetric
cathodes. I guess he can just make those caps really big, so
their effects are both well below a bandwidth determined
elsewhere.

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.

And by this time your amp weighs half a ton and costs a
fortune.

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.

Interesting, thanks. A better proposition now than then,
because of the improvement in price and performance of
electro caps.

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.

Conventional PP amps throughout history haven't bothered
much about DC balance, beyond offering a means of measuring
and adjustment. It shouldn't be hard, these days, to
automate that process of periodic maintenance.

One interesting thing about this idea of cap coupling, when
applied to valves operating in DC parallel, is that the idle
currents of the two valves can be independently adjusted for
optimum AC performance.

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.

Anyone remember the chap here who made *massively* parallel
amps with hundreds of EL86? On the grounds, IIRC, that OPTs
become relatively smaller as amps get bigger, so in terms of
kilos per kilowatt, the bigger the better? Now EL86 aren't
as cheap as they were, and electricity is much more
expensive .

cheers, Ian


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