Lil' Devil P-P Amp

[wrenchone]

A given - one, I'm always working on something or the other, and two - I can never leave well enough alone. Given those two postulates, this is a description of my latest heresy, an amp I've christened "Lil' Devil" (the color scheme will match the name, in a subdued sort of way). The form factor will be the same as my SE "Shrine" amp, 6" by 6" by 6" (tubes on top), based on a standard Hammond utility chassis. I started out with the assumption that the front end would be a differential version of the triode-cascoded JFET stage I used in the Shrine amp, using a 12AX7 instead of 6C4. Output tubes will be 6HE5/6JB5/6JC5 vertical sweep tubes, whose characteristics more closely match your usual pentode audio output tube than a typical sweep tube (the screen grid is less sensitive and has a higher voltage rating).
I had trouble spotting all the tubes in an aesthetically pleasing manner on the top deck, so I took another look at the input section and asked myself why I needed a whole triode with all the associated trappings just to cascode a JFET. The triodes went, and are replaced by a pair of Zetex 400V bipolar transistors. This frees up more space on the top deck for the output tubes, pleasingly placed.
The amp will be powered by a custom SMPS (already up and running), compounding heresy on heresy.

Current schematic (It may change a little) is shown. Pictures will follow as I get the little sucker built (input PC boards are mostly stuffed, and the chassis has almost all the holes it needs). I want to get the SMPS in place with a shield before I spot the holes for the output transformer, which will be tucked away inside the box, just like the Shrine amp. Output iron is from an old Baldwin organ or two, with ~6K P-P impedance, and a single 8 ohm output winding.

[tinitus]

I dont have skills to judge it, but that looks smooth

[Cassiel]

that looks smooth

It looks too complicated for me.

[tinitus]

Originally Posted by stalker

It looks too complicated for me.

I suppose you refer to curcuit design, because it looks to me like everything can be placed on a 3"x3" board

[Cassiel]

I suppose you refer to curcuit design

Right, an engineer's amp. I'm not so I'll pass.

[Arnulf]

Originally Posted by wrenchone

Current schematic (It may change a little) is shown.

Make R11 adjustable ? You're likely to have to tweak the feedback ratio.

[revintage]

Why use a high THD high-gain driver in front of the output tubes?

As in sand-amps this is only helped by massive amounts of NFB.

But prove me wrong, I haven´t dug so deep in this particular design.

[OneyedK]

Originally Posted by revintage

Why use a high THD high-gain driver in front of the output tubes?

I don't see the high gain in the driver/phasesplitter?!

[wrenchone]

I've had nothing bet good results so far using JFETs as front ends in my amp designs. The PN4393 will probably squeeze out 10k umhos (10 milliSiemens) if you bias it real hot, but at 2 mA bias current, it's pretty docile. I cam precisely match JFETs much easier than I can with their tube counterparts, as well.

[wrenchone]

Here is a piccture of the top deck (underside) of the amp, with most of the wiring finished. Tasks remaining are to wire the screens and the balance pots. The top deck is made of single-sided 2oz FR-4 circuit board, with the unclad side facing up (it'll take paint better), and the copper clad side serving as ground plane for the amp.