Re: Alternative OPT current balance method
- From: John Byrns <byrnsj@xxxxxxxxxxxxx>
- Date: Thu, 18 Jun 2009 17:58:05 -0500
In article <6%u_l.15485$Yf.964@xxxxxxxxxxxxx>,
"Ian Iveson" <IanIveson.home@xxxxxxxxxxxxxxxx> 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.
I'm sure Patrick is correct; I would be surprised if it hadn't been done
before, there is nothing new under the sun.
Anyway, what are the unusual design features?
The actual amplifier design features are largely orthogonal to this
idea, anyway I discussed them in this forum a few years back but never
built the amplifier because of potential problems with the output
transformer from my junk box.
Why are you planning to use an intolerant OPT?
Because I have one in my junk box and the original plan was to build the
amplifier largely with parts I already had on hand.
It seems perverse to deliberately create a problem to fit a solution.
I'm not sure I follow you here, I had the problem and tried to create a
solution to fit the problem, not vice versa.
If there is a good alternative reason for your intolerant
transformer plan, perhaps you could say what it is?
See above, I already had the transformer in my junk box.
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?
Purity of design.
Also, there may be other alternatives.
Probably, however I would like a set it and forget it solution that will
work for years without the need for readjustment when tubes are replaced
or between tube replacements.
Perhaps routine manual bias resetting?
See above, I don't like the requirement to have to reset things.
Just how intolerant are you planning this transformer to be?
I'm not planning how intolerant the transformer will be, it is what it
is, it was hot designed as an output transformer and I would like to
minimize any potential problems with core saturation.
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?
Could you identify the "odd" resistor, it isn't clear which resistor you
are speaking of?
Don't it need adjusting for Vak balance?
No, that is why the grid bias voltage on the upper tube is set to
approximately one half of the B+ voltage by the resistive voltage
divider so that the Vak is balanced.
What's the penalty for Vak being out of balance as the valves drift?
The Vak should not drift by more than a couple of volts as the tubes
drift due to the way the upper tube is biased. Therefore any penalty
for Vak being out of balance should not be invoked.
Are there any other problems that might arise from asymmetrical
grid resistances?
The grid resistances are equal so I haven't considered what problems
might arise if they weren't equal.
The only good reason for planning transformer intolerance is
the hope of achieving better low frequency performance
AFAIK.
Another good reason is the desire to make use of a part you already have
on hand.
That is, you sacrifice flexibility by maximising primary inductance.
I wouldn't know about that one way or the other.
I wonder therefore how good your design
is at maintaining perfect full power AC balance at LF?
Maintaining perfect full power AC balance at LF is not an issue that I
am concerned with, my design is not atypical of push-pull valve
amplifiers in this regard.
Particularly if it's running in AB. If LF AC balance isn't
perfect, then surely your intolerant transformer will
complain?
It is a class A design.
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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