High(er) gm octal pentodes?

dgta · 30th January 2012, 01:05 AM

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?

tubelab.com · 30th January 2012, 01:16 AM

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.

dgta · 30th January 2012, 01:36 AM

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?

radiotron · 30th January 2012, 06:17 AM

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.

DF96 · 30th January 2012, 09:03 AM

It is in the nature of pentodes to have high rp; that is what they do!

Bandersnatch · 30th January 2012, 11:25 AM

hey-Hey!!!,
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!!
cheers,
Douglas

Michael Koster · 04:09 PM

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?

kevinkr · 30th January 2012, 05:43 PM

You might want to also consider the C3G German loktal type.

richwalters · 30th January 2012, 07:45 PM

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..

tomchr · 30th January 2012, 08:02 PM

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.

~Tom

30th January 2012, 01:05 AM	#1
dgta diyAudio Member Join Date: May 2011	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?

30th January 2012, 01:16 AM	#2
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	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. __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

30th January 2012, 11:25 AM	#6
Bandersnatch diyAudio Member Join Date: Jul 2003 Location: Ann Arbor, MI	hey-Hey!!!, 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!! cheers, Douglas __________________ the Tnuctipun will return

30th January 2012, 04:01 PM	#7
Michael Koster diyAudio Member Join Date: Jan 2008 Location: Eureka, CA	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.

30th January 2012, 05:43 PM	#8
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	You might want to also consider the C3G German loktal type. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan


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30th January 2012, 01:36 AM	#3
dgta diyAudio Member Join Date: May 2011	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?

30th January 2012, 06:17 AM	#4
radiotron diyAudio Member Join Date: May 2007	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.

30th January 2012, 09:03 AM	#5
DF96 diyAudio Member Join Date: May 2007	It is in the nature of pentodes to have high rp; that is what they do!

30th January 2012, 07:45 PM	#9
richwalters diyAudio Member Join Date: Sep 2003 Location: Alps:Tube amp designs over 150W, SMPS guru.	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..

30th January 2012, 08:02 PM	#10
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	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. ~Tom __________________ 21st Century Maida Regulator, Universal Filament Regulator, 300B Driver PCBs, and more... Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right

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