Side Project: A lower-power power-amp

I'm somewhat obsessed with saving power - just ask my wife who calls me the "Heat Miser". 

So, the idea building power hungry, Class A, Single Ended tube amps seems a bit contrary to my lifestyle.  Well maybe.  My prototype 6B4G Triode based amp draws just over 40 Watts... not too bad and I'm still satisfied with that design.  But while waiting the postman to deliver a few extra supplies for that one, I prototyped a simple Single Ended amp based on a 6V6 pentode.  I chose that tube because it has a rather low heater draw for the power output it produces (2.8 Watt to heat it for up to 4 Watts of output).  I used a 6SN7 for a driver.  It's renowned for it's low distortion characteristics.  After a couple hours crunching numbers to bias the tubes, another hour to scrounge parts, another hour to build it, and 10 minutes to find one missing ground, I had a working channel.  It sounded pretty good, so I copied it to a second channel.


Now it sounded really good, especially on semi-acoustic music with nice vocals (Neko Case, Fleet Foxes, Yes, CSN, Jim White, Iron & Wine, etc.). The bass end was great.  After first spark up around 8pm, I literally listened to various tracks all evening - quite a break-in.  It was somewhat unforgiving on bad or old recordings and didn't move me so much on rock tracks, but overall it was very listenable.  Even before measuring distortion, I knew it had more 2nd order distortion than my Triode amp.  Sure nuf, it measures 6.8% THD @1kHz (-23dB 2nd order, -46 dB 3rd order).  Not super, but there's still some room for tweakin.

Here's the kicker - It draws only 22 Watts while putting out about 1.5 Watts per channel - plenty for living room levels on efficient speakers (90+ dB/m).  So for the same carbon footprint of my wife's 5 minute daily hair drying (1200 Watts), I could listen to music for 4.5 hours!  I'll let the speaker draft and tube heat dry my (orange spikey) hair.

More wood fun

My original plan was to use a pair of dual plastic binding posts for speaker connections.  I drilled appropriate holes in the panel for them, then decided that I'd rather use nicer (single) gold posts.  All the ones I found online required smaller diameter holes than the ones that I already drilled.  They also all seem to have cheesy plastic isolation washers.  To replace those and make up for the large holes in my panel, I decided to make my own insulation blocks out of scrap Ipe.  A simple block would float on the panel surface, so I needed a block with built in shoulder washers. Perfect job for a milling machine...

I started with blocks about 7mm thick and milled off about 2mm of thickness except where I needed the 2 donut shaped shoulders.  The workpiece is mounted in a vice and moved under the milling bit using X/Y screwdrive knobs.  Milling a circular pattern with X/Y knobs reminds me of drawing circles on an Etch-a-sketch.  I guess it's good for the hemispheres of my brain to work together once in a while ;-)

I finished the day by doing the final chassis sanding.  First, I used a pad sander with 100 grit on all surfaces.  Some joints also needed some leveling with a hand block to match their connecting piece.  I then rounded all corners with a long strip of glue backed 100 and 150 grit stuck to a work board.

I finished all surfaces with 220 grit.  Finally, I can clean up all that nasty to breathe Ipe dust.  Love the wood, hate the dust.