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Old 11th May 2011, 05:57 PM   #1
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Default Extreme plate load

This board has always been very informative and knowledgeable, so I'm confident you'll be able to help me with this design.

I need a several hundred kohm (preferably closer to 1M) plate load @ about 30 ... 40 mA, that can withstand voltage swings in the kilovolt range. Idle voltage (DC) accross the plate load can be at most 300 volts, I plan to have a B+ around 1000 volts.

So, there seems to be three ways to go:

1) choke
2) SS
3) pentode

Choise number 1 seems impractical, I don't think such choke exists, or if it does, it'll cost (and weigh) way too much.

As for SS, I don't know much about them. There seems to be some SS devices with reasonable prices and 900 V ratings, perhaps it would be possible to stack them somehow? As I said, SS is not my expertise.

The most interesting choise at the moment seems to be the pentode, perhaps some TV sweep tubes. They seem to have good voltage ratings, and with the low current I'll be running they won't be under strain, but I don't really know if they're suited for the task - like I said, I'd need 500 - 1000 kohms of impedance. I read from some tutorial, that with pentodes, you can approximate the generated impedance as being the rp of the pentode - well, the sweep tubes I looked at have rp's in the 10 - 15 kohm range.

Any suggestions on how to proceed?
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Old 11th May 2011, 06:23 PM   #2
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What is the frequency range of your application? Staying above 1M at high frequencies is going to make capacitance a big issue.
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Old 11th May 2011, 06:26 PM   #3
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Audio range, 20 ... 20 000 Hz.
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Old 11th May 2011, 06:28 PM   #4
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Have you already settled on a common cathode (tube) for the bottom or are you still deciding about the whole thing?

10pF at 20kHz is already 800k.

Last edited by Andrew Eckhardt; 11th May 2011 at 06:34 PM.
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Old 11th May 2011, 06:35 PM   #5
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I'm going to look into using a 808 triode, in A1, as a voltage amp. The A1 curves I'll have to draw myself, roughly looking at the published curves 700 V idle seems to be in the ballpark for a good voltage swing.
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Old 11th May 2011, 07:01 PM   #6
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A stack of depletion mode FETs?
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Old 11th May 2011, 07:18 PM   #7
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Old 11th May 2011, 07:27 PM   #8
DF96 is offline DF96  England
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With an unbypassed cathode resistor the anode impedance is boosted so Rp becomes Rp+(mu+1)Rk. Mu for a pentode is high, so high impedance is not a problem. Maintaining it to 20kHz is the problem.
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Old 11th May 2011, 07:44 PM   #9
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Wavebourn, you are referring to SemiSouth SJDP120R085, right?

http://www.semisouth.com/products/up...085_rev1.0.pdf

The simplest CSS I know how to make with that would be a transistor, with a resistor under the source, and the gate connected to the bottom of the resistor. How would I go about calculating 1) the impedance that is generated 2) how much voltage the CSS would drop?
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Old 11th May 2011, 07:50 PM   #10
SY is offline SY  United States
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Quote:
Originally Posted by MrCurwen View Post


Choise number 1 seems impractical, I don't think such choke exists, or if it does, it'll cost (and weigh) way too much.
Had you considered using the secondary of an ESL step-up transformer?
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