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Old 19th July 2008, 01:28 PM   #11
SY is offline SY  United States
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I probably posted the driver at some point, but can't remember where. Tubelab's circuit is pretty much the same thing; we seem to have independently arrived at the same answer. Nothing complex, just an IRF820 as a plain vanilla source follower, with the drain connected to +200, and the source connected to -200 via a 100k resistor, then direct coupled to the screen. Bias supply feeds the gate via a large resistor (I think it was 1M). The driver is not class A, strictly, but the distortion is low nonetheless.

The problem with deep bass is the output transformer. And when the OPT is driven from a high source impedance (like a pentode), the LF problem gets much, much worse.
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Old 19th July 2008, 03:33 PM   #12
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Thanks SY, I had forgotten that it was bi-polar (manic-depressive?) power supply and fixed bias. Will do some searching and have a peek.
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Old 19th July 2008, 03:34 PM   #13
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Quote:
Originally posted by mashaffer
Actually I was thinking gas regulator for screen supply in pentode mode.
A well bypassed 0C2 might drop a good 75 volts from plate to
screen in a triode strap. As long as screen current stays within
the 5-30mA range. And never drops low enough to let the pilot
light go out... Small current source from screen to to ground
could insure the gas regulator always remained lit.

My screen saver 0A2 above, intended to be almost always off.
Remove itself from the audio circuit as safe operating voltages
for G2 are realized... Definately not the normal way to abuse
such a regulator tube.

Important to keep in mind that gas tubes are noisy, have internal
resistance, and imperfect regulators of voltage where current varies.
All quite manageable in the right circuit.
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Old 19th July 2008, 03:45 PM   #14
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Well I wasn't thinking of triode mode but a regulated screen supply for pentode mode. So a screen supply of say 175 volts would be fed to 150V worth of regulator tubes and then to the screens. Of course that would demand a regulated B+ too I suppose. At that point one has to start thinking about SS regulators.

Is it normal practice to shunt gas tubes with a cap to kill the noise?
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Old 19th July 2008, 04:13 PM   #15
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I know it is expensive and so I would be unlikely to use it in the near future but has anyone tried or seen inter-stage tranny used as a driver for screen drive? I know that tubelab tried one briefly in his power drive work but I not found anything in googling about that shows it's use for screen drive.
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Old 19th July 2008, 04:38 PM   #16
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How's this Screen Driven Cascodlington for low parts count?

Triode in "4th circuit" should be regulating the Sandy/Pent cascode,
making all behave as-if one really big triode? Or at least I think so...

Its got your 4th circuit. Its got your gas tube. Its got your LED.
Its got yer screen drive. Its got your A2. Its got your Cascode.
Its got your current multiplier. Its got your herbs and spices...
Its in there...

If it ain't a spaghetti sauce, its Circuit Succotash for sure.
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Old 19th July 2008, 04:41 PM   #17
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Quote:
Is it normal practice to shunt gas tubes with a cap to kill the noise?
Only small values, not enough to make a big difference. Otherwise, Google "relaxation oscillator."
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Old 19th July 2008, 04:47 PM   #18
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That could be a problem....

But only if the current isn't high enough to keep it lit continuously.
There should be no negative resistance concerns above 5mA...
A positive resistance won't oscillate... Well, at least it shouldn't...
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Old 19th July 2008, 04:53 PM   #19
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I dunno. How serious is the noise at these levels, taking into account the various shunting impedances?
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Old 19th July 2008, 05:51 PM   #20
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I am an idiot!

Screen tied to Plate. Grid tied to Cathode.
Not an active amplifying device wired this way.
Its nothing but a big power wasting Diode.

It probably still works, but not as advertised.

-------------------------------------------------------

If I were to ground the Pentode's grid though a
suitable resistor. Then perhaps it does something
meaningful again.

The mode would be Triode, and not even A2...

Though the screen is still delivering a current,
it can't be "screen driven" when G2 has clearly
been bypassed and strapped to the plate...

I'll re-draw with a correction in a minute.
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