Single-ended output using fixed bias

[Malcolm Irving]

Oh yes, my latest build, the SE-30, utilizes a fixed-bias, single-ended KT-120, with an EF86 driving it (behind a normal 2-stage 12AX7 preamp), and also uses the aforementioned "RMS" control (which I label "touch"). I tuned it for about 27-Watts, and the sound is amazingly complex and quite loud through a pair of 10" Celestions.

That sounds like a very interesting design. I am guessing the 2 stages of 12AX7 have enough gain to overdrive the EF86 if you want to, but probably the KT120 will be overdriven first? (Not sure if you have a master volume somewhere.) I have built an amp with an EF86 as the third gain stage after 2 stages of 12AX7, followed by a 12AX7 LTP PI and 4 EL84s in push pull. The EF86 seems to have a different and interesting overdrive characteristic compared to the usual 12AX7 pre-amp overdrive sound. I think most EF86 designs are missing a trick, by using it as V1 in a very high gain circuit (where microphonics can show up badly). Using an EF86 at a later stage greatly reduces any microphonics issues, and allows its interesting distortion sound to become available.

[jbefumo]

Yes, the two ax7 stages CAN overdrive the EF86 if desired. I use a dual-ganged 1M pot to simultaneously attenuate the signal both before the second 12AX7 stage and between it and the EF86. I also wire the EF86 with a variable control to continuously sweep it from pentode, through ultralinear, to triode mode (from Valve Wizard's book), that really alters the tone. No master volume at the moment, but I do have a Pentode/Triode/UL switch on the power circuit, and will be using an integrated reactive attenuator as a sort-of post-OT 'Master Volume'. I also run the EF86 configured for relatively low gain, and have never had any microphonic problems with them, even in a VERY tiny (smaller than a Champ), 40W combo.

I've done some prototypes using the EF86 as V1, and also had no issues. I think the problem is when people blindly copy the old VOX circuit, where it was the only gain stage, running at ungodly gain levels.

If you configure it for approximately the same gain as a typical ax7, you get the increased touch sensitivity and nuanced tone when used as V1.

SO much to learn! I LOVE this stuff!