Final 2 evenings of building...

All that was left was the signal path:

• Shielded cables from the RCA jacks to the driver board (black)
• Driver board to driver tube connections (upper center)
• Coupling capacitors (yellow) from the driver to the power tubes
• Power tube to power transformer (green wire)
• Power transformer to speaker binding posts (blue/brown twisted pairs)

From the bottom

From the top (power transformer cover still unfinished)

After inserting the tubes and powering up, I was shocked when the high voltage fuse blew, but not the mains fuse.  It took me hours of looking for a short in the tube sections before I realized, it was the rectifier. I hadn't realized the hexfred heat sink lugs were not floating but tied to the fricken cathode!  Thank god I decided to add the high voltage fuse or my power supply would have been toast.  I added nylon isolation spacers between the panel and the hexfreds and then all was good.  Voltages were within 1% of my original prototype.  This was my first test of the dual channel biasing knob.  In hindsight, a multi-turn pot would have offered finer adjustment, but I probably would have paid $80 for a stereo multi-turn pot.

It was late, but I hooked up some speakers and a CD player, popped in "Are you going with me" (Pat Metheny), turned off the lights and sunk into my comfy chair and blissed out watching the blue glow of the output tubes dance to the repetitious two note groove and then Pat's explosive climax.  Sorry, that started to sound like a penthouse forum.  60 cycle hum was more that I would have liked, but par for the course in a directly heated output tube with AC heaters.  Other than that, I'm very happy with this amp.

Remaining work is mainly aesthetic:

• Add venting and paint the power transformer cover
• Final sand and oil the wood frame
• Glue the frame
• Add corner blocks, bottom mesh and vibration dampening feet
• Finish and install the aluminum tube socket covers

Day 3 of building...

Added AC mains fuse and switch (upper left), Bias supply (circuit board - upper right), Bias pot and jack (center).

Day 2 of building...

After a couple of evenings building, I've completed the power supply and routing of the heater lines.  The power transformer is on the top of the plate with wires passing through grommets to this side.  The components you see on this side (from top-left to bottom-right): Power Switch, Power Lamp, Mains Fuse Block, Tube sockets, Power Supply Choke, Secondary Fuse Block, Rectifier Bridge (4 hexfreds + snubber caps), Smoothing and Reservoir capacitors with snubber caps)

Wiring Legend:

• Yellow-orange = Neutral leg of the transformer primary
• Red to fuse block and Orange to rectifier block = High Voltage Secondary
• Blue = 5v winding for Bias supply
• Grey = 6.3v windings for tube heaters
• Red/Black = Driver B+ and Ground

Purple Haze

For 2+ months, I've been unable to make any progress on my main 6B4G based amp cuz the aluminum plate need coating.  Bare aluminum needs a coating so it doesn't oxidize - that white powderish look.  I had first planned to anodize, but the best deal I could find locally was $150.  Too high, although I could have done 6 to 10 panels for that (next time!).  Alodining, I was told, was the next best solution (no pun intended).  I soon found out that stuff contains hexavalent chromium, the toxic stuff that Erin Brokovich exposed.  I wasn't about to use that, but found an non-chromium version online ("Alodine 5700").  It was supposed to take 7 days, but ended up arriving 7 weeks later.

I set up my basement bathroom with the following:

• Heated to 70+ degrees F
• Installed hooks for drying the panel in front of a heat lamp
• Wore a VOC mask and solvent proof gloves
• Worked with all chemicals in the tub in case of spillage. 

Then I did the following:

1. Prepped the panel with an alcohol rinse, vinegar rinse, water rinse, the air dried.
2. Filled the tank (a shallow Rubbermaid box lid) with Alodine 5700.
3. Immersed the panel in the Alodine for 2 minutes.
4. It didn't look done, so I re-immersed for another 2 minutes.
5. Rinsed in RO filtered water.
6. Funneled the Alodine back into the container (it's reusable)
7. Hung and air dried in front of a heat lamp

As it dried, I knew I wasn't going to like the end result.  Purple haze and rather streaky...

... probably from not prepping the surface with the recommended acid bath.

Oh well.  But now what?!  I considered just painting it silver since now it would hold paint - that would look cheesy.



So I scubbied down the Alodine coating with silver polish.  Here it is 1/3rd polished:

It looked pretty good - probably nearly down to bare metal again.  But now I was ready to just chock up the Alodine waste of time as a learning experience and do what I should have done in January, and slap on a clear coat of engine enamel. 

The clear coat did somehow capture some small dust particles while it was drying outside.  But overall, I'm pretty happy with it.

Finally... TIME TO START BUILDING!!!!

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.

Knowing the drill

I've spent my last few evenings marking and drilling the aluminum panel.

Tools: Caliper and rule for measuring, square and scribe for marking, center punch and hammer for making center divots for each hole.

Once marked, it's time to drill...

Center Dill #4; Stepped Unibit; Large Unibit, Carbide tipped Hole Saw

All holes (nearly 100 of them), regardless of size, are started small and are opened larger with progressively larger bits.  All are started with a Center Drill.  I have a #2 and a #4.  For small holes or ones which need very precise placement, I use the #2.  The #4 is great for #4 screws, venting holes, etc.  Larger holes then get the small Unibit.  Stepped Unibits are an amazing time saver.  There's no need to swap out bits in the chuck, just keep going deeper until the step is the right diameter.  If you just graze the hole with the next biggest step, it even deburrs the hole.  For the tube socket and output transformer holes, I use a 35mm Carbide tipped Hole Saw.  It takes a lot of torque to cut a hole that size.  My drillpress really bogged down, but with a bit-o-lube and unclogging the cutters every 100 turns or so, I could create a nice hole in about 10 minutes. 

Now for the irregular hole needed for the AC power connector:

Marking with template; Corners Drilled; More holes; Final hole after filing

Then a final scrub with steel wool and a green pot scrubber.

Final Panel