12ax7 for 1st and 3rd stages.

[RockinRocket]

I have been thinking about using a Ef86 and don't want to deal with microphonics as the first stage. So second stage it will be.
Can I uses a 12ax7 for the first stage cascaded in to the Ef86 and come back and use the other half of V1 for a cathode follower?

[Reeltarded]

Yep. Been thinking about it for literally years. Check out Merlin's pentode thing on his tech pages.


http://www.valvewizard.co.uk

[10thTx]

I have not done that in the first gain stages, but did use a similar idea in an overdrive stage. It sounded great to me. It was a 5879 instead of the EF86.
The cathode follower after the 5879 added some smoothness to the tone, IMO.

with respect, 10thtx

[TUBEDUDE]

I've used the 5879 as the input in several amps with no noise or microphonic issues.

[Reeltarded]

If you look at Weber's add an Ef86 kit at the values of caps on the 6 position switch you find worthwhile values for tone carving. I would use that same setup but delete the dullest leaving that position on a similar switch open.

Another idea is a full tone circuit for that stage and a bypass. It has such a massive gain factor that you could easily get away with killing it and still having headroom.

I haven't tried any new Ef86s but my pile of spares are Mullard and Mazdas with a 'dont use this tube, ever' failure rate of about half. That failing half would probably 70% work out as a middle stage. A bad one turns an amp into a very good boundary microphone and also a great method of mimicry for fake shortwave voice work.

"Gozirra! Gozirra!! Come in, Tokyo!!"

[tubeswell]

A few years back I used EF86 and 5879 with CF buffer and a triode pentode morph control in a couple of amps (when I first got Merlins 1st pre-amp book). I tried a few variants and I found the control worked best with the log taper pot wired bass-ackwards and fed by the low impedance output from the buffer. YMMV

Here's a few schematics

[Orson Maxwell]

Nice screen compression pot! Might wanna wire one in on the test amp as well! Thanks for sharing.

I myself am working on a last stage pentode design, however I've found the ECF80 tube to be really neat in terms of a pentode+CF in one envelope. We also have a nice abundant replacement model here in Russia for ECF80.

Anyone used those?

OP, sorry if I'm intruding on your thread. One last remark I had: don't forget to decouple the supply for the triodes since they will amplify signal in phase.

Cheers!

[Reeltarded]

Oh, would you look at that! Right up my alley. I shall try that morph in my lil pentode monster.

And a warm welcome, Orson! Best amp corner on Earth!

[Orson Maxwell]

Cheers, guys! Finally I'm able to converse with someone who had read Merlin's books! Glad my Dumble research led me here