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Old 19th August 2003, 05:30 PM   #1
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Default Pentode cathode follower with regulated screen voltage?

Hi All,

I'm still exploring possibilities as far as a line stage goes. I started looking at cathode followers (since I don't really need any gain) to use as a buffer/volume control after my cd player. I came across a diagram of a proposed cathode follower using a pentode (6V6 is what I have in mind) but the screen 2 voltage is regulated (?) using a triode cathode follower (5687, I have lots of them)and taking the voltage off the Rk of the triode. I'm not ashamed to say that I'm not all that familiar with biasing cathode followers and this just makes the picture a little more fuzzy. I can do a little reading up and calculating but does the potential of this circuit justify the added complexity and work? Thanks.
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Old 19th August 2003, 05:33 PM   #2
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This looks wrong. In a proper pentode CF, the screen grid is held at a constant potential with respect to the cathode.
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Old 19th August 2003, 05:48 PM   #3
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Hi Sy. It is held at a constant potential. The reference is the triodes cathode though. I just don't see the advantage of that though. The article can be found here:

http://www.glass-ware.com/tubecircui...llowers_2.html
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Old 19th August 2003, 06:41 PM   #4
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Yes, it is held at a constant potential but you gain the same effect (minus 300 ohms or so series resistance due to the CF) by connecting screen to B+. And that means the pentode is in triode mode. To be in pentode mode, the screen must be held at a constant potential wrt. the cathode. But that's no problem, move that coupling cap from ground to cathode.

<Checks link>
Frankly, I can't believe a publication would actually say that! Their pentode is in triode, not pentode mode. All they did was give it a 1/2 B+ filtered supply.

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Old 19th August 2003, 06:45 PM   #5
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Just goes to show that you can't trust everything you read on the 'Net. It's not a good circuit.
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Old 19th August 2003, 07:57 PM   #6
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How about using an emitter follower:

More 6158/13D3
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Old 19th August 2003, 09:05 PM   #7
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Old 20th August 2003, 03:21 AM   #8
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That'll work too. But that uses crap (aka SS).

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Old 20th August 2003, 08:23 AM   #9
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Default Smells of roses...

I'm not sure I'd have put it quite like that, but what goes on at the screen grid of a pentode is second-order.
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Old 18th November 2013, 04:35 PM   #10
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I've experienced a cathodyne with an EL 84 the results are excellent. You must maintain the voltage of the screen constant.
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