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Old 30th January 2012, 01:05 AM   #1
dgta is offline dgta  United States
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Default High(er) gm octal pentodes?

Does anyone know/use any octal pentodes suitable as drivers? The 6AC7 is ok at gm=9,000 but has high rp. Still, I may try it.

Anything else out there?
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Old 30th January 2012, 01:16 AM   #2
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The 6AG7 is similar but claims 11000 Gm. It's been several (like 10) years ago but I had some 6AG7 and 6AC7 and wound up using 6V6 or 6K6 for my driver, much lower Gm but it just sounded better.
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Old 30th January 2012, 01:36 AM   #3
dgta is offline dgta  United States
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Agreed. Power tubes are alright, but currents and heater power starts getting to be a bit much when you're regulating everything

Anybody have any experience with 6SD7?
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Old 30th January 2012, 06:17 AM   #4
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How about buffering the pentode with a MOSFET source follower? Having some success here with a 6SJ7 feeding a 2SK2717 follower to drive triode strapped 6AV5GA, audibly superior to r-c coupling pentode directly to 6AV5 grid, no significant HF rolloff, measures nicely, -3dB point is ~60kHz if I can trust the stability of HP200CD generator. Yes, I have been reading many threads re the 6AV5 and MOSFET followers, many thanks to George (Tubelab) for the interesting posts about those.
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Old 30th January 2012, 09:03 AM   #5
DF96 is offline DF96  England
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It is in the nature of pentodes to have high rp; that is what they do!
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Old 30th January 2012, 11:25 AM   #6
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While it isn't octal, have you considered the EL84?

George, thanks for that mention of using the 6V6; I have an amp running a 6AC7 LTP up front and modifying it to take 6V6 would be not so difficult... I don't think the 6AC7 is a bad one at all; if the 6V6 is better, than get outta my way!!
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Old 30th January 2012, 04:01 PM   #7
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I do like 6V6 as a driver. I haven't found a compelling need for high transconductance in a pentode driver yet (driving another pentode). What's the issue with Rp of a pentode? don't you have a load resistor in the circuit? Are you building a single stage driver for a low mu triode?

Last edited by Michael Koster; 30th January 2012 at 04:09 PM.
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Old 30th January 2012, 05:43 PM   #8
kevinkr is offline kevinkr  United States
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You might want to also consider the C3G German loktal type.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 30th January 2012, 07:45 PM   #9
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6KD6 got a full box (not octal)...and not the last reply in TV scan current dump monsters before SS tookover. Fairly high gm too, but gain of 4 in triode config needs some circuit tweaking..
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Old 30th January 2012, 08:02 PM   #10
tomchr is offline tomchr  Canada
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Funny. I just found myself in the same situation yesterday. I ended up adding a source follower a la tubelab's PowerDrive. Yeah, yeah.... I know. Sand in the glass... But for a low output impedance driver it's pretty tough to beat. Only takes 2~3 components.

6J5 + source follower + 300B = Audio Nirvana.

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