R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

News:


Author Topic: "not to be used in cathode follower"  (Read 3438 times)

Offline YZ

  • Hero Member
  • *****
  • Posts: 868
"not to be used in cathode follower"
« on: November 29, 2008, 01:51:49 am »
Could someone explain to me why this 12AX7 tube is not recommended for cathode-follower usage?

http://www.kcanostubes.com/products/377/Tung-Sol-Reissue-12A X7-15.htm

regards,

YZ

Offline Stephen Payne

  • Jr. Member
  • **
  • Posts: 67
Re: "not to be used in cathode follower"
« Reply #1 on: November 29, 2008, 08:05:27 pm »
I've always heard that all 12ax7s are bad for cathode follower.
Steve

Offline Jim Williams

  • Hero Member
  • *****
  • Posts: 1105
Re: "not to be used in cathode follower"
« Reply #2 on: December 01, 2008, 10:46:22 am »
There are simply better choices. A follower is a unity gain configuration, the 12Ax7 is a high mu (gain) tube. It has poor high impedance output current. The better choice is a 12AT7 as a follower. It's a poor choice for gain as it doesn't have nearly as much.
Jim Williams
Audio Upgrades

Offline Dave Hecht

  • Sr. Member
  • ****
  • Posts: 457
Re: "not to be used in cathode follower"
« Reply #3 on: December 02, 2008, 02:32:10 am »
12ax7 cathode follower is pretty common in guitar amps. Fender, Marshall, Hiwatt, Orange have all used a 12ax7 follower on many of their models. Can't see any reason why this one wouldn't work as well as any other new 12ax7 in those type of circuits. For other applications, as Jim mentioned, 12at7 or 12au7 would be a better choice.

Dave Hecht

Offline YZ

  • Hero Member
  • *****
  • Posts: 868
Re: "not to be used in cathode follower"
« Reply #4 on: December 17, 2008, 03:11:34 am »
Dave Hecht wrote on Tue, 02 December 2008 05:32

12ax7 cathode follower is pretty common in guitar amps. Fender, Marshall, Hiwatt, Orange have all used a 12ax7 follower on many of their models. Can't see any reason why this one wouldn't work as well as any other new 12ax7 in those type of circuits. For other applications, as Jim mentioned, 12at7 or 12au7 would be a better choice.

Dave Hecht


What got me stumped was the fact that only that particular brand of 12ax7 has the 'not for cathode follower' notice on that website.

I'm still curious; that notice would make me wary of using that tube for anything.
regards,

YZ

Offline Jim Williams

  • Hero Member
  • *****
  • Posts: 1105
Re: "not to be used in cathode follower"
« Reply #5 on: December 17, 2008, 12:42:22 pm »
All the classic Marshall amps used a 12AX7A cathode follower before the passive EQ section. That never seemed to effect those poorly. Maybe try this brand (what was it?) in a Marshall and see.
Jim Williams
Audio Upgrades

Offline Dave Hecht

  • Sr. Member
  • ****
  • Posts: 457
Re: "not to be used in cathode follower"
« Reply #6 on: December 17, 2008, 01:19:48 pm »
YZ wrote on Wed, 17 December 2008 00:11

Dave Hecht wrote on Tue, 02 December 2008 05:32

12ax7 cathode follower is pretty common in guitar amps. Fender, Marshall, Hiwatt, Orange have all used a 12ax7 follower on many of their models. Can't see any reason why this one wouldn't work as well as any other new 12ax7 in those type of circuits. For other applications, as Jim mentioned, 12at7 or 12au7 would be a better choice.

Dave Hecht


What got me stumped was the fact that only that particular brand of 12ax7 has the 'not for cathode follower' notice on that website.

I'm still curious; that notice would make me wary of using that tube for anything.


That note's on a dealer's website - what does the manufacturer recommend? Might just be the dealer's preference. If it is in fact a true 12ax7, there should be no reason it can't be used in place of any other 12ax7.

Dave Hecht

Offline Gustav

  • Jr. Member
  • **
  • Posts: 70
Re: "not to be used in cathode follower"
« Reply #7 on: December 22, 2008, 06:53:28 pm »
Could it be a heater to cathode voltage limit?  Would they be OK with a raised heater?

Offline Brian Kehew

  • Hero Member
  • *****
  • Posts: 2914
Re: "not to be used in cathode follower"
« Reply #8 on: December 30, 2008, 02:54:32 am »
Maybe a gain (voltage, as you say) issue - as the 12AX7 has so much more gain than the AT or AU versions...

BTW - the new book on VOX amps has a wonderful section about amp designs and how they work. Our friend knew nothing about electronics as he was writing it, and he learned in order to present the material so ANY layman could understand. First book I've seen do that - I learned a lot myself.

http://www.voxguidebook.com
Relax and float downstream...

Offline Dave Hecht

  • Sr. Member
  • ****
  • Posts: 457
Re: "not to be used in cathode follower"
« Reply #9 on: December 31, 2008, 02:26:34 am »
[quote title=Brian Kehew wrote on Mon, 29 December 2008 23:54]Maybe a gain (voltage, as you say) issue - as the 12AX7 has so much more gain than the AT or AU versions...

Gain won't be an issue in a cathode follower circuit - gain is essentially unity. Most 12ax7s work fine in a follower, so this issue seems to relate to just this particular tube. Again, if it's truly a 12ax7, it should work in place of any other 12ax7.

Dave Hecht