Number 5?

I've always been curious about low wattage amps, or "Micros" as they're called - in the 1 to 5 watt output range.  Online demos make them sound OK and they break into distortion at lower volume, so probably make ideal gritty to higher gain amps for home use. Considering my amps range from 40 watts (Scarlett push-pull 6L6s) to 10 (AC4 dual single ended EL84), and they're all too loud to play routinely at full volume, I can appreciate the utility of a Micro. And while I haven't been that interested in the triode-powered versions of the DIY Micro projects I've seen, I've recently learned about some that use a 1-watt pentode tube,  the EF80, developed in the mid-50s for VHF / TV uses, and those seem likely to sound better. 

Also, ever since my friend Michael sent me chassis photos of one of his favorite amps...

... I've been itching to try out a cathode-biased "quartet" power section (pictured above): 4 power tubes in push-pull configuration (2 at each end of the output transformer), with a simple resistor to ground at the cathode ends of the tubes, to provide grid biasing (rather than the more complicated bias designs of the R/J and Scarlett amps).

So, with an idea of the power section I wanted as a starting point, I started thinking of other amp features I've wanted to include in a build but haven't: a cathodyne phase inverter for example (I've only implemented LTPs), new and different tonal shaping, buffered fx loop, etc. I have some 12" unused speakers around, and wouldn't mind having another lightweight combo, so maybe this one can be that.  Several well-known brands/builders have 4 x EL84 power sections to draw from: Vox, Komet, Matchless, Peavy, and they provide useful background on both fixed and cathode-bias versions, but researching these and others, I found I kept going back to the 70's era Orange amps. 

Orange 100 preamp schematic 

Some models use a DC coupled cathodyne PI, which I like, and included some other pre-amp features that interest me like the late stage variable gain boost and the "Depth" selector for shaping the input signal (main interest here is in using up at least one of the several 6-position switches I have laying around), yet are also fairly straight forward circuit-wise with a no-nonsense front panel. 

Oh, and given my recent and satisfying experience converting the R/J power section from a pair of EL84s (~18 watts) to a pair of KT66s (~25 watts), I'd like to design the layout and circuit with an eye toward a future conversion, from EF80s to EL84s, in case I'm not happy with the low watt outcome. 

So, now the task is to stitch these requirements together in a way that will sound good and work well, ideally as whole, greater than the sum. 

... and here're the fruits of that effort: 

Low power power-supply modeling 

(to figure out what transformers and filtering will be appropriate)

Modeled draft schematic

(the nitty gritty of how it will work and what parts are needed, 

successfully modeled using software)

Draft layout

(how might it all fit together in the chassis)

Cabinet configuration

(how might it all fit together in the cabinet)

It's hard to precisely identify the point at which various curiosities and puzzle-solving desires coalesce into the critical mass of motivation required to launch a project into a committed stage. Even though I've worked out enough of it to start building, I don't know if I'm quite there... 

 

 

AC4 Refinements

So, the Chekhov has survived its first gig, and it did a fine job projecting me with a jazz quintet playing in a ~250 seat former church... as far as I could tell from where I stood. 

I've since re-configured the overdrive circuit to more or less match the 2SE - that is a Dumble-inspired two triode affair - and that aspect of the amp is now usable and pretty nice sounding. It required significant changes to the board and the addition of a 3-pole tag strip to accommodate all the additional components. 

What was originally implemented as this:

... is now this:

I've read more about the reverb chip I got, and while it does seem to be the only choice, in most cases the person posting about it has reported lots of complicated circuitry to accommodate it and its integration into a tube amp. I think I'm resigned to nabbing an inexpensive pedal that just lives with the amp, and to letting go of implementing reverb in this amp. 

I'll have to re-do the panel labels to match the new controls and cover over the remaining hole. 

Left to do:

• Build new cabinet in 1/2" Birch to shed 8 lbs. 
• Front panel label. 
• Install power tube retainers.