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R/E/P => R/E/P Archives => Bruno Putzeys (Designer) - Dave Hecht (Master Tech) => Topic started by: YZ on November 29, 2008, 01:51:49 AM

Title: "not to be used in cathode follower"
Post by: YZ 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

Title: Re: "not to be used in cathode follower"
Post by: Stephen Payne on November 29, 2008, 08:05:27 PM

I've always heard that all 12ax7s are bad for cathode follower.

Title: Re: "not to be used in cathode follower"
Post by: Jim Williams 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.

Title: Re: "not to be used in cathode follower"
Post by: Dave Hecht 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

Title: Re: "not to be used in cathode follower"
Post by: YZ on December 17, 2008, 03:11:34 AM

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

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.

Title: Re: "not to be used in cathode follower"
Post by: Jim Williams 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.

Title: Re: "not to be used in cathode follower"
Post by: Dave Hecht 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

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

Title: Re: "not to be used in cathode follower"
Post by: Gustav 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?

Title: Re: "not to be used in cathode follower"
Post by: Brian Kehew 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

Title: Re: "not to be used in cathode follower"
Post by: Dave Hecht 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