The AM Forum

Hi All,

 I need to buy a set of tubes for a R-26/ARC-5. The tube in question is a 12SK7. It seems I can substitute a 12SG7 as both are remote cut off pentodes with the same pinout and similar voltage/current ratings. The big difference is the Gm. The 12SK7 has a Gm of 2300 while the 12SG7 has a Gm of 4000. Is this OK?

Thanks, Rol.........

It'll work, but the AVC action my be goofy.

No harm, tho.

73DG

Hi Roland..I decided to wade in here..probably because in my experience seeing a sub or "close enuf" gonna be tried kinda throws up a signal flare..you might want to rethink that Roland..the 12SK7 uses a basing of 8N..the 12SG7 8BK..both my Howard Sams and RCA and Sylvania sub books list no direct drop in for the 12SK7..however a pamphlet from 1945 by Richard Crane says you CAN sub in both a 12K7 and 12SG7 but but only if you rewire the pin connections..I'm thinking that after looking at the basing difference he's probably concerned about pins 3 and 5 as a guess..??..anyhoo do what you want but at least I wouldn't but hey if for some strange reason it works out OK let us know...gud luk!   73 de DAVE

Thanks for the replies. I'm aware of the pins 3 and 5 being internally connected in the 12SG7. It's no issue as the R-26 has these pins connected already. I'm just being cheap. The 12SG7 is 1/3 the cost (in bulk) of the 12SK7.

Rol.......

Rol,

E-mail me your addy and I'll send you a couple SK7's gratis.

73DG

DG,

Good man,  SK7s are fairly common and still plenty around.  The right tube in the IFs keeps the S meter working right.

Fred