The Marv II

My second Hi-Fi amp

Amp porn (click to enlarge)

 "The Marv II" (left) and "The Marv" (right).  Both are single ended.  Same power supply and output transformers, but rather different otherwise.  The Marv uses 6B4G's (directly heated triodes), fixed/adjustable bias, and a 6SL7 un-bypassed driver.  The Marv II uses 6V6 pentodes and is auto biased with a 6SN7 bypassed driver. I designed/built The Marv II for a friend who lives off the grid. Low power consumption was a must. Drawing about 22 Watts, this thing rocks his supper efficient Klipsch La Scala's.

	The Marv II	The Marv
Power Supply	222v R-core transformer, Hexfred rectified, Cap-Choke-Cap filtered, Low ESR caps	253v R-core transformer, Hexfred rectified, Cap-Choke-Cap filtered, Low ESR caps
Power Supply Ripple	32mV < 0.01%	36mV < 0.01%
Controls	3 position switch Off/Standby/On	3 position switch Off/Standby/On, Bias Adjust
Driver	6SN7 bypassed with Oscon caps	6SL7 unbypassed
Power Tube	6V6 pentode, auto biased, Class A operation	6B4G directly heated triode, fixed bias via separate supply, Class A operation
Output Transformer	James 5kohm	James 5kohm
1kHz THD	3.0% THD, 2nd harmonic: -30dB, 3rd: -41dB	1.4v pk-pk input => 3.5 Watts RMS, 2.5% THD, 2nd harmonic: -32dB, 3rd: -75dB
Damping Factor into 8ohms	2.44	4.54

Some final pics of "The Marv"

(click on a pic to enlarge)

"The Marv".  Named after my dad who instilled in me a love for building things.  Most of the tools I used (drill-press, mill, punches, bits, etc.) were his, as were a handful of the parts (the biasing socket, lamp socket and jewel).  Not an hour went by working on this when I didn't think of him and yeah, that's one of the main reasons I built it.  And now, I think of him every time I fire it up.

OK, my first amp has been usable for over a year.  It's provided plenty of aural joy and thermionic warmth to my chilly basement, however, I hadn't actually glued or oiled the wood frame, added feet or covered the power tranny.  Finally, I made time.  To add a mounting surface for a set of Vibrapod feet, I decided to add corners to the frame.  To add strength, I dowel jointed one side of each triangular corner.

The corners are inset about 3mm to allow a perforated aluminum panel to sit inside the frame.  Drill a hole in each triangle, ream a corresponding hole in each corner of the perforated panel, screw it all together and voila:

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!!!!