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Old 16th December 2012, 11:13 AM   #11
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This is the basic topology(as I remember it) that was shown at Intactaudio/forum/circuits (in the JC thred) by FB. Would call it a pentode on steroids.

Sy, suppose you mean pentode, not triode on bottom?
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Old 16th December 2012, 11:17 AM   #12
SY is offline SY  United States
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Both, really. It's a pentode tube used as a triode with the screen as the plate. The plate is a Dunsel.
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Old 16th December 2012, 11:24 AM   #13
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Please explain "Dunsel". Can have nothing to do with the circuit he showed and also the JC-fellow showed at intactaudio? See my post above, the screenfeeding device could have been BJT.

Last edited by revintage; 16th December 2012 at 11:35 AM.
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Old 16th December 2012, 11:29 AM   #14
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Urban Dictionary: dunsel
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Old 16th December 2012, 12:05 PM   #15
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Sorry SY, this makes no sense as both JC and FB refers to the GM-amp topology I showed above, where the anode is working as anode. Or are you refering to a completely different topology?
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Old 16th December 2012, 12:22 PM   #16
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No, that's the same topology, though with a MOSFET on top rather than a bipolar. If you do the AC signal analysis, you'll see that the plate hardly enters into it and that (with an adjustment to the DC current), it can be disconnected with the circuit still operating the same way.

If you can tell me the plate voltage on the circuit you drew, I'll walk through the calculations.

edit: One more question- in his ETF lecture and the Linear Audio article, Frank used a resistor load. Has this changed or was the CCS your addition? It would seem to make the gain and output impedance crazy-high.
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Old 16th December 2012, 01:03 PM   #17
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And yet one more question... what is the impedance looking in to the source of the MOSFET (since you have your simulator out and running)? I did my original analysis and measurements with the resistor plate load and bipolar driving the screen, as in Frank's original presentation, where the calculations were much easier!
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Old 16th December 2012, 01:22 PM   #18
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The anode/plate does play a role: it collects electrons. If you disconnect it then g2 has to collect all of them, which in some cases might cause it to overheat. Of course, this won't be a problem if you are running at low current.

I think this pentode/cascode hybrid was invented too late. It is of limited value for audio because of 1/f noise, which it does not improve. It might have been useful for HF radio front-ends, where partition noise does matter, although it is unclear to me what advantage it has over a normal cascode.
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Old 16th December 2012, 01:49 PM   #19
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Quote:
Originally Posted by DF96 View Post
The anode/plate does play a role: it collects electrons. If you disconnect it then g2 has to collect all of them, which in some cases might cause it to overheat.
Thus my point about DC. If a D3a is running 20mA plate current, then g2 current is going to be about 6mA. So disconnect the plate, run 6mA to the screen from the top cascode device, and the performance is about the same. No overheating.
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Old 16th December 2012, 02:08 PM   #20
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Lars, I remember that discussion over Dave's forum and I believe your schematic is pretty much what was proposed.

What would mr Miller say about the input C with screen-as-plate pentodes?
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