Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chaswahl on December 02, 2020, 04:51:26 pm

Title: One triode of preamp tube favored for lower-signal side?
Post by: chaswahl on December 02, 2020, 04:51:26 pm

There's a recent thread on the TDPRI Shock Brothers forum ( https://www.tdpri.com/threads/any-difference-between-section-1-and-2-of-a-double-triode.1055822/ ), that I started by asking why some schematics that indicate pin numbers have the triode with higher pin numbers used for the stage that deals with the lowest signal (first, rather than second in the signal chain). This led to my realization that the 678 triode of a 12A*7, or the 456 triode of a 6S*7, is generally called "section 1" or the like in datasheets. Opinions in the thread there favor the idea that the triode with lower pin numbers is more susceptible to hum than the "section 1" triode with higher pin numbers. Along the way, I've found that GE, RCA and Sylvania datasheets for, say, a 12AX7-type tube all show pretty much the same diagram (whether schematic only, or really a representation of how the tube is wired internally), while one from Philips, which happens to be the only datasheet I know of that mentions (or suggests) a more "favourable" triode regarding hum, has a somewhat different diagram, with the tube elements mirrored with respect to pin locations, when compared to the datasheets of the USA manufacturers mentioned. The Philips sheet does not give a reason for that statement, unfortunately.

What I'd like to know from you knowledgeable folks is whether you could give reasons why the 678 (or 456) triode is shown in the "primary" position.

I'd be happy to repost some of the relevant material abovementioned, if that helps.

Thanks,
Charles

Title: Re: One triode of preamp tube favored for lower-signal side?
Post by: vampwizzard on December 02, 2020, 10:02:14 pm

any change would probably be in regards to the internal distances between AC carrying lines inside the tube. Going by layouts of the AB763's, the 123 triodes got the input signal.. if they were worried about amplifying noise they would've found a way to swap that around.

Folks routinely get matched triodes for PI sections. Ive never noticed a difference.