Question of Curiosity + an odd-ball tube

[Shrapnel]

I have a bunch of 6X8's floating around here from an organ pull a while back, and I got to thinking. (I don't have a project at this time to even think about actually testing.)

the 6X8 is a pentode and triode in a single bottle, sharing a cathode, which can make this harder to use in a geetar amp. One possible solution is this:  A CATHODE-COUPLED PARAPHASE http://www.dogstar.dantimax.dk/tubestuf/paraph.htm

WHAT IF: Instead of that, the pentode section was wired like a big bottle for triode mode, and make it a long-tailed PI, would this work, or is that Pentode, even wired in a triode mode just too different?

[kagliostro]

It will be interesting if it is usable as a paralleled pentode/triode V1

don't know that tube so can't say if that can be done ..........

may be a little PP amp (don't use the triode section) ?

Kagliostro

[HotBluePlates]

6X8 datasheet

The triode section is mu = 40, pentode section (wired triode mode) mu = 42. That's plenty close enough on that account.

However, the triode section has higher Gm and lower plate resistance than the pentode section. Depending on which you take as your baseline number, the pentode section's Gm is about 30% mismatched (below) the triode section.

This might be a problem, because the long-tail topology tries to force equal/opposite currents for each half. Try it and see what happens.

[jjasilli]

I have a bunch of 6X8's floating around here from an organ pull  Me too.  One obvious use is to go with the paraphase PI. I think that's the site you listed.

Other options:  use the pentode as an input stage to overdrive the triode section.  Or, per Merlin's Book on Preamps, use the triode as an input stage to overdrive the pentode.  Merlin's book shows how to draw load lines and design such circuits.  That way you can use up 2 of these tubes in one build -- one at the input, the other at the PI.

The pentode has a very high plate impedance, 750K I think.  Output impedance will depend on the value of the plate resistor, which is in parallel with that.  I need to do the math per Merlin's book.  E.g. if a 47K plate resistor works for the pentode, that's a 44K output impedance -- very good.  Otherwise, if necessary, the pentode could drive a triode, either through the plate or as a cathode follower, to serve as a buffer.  (The 6X8's inner triode would not work as the cathode follower, due to the shared cathode)

BTW I can highly recommend Merlin's Preamp book -- I'm about 2/3's done reading it.