I need to figure out the plate current versus control grid voltage curve of
a 4X150A that I intend to use as a voltage controlled current sink (class
A). I have not found any transfer curves in datasheets of the tube.
What's the best way to determine this?

Also, is it possible to linearize by varying, say, screen grid current or
voltage as well?

Thanks.

Prune wrote:

I need to figure out the plate current versus control grid voltage curve of
a 4X150A that I intend to use as a voltage controlled current sink (class
A). I have not found any transfer curves in datasheets of the tube.
What's the best way to determine this?

To determine the transfer curves, you set up a sample of the tube
and measure Vg1 input and Va output for a given load,
and plot the graph.
If it involves thousands of volts, beware, its dangerous.

The transfer curve can be seen with a dual trace CRO in X-Y mode.

Its possible to use a tube tracer to draw the Ra curves for values of grid
voltage,
but unless you have the piece of gear, its a lotta work to make one.



Also, is it possible to linearize by varying, say, screen grid current or
voltage as well?

Linearizing a tetrode can only be done by applying NFB.

But your aim is to use the tube for a current sink, ie, a high impedance
sink.

What for?

If its for a DC plate supply so that the anode can then be cap
coupled to an OPT, then the non linearity of the current source may
have very little effect on the non linearity already
present in the signal, since the current source load on the tube may be
over ten times the value of the OPT primary load.
So a 25% change in actual current source load may make almost no difference.

We would have to know what your schematic is before offering
more accurate advice.

Patrick Turner.



Thanks.

We would have to know what your schematic is before offering
more accurate advice.

Right, well, I didn't describe it before as it's difficult to explain,
and the application is nonstandard.

I've posted about this here quite a while ago, so if it seems familiar,
that's why. The tube is driving a glow discharge (Hill Plasmatronic
speakers derivative, except he used TV sweep tubes, one per cathode;
also, I'm using a complex cathode geometry, microhollow cathode
discharges feeding the main discharge, to avoid the need for helium and
provide discharge stability). Hill's patent is 4,219,705, with fig.8
being essentially what the actual Plasmatronics used (the fan just cools
the heatsink's back and doesn't blow through). MHCD sustained discharges
are described in various plasma physics papers; Hill didn't use them as
they weren't known back then.

Single channel description: I've got about 2500 V at the plasma anode (I
plan to regulate it, just pi-filtered right now), and the tube is at the
plasma cathodes as a current sink, to be fed the appropriately amplified
audio signal (class A in the sense that maxium waveform will never cause
current to go below a certain value, so it's always conducting). There
are potentiometers for fine adjustment from the tube plate to the bottom
halves of the five microhollow cathodes, and resistors between the top
halves and plasma anode. I haven't actually run the tubes (I'm using
salt-water ballast resistor for testing) as the cooling blowers haven't
arrived yet.

The microhollow electrodes I had made with copper-sandwitched mica, but
it erododes too fast. I finally got free samples of sapphire wafer for
the insulator, and I'm working on platinum plating some tungsten for the
conductors. That should last very nicely. In the meantime I've been
thinking of how to get good performance out of the output tetrode. One
restriction is that the discharge should drop most of the voltage,
otherwise power is just wasted in the tube.

Sound generation -- instantaneous change in pressure at any time is
proportional to the instantaneous change in power being dissipated in the
discharge. A normal glow discharge is constant voltage within a range of
current and is why Hill used current drive, but the I-V characteristics
of an MHCD discharge are not so simple and vary based on the specific
arrangement (they can also be resistive or negatively-resistive (+ an
offset of course so there's always a minimal voltage drop across the
discharge)). In either of the three cases, current drive works fine, and
prevents a problem in the fault case of a glow-to-arc transition (if that
happens, the arc extinguishes immediately due to insufficient current,
the top half of the MHCDs are biased from the resistors from the plasma
anode, and the discharge self-restarts). However, if the operating range
encompasses more than one of these regions, then nonlinearity of the
discharge itself becomes a problem, so I guess one way to deal with that
specifically is to have feedback proportional to power rather than
current or voltage. Current through the discharge and voltage drop
across it can be sampled and with an analog multiplier gives such a
feedback signal, but I hope it wouldn't be needed as it gets too
complicated, and the best analog multipliers are only about 0.1%
accurate. But before I worry about that, the 4X150 nonlinearity when
used as a transconductance amplifier is what I'm trying to figure out
now.

Prune wrote:

We would have to know what your schematic is before offering
more accurate advice.

Right, well, I didn't describe it before as it's difficult to explain,
and the application is nonstandard.

I've posted about this here quite a while ago, so if it seems familiar,
that's why. The tube is driving a glow discharge (Hill Plasmatronic
speakers derivative, except he used TV sweep tubes, one per cathode;
also, I'm using a complex cathode geometry, microhollow cathode
discharges feeding the main discharge, to avoid the need for helium and
provide discharge stability). Hill's patent is 4,219,705, with fig.8
being essentially what the actual Plasmatronics used (the fan just cools
the heatsink's back and doesn't blow through). MHCD sustained discharges
are described in various plasma physics papers; Hill didn't use them as
they weren't known back then.

I remain entirely un-informed.......



Single channel description: I've got about 2500 V at the plasma anode (I
plan to regulate it, just pi-filtered right now), and the tube is at the
plasma cathodes as a current sink, to be fed the appropriately amplified
audio signal (class A in the sense that maxium waveform will never cause
current to go below a certain value, so it's always conducting). There
are potentiometers for fine adjustment from the tube plate to the bottom
halves of the five microhollow cathodes, and resistors between the top
halves and plasma anode. I haven't actually run the tubes (I'm using
salt-water ballast resistor for testing) as the cooling blowers haven't
arrived yet.

The microhollow electrodes I had made with copper-sandwitched mica, but
it erododes too fast. I finally got free samples of sapphire wafer for
the insulator, and I'm working on platinum plating some tungsten for the
conductors. That should last very nicely. In the meantime I've been
thinking of how to get good performance out of the output tetrode. One
restriction is that the discharge should drop most of the voltage,
otherwise power is just wasted in the tube.

Sound generation -- instantaneous change in pressure at any time is
proportional to the instantaneous change in power being dissipated in the
discharge. A normal glow discharge is constant voltage within a range of
current and is why Hill used current drive, but the I-V characteristics
of an MHCD discharge are not so simple and vary based on the specific
arrangement (they can also be resistive or negatively-resistive (+ an
offset of course so there's always a minimal voltage drop across the
discharge)). In either of the three cases, current drive works fine, and
prevents a problem in the fault case of a glow-to-arc transition (if that
happens, the arc extinguishes immediately due to insufficient current,
the top half of the MHCDs are biased from the resistors from the plasma
anode, and the discharge self-restarts). However, if the operating range
encompasses more than one of these regions, then nonlinearity of the
discharge itself becomes a problem, so I guess one way to deal with that
specifically is to have feedback proportional to power rather than
current or voltage. Current through the discharge and voltage drop
across it can be sampled and with an analog multiplier gives such a
feedback signal, but I hope it wouldn't be needed as it gets too
complicated, and the best analog multipliers are only about 0.1%
accurate. But before I worry about that, the 4X150 nonlinearity when
used as a transconductance amplifier is what I'm trying to figure out
now.

I do wish you all the very best of luck,
but remain in a mood of dejected despair and
am a complete and utter loss to be able to help you.

I bet it all looks spectacular when its turned on.

Patrick Turner.

Patrick Turner wrote in
:

I remain entirely un-informed.......

I know I'm not good at describing things. I'll sketch a diagram later
today.

I bet it all looks spectacular when its turned on.

Doesn't matter very much what it looks like, it's about what it sounds
like. There was a website (no longer functional) of a Plasmatronics owner
who took measurements. The pulse response and waterfall plot looked unlike
anything else, much cleaner than any tweeter I'd seen. And the frequency
response was very flat. I wish I had saved the graphs on my PC. I
completely forgot about my plans to build electrostatic speakers, I knew I
had to go for a non-solid sound driver.

Hi,
I have an 8-page Eimac data sheet for the 4X150A and D if that
would help. It has two full-page plots of transfer characteristics for
screen voltages of 250V and 350V. I can xerox and mail this, for the
cost of postage. Email me, adouglas at gis.net

Alan

On Fri, 03 Jun 2005 13:17:24 GMT, Prune
wrote:

all clipped for amazing coolness and complexity

Are you working for a scientific understanding, or for a
working solution? If the former, cool, good fortune, but
nobody here will have a clue. If the latter, maybe you
could do an impulse (or the mathematical equivalent)
response test of a mockup and apply the response to your
signal path.

Big fun,

Chris Hornbeck
"He thought so little they rewarded he,
By making him the ruler of the Queen's Navy".

Well, I was going to say both, but...


Chris Hornbeck wrote in
:

On Fri, 03 Jun 2005 13:17:24 GMT, Prune
wrote:

all clipped for amazing coolness and complexity

Are you working for a scientific understanding, or for a
working solution? If the former, cool, good fortune, but
nobody here will have a clue. If the latter, maybe you
could do an impulse (or the mathematical equivalent)
response test of a mockup and apply the response to your
signal path.

Big fun,

Chris Hornbeck
"He thought so little they rewarded he,
By making him the ruler of the Queen's Navy".

Actually I came across this:
http://www.webace.com.au/~electron/tubes/FIG23.jpg
What sucks is that if I wanted to be in a linear region, it means a
positive grid, and so grid current, which makes driving properly harder,
not to mention that my tubes have gold-plated grid and grid current above
10 mA is a no no (I was thus told by Eimac tech, as I have special (read:
obscure) audio versions of the 4X150A, but I can't complain given I paid
$10 for a perfect condition NOS pair, and ironically the used air system
sockets cost far more than the tubes themselves).

I was thinking about ultralinear circuits, where the screen voltage is
modulated (antiphase with control signal; doesn't necessarily have to be
from an output transformer; e.g. OTL ultralinear circuits at tubecad
journal). But that won't work for me, as it makes the tube more like a
triode, away from my application as a current sink. If the modulation is
instead in phase with the control signal, I'm not sure what it will
do...I think it's kind of like positive feedback, just as the way the
ultralinear connection is somewhat like a local NFB.

In that graph, if g1 and g2 are interchanged, one would again obtain S-
shaped curves. If the overall plate current function is (approximately
at least) linearly separable, as in (for fixed Ea as this is a current
sink) Ia = f(Eg1, Eg2) being expressable as Ia = f1(Eg1) + f2(Eg2) *,
then modulating the screen as well should be able to be set up as to give
a straight curve (if it's not separable, then the relationship between
the needed signals is nonlinear and so impractical). Having considered
this, it just doesn't look like any possible combination is possible
anywhere near the operating range I'm looking for.

I guess predistortion is the reasonable option I have, and then Hawkford
EC and/or NFB.


*of course E is potential, I current, a anode, g1/g2 the grids



Alan Douglas adouglasatgis.net wrote in
:

Hi,
I have an 8-page Eimac data sheet for the 4X150A and D if that
would help. It has two full-page plots of transfer characteristics for
screen voltages of 250V and 350V. I can xerox and mail this, for the
cost of postage. Email me, adouglas at gis.net

Alan

Hi,
I misread the Eimac plots; they're actually plate voltage vs. grid
voltage, for plate currents from 10mA to 1.2A. You'd have to replot
point by point to get transfer characteristics.

A couple of operating points I checked, don't match this other
plot that you referenced. I don't know where that one came from, or
how they could run at up to 8A plate current. The Eimac sheet doesn't
rate it beyond 0.25A.

73, Alan

"Prune" wrote

I was thinking about ultralinear circuits, where the screen
voltage is
modulated (antiphase with control signal; doesn't necessarily have
to be
from an output transformer; e.g. OTL ultralinear circuits at
tubecad
journal). But that won't work for me, as it makes the tube more
like a
triode, away from my application as a current sink. If the
modulation is
instead in phase with the control signal, I'm not sure what it
will
do...I think it's kind of like positive feedback, just as the way
the
ultralinear connection is somewhat like a local NFB.

The UL connection is voltage derived, voltage applied feedback,
which reduces Ra, which is what you don't want.

If you use *current derived*, voltage applied negative feedback, the
effect should tend to linearise the current, and hence increase
effective Ra.

However, assuming you are feeding a varying impedance which you
cannot use as a sense resistance, and that you don't want the
necessary additional sense resistance to be huge, you will need to
amplify the feedback signal before applying it to the screen. Easier
to use as pentode and arrange feedback to the grid.

You need to be careful about screen current, which has a max rating,
and which tends to rise sharply as anode current drops at low Vak.
Under those circumstances, feedback trying to raise the screen
voltage to counter the fall in cathode (or anode) current would put
the screens in double jeopardy.

cheers, Ian

"Ian Iveson" wrote in news:F61pe.106842
:
If you use *current derived*, voltage applied negative feedback, the
effect should tend to linearise the current, and hence increase
effective Ra.

Indeed, I can take care of the plasma load I-V nonlinearities by
multiplying this current feedback with the voltage drop across the
discharge. The problem with that is that I've not seen an analog
multiplier more accurate than 0.1%.

Can Hawksford error correction be adapted to this kind of output stage?
The problem is that for a transconductance amplifier it would have to be
a current error, and this stage has a big current gain.

I was thinking about the parallel between this and typical class A,
single ended amplifiers. They usually have a voltage output stage with a
load parallel to the output device/ground column, supplied by a constant
current source. Here I have a current output stage with a load in series
with the output device, and with what should be a constant voltage source
on the other end. Now, I've not built a regulator for the power supply
yet (currently CLCRC filtered, handwound choke, godam heavy bulky
capacitors..), but I have some appropriate high voltage solid state
devices. I remember one discussion at tubecad.org where Broskie found
that removing capacitance AFTER the regulator improved sound. I wonder
about that. One advantage I can see in my case of not having a
capacitance at the voltage side is that there won't be an out-of-phase
voltage variation on the high side of the load, but if the regulator is
very low output impedance, it shouldn't matter. I'm also thinking of
doing a capacitance multiplier instead of fixed-reference regulator, so I
can both have a small dropout and avoid problems from long-term mains
voltage variation.

Simulation of my current power supply shows ripple in the tens of
millivolts, but say with a 500 Hz (my crossover frequency) load drawing
DC offset 175 mA with 150 mA amplitude, times two channels, I'm getting
about two volts of variation at the output. (BTW, what is the actual
value of the primary inductance of a 1500 W transformer, is 50 mH a good
estimate?)

Oh crap, forgot to mention it's a microwave oven transformer, rebuilt core
(interleaved laminations), 20% extra turns added to get rid of magnetic
leakage (those cores are undersized). About 1 turn/volt initially, 1.2
now. About 16 lbs. 2100 VAC original output, I added also to the
secondary winding to keep the voltage. I talked about it in this group
some months ago. In the simulation I'm using 65 mH primary and 20 H
secondary. Do these sound like reasonable guesses, or am I way off?