6DJ8 transconductance at different currents

[Shannon Parks]

Scrapped my hybrid cascode preamp as soon as I finished building it. The biasing and oscillation problems combined with no PSRR seemed to steal all the joy. The fact that it was a 1980's topology and out of style made me shelve it.

Back to the high mu single triodes for testing! I ordered several 6GK5s. The 6C45S tubes seem a bit pricey and no tube rolling is possible. If the 6GK5 pans out, I think I may have a *very* cool idea planned. 417A, NOS 12AY7 and NOS 6DJ8/6922 are just too expensive.

The carrot: ordered a Denon DL-103 yesterday and I want 60dB of tube gain. 'Cuz if it heats up, it puts out.

-Shannon

[Shannon Parks]

Cool - I found a 3KG5 in a junk box on my desk at work (don't ask). Can't wait to get home and test this little guy.

--Shannon

[EWBrown]

The Philips (and other) 5824s are a WE417A substitute, they may be more available and reasonable. They aren't as "screany" as the Russki 6C45P-Es, with "only 24 mA/V instead of 45 mA/V transconductance. The 477/5842 were RADAR IF (and VHF RF) tubes, intended for grounded grid service, the 6C45Ps were designed as radar pulse modulators and they needed the high gm in order to generate very fast rise and fall transition times. They can be "tamed", but with involving just about the same amount of effort required to "tame" one of the wild "mustang" horses from the southwestern desert states.

I have two different phono stages (bottlehead Seduction) and a more recently built SS design, using 2SK170s, and both have nearly the same circuit topology, and the passive RIAA section is nearly identical. These have around 50 dB gain, but the lower-output MC pickups wll need more gain to play properly Now all I need is to get ahold of a semi-reasonable turntable, to play the 400 LPs that I have accumulated over the decades.

/ed B

[20to20]

Your work on this has set my mind at ease about a quad of KT66's I just got. Seeing how Gm can change with different Ip explains why the Gm numbers that came with the tubes seemed very low. AES did the matching @30ma and the Gm was 3.5. I thought they had sold me some low spec tubes ''On Sale''. The data sheets call for >6 Gm as typical. Since no data on these shows a Gm/Ip curve I was mystified, and a bit worried. I found a data sheet that showed the typical Gm was measured at 85ma. and that seems to suggest there is a real Gm/Ip curve and that at a lower Ip point Gm will be low too. Still looking for confirmation of the theory though. Thanks for the work.

[20to20]

Found this quote from Roger Modjeski..

..''Transconductance in triodes and pentodes increases quite dramatically with plate current, which is a source of distortion because it increases as the signal swings negative and decreases as it swings positive (at the plate). This is the basic distortion mechanism in all active devices, vacuum and solid state...''

[Geek]

Found this quote from Roger Modjeski..

..''Transconductance in triodes and pentodes increases quite dramatically with plate current, which is a source of distortion because it increases as the signal swings negative and decreases as it swings positive (at the plate). This is the basic distortion mechanism in all active devices, vacuum and solid state...''

Mu-follower the things and kick butt with Gm and Vo swing

Cheers!

[20to20]

Mu-follower the things and kick butt with Gm and Vo swing

Cheers!

Sounds like something a Kanuck winger would say....