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!!!!

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

Woodworking can test my tolerance

I spent 3+ hours obsessing yesterday over mere hundredths of an inch.  My dovetailing jig is cool, but when few hundredth's of an inch make a significant difference in the tightness of a joint, I think Rockler could have done a better job of making their jig more precise.

The general idea is that once you have the jig set up for a given width and thickness of blank, you should be able to clamp a new set of blanks in, route away and Bingo... perfect joint.  One problem is that the fence that sets the depth of the cut is attached to the upper clamp, but the clamp has a substantial amount of play.  So the fence can shift up to about 0.05" between clampings.  It took me a while to figure out that even though two passes had identical adjustments, things were shifting around every time I reclamped.  Ok, so I'll remeasure and readjust the fence depth every time.

The next problem is alignment/squaring of the blanks.  The main magic of a dovetailing jig like this is that you route both blanks at the same time.  There's a little alignment stop which is supposed to keep the two blanks aligned relative to each other and the jig.  My first beef is that it is free to float way out of square to the jig.  Ok, I'll square it each time I adjust it (3 screws).  But wait, it's made of fricken no-so-hard plastic and it flexes!  The upper part of the stop has 2 screws so squaring it pretty much keeps the upper blank square.  The lower portion has only one screw.  Trying to square it is like trying to benchpress with one arm.  Without a second tie point the stop rotates around the single screw and flexes based on the position of the upper.  That would be just fine if it was metal which doesn't flex, but as is, the lower blank is almost never square if you simply rely on the stop to keep it square.  In the end what should take about 1 minute of set up per joint takes about 10 and still may result in a bad (loose, tight or misaligned) joint.

To make up for looseness of tightness, the height of the router bit can be adjusted.  I found that the difference between loose and tight could be as little as 0.015".  You girls take note.  Yeah right, like any girls would read this blog.

Anyway, enough bitchin'.  After about 16 test joints and wasting about 10+ inches of length of my good blanks, I made the final cuts and I'm pretty satisfied with results.  It holds together without glue and only one of the 4 joints needs some filing to get the pins and tails to align.

The next step is to saw a kerf (slot) just below the top of the frame for sliding in the aluminum panel.

Enclosure layout design and mockups

Enclosure layout design, despised by many builders, is actually something I've always enjoyed.  Probably because I started making enclosures for my projects with my Dad when I was about 6 years old.  From cardboard with holes cut with scissors to drilled aluminum.  Those projects usually had just a few switches and lights, buzzers and bells, but the basic concepts are the same.  The main work is drilling the holes, but planning where to put the holes is definitely an art.  It involves ergonomic evaluation to make the device usable, but part placement must consider the functional constraints of the innards.

In an amp, that generally means making sure that components which connect to each other are close to each other and things that could induce noise to each other are far from each other.  Since most components connect to multiple other components, various placement permutations result in the need to consider oodles (technical term) of tradeoffs simultaneously.  Unlike semiconductor block/module placement and routing (which I did professionally for 10 years), the amp layout problem is solvable in polynomial time.  In fact, the problem space is so relatively small that even the meager human brain can figure out a set of component placements which are nearly ideal.

For me, noise is the number one consideration.  Electromagnetic induction is used to our advantage in many components of an amp.  Power transformers, output transformers and chokes all work because of electromagnetic induction.  However, that phenomena is also the leading cause for noise in an amp.  Any conductor (wire, resistor, capacitor, transformer, etc.) is susceptible to induced current from another conductor. To be induced, that current must be oscillating and thus has a frequency which can be heard if it or it's harmonics are within the audible range.  Most hum heard in an amp powered in the US will be either 60Hz like the incoming AC line voltage, or 120Hz (2 x 60Hz).  Either is certainly audible and annoying.

It's near impossible to eliminate the source of electromagnetic induction, but we can shield sensitive components from it, or keep sensitive components far from problematic sources.  Fortunately, induction has an inverse square relationship to distance.  So a small bit of distance between components can have a dramatic effect in the amount of induced current.  Thanks Physics!

Since transformers are the #1 source of EMF in an amp, I feel it's most important to place those first starting with the power tranny.  In this amp, I decided centralize all the power components in the center rear of the amp (see below).  Then the audio portions of the system can exist "far away" on either side. I did allow one twisted pair carrying AC to run along the side, to the front to the power switch and lamp.  This was the single case where I let ergonomics and aesthetics trump performance.

My other layout goals were:

• Arrange all inductors (transformers and choke) for minimal electromagnetic interaction (eg. Orthogonally oriented cores, spaced as far as possible or on opposite sides of a grounded shield)
• Keep AC lines > 2cm from DC rails or signal lines/components (AC lines include power entry/fuse/switch/lamp and tube heater lines)
• Keep AC lines as short as possible (power switch and lamp exempted)
• Arrange signal path components so the path is roughly linear without recrossing (ie. start from front center, out to the power tube, back to the output tranny, then back to the speaker terminals).
• Employ a "Star" grounding scheme with the lug very close to the amp center and close to the power channel
• Optimize heat dissipation.  Keep heat generating components (transformers, power resistors and tubes) away from each other and away from heat sensitive capacitors.

I decided to use Google sketchup for doing my layout.  It's a trip constructing in 3-D and took me a few nights to become productive.  It's an awesome tool though and one of the coolest things is that there's oodles of models for most everything you can think of online.  I found switches, jacks, tubes, etc.  The things I was left to construct myself were the James output transformers, power supply caps, power transformer cover, and circuit boards.  Here's what I came up with for the top view:

The output tubes don't quite look like 6B4G's, but good enough for government work...

... and after lots of jockeying trade-offs, the underside view:

... now "eyeballing" with real (top and bottom) parts:

Time to start planning the final build

With most of my design work complete and measurement and listening tests of my prototype showing nice results, it's time to start planning the final build.  That means final part selection, wood frame prototyping and layout planning.  I've made probably 15 orders in the last 6 months from 6 different US vendors, 1 Chinese, and the rest on Ebay.  Lots of part swapping means my parts bins are brimming with parts to be used on future prototypes and projects.  My current prototype uses all of the components of my most resent design revision except for the power switch, jacks, etc. and it still uses carbon resistors which will be swapped with metal film.

For the wood frame, I've decided to use some Ipe (aka Brazilian Walnut or Iron Wood) boards which my coworker had left over from his very nice deck (Thanks Jeff!).  It's super dense, fairly unforgiving to work and it creates a nasty dust which I shouldn't have been breathing, but it's fricken beautiful wood.  The boards were roughly 6" wide by 3/4" thick.  I had a 2.5' board and a 1.5' board.  I ripped them (roughly) in half and had 4 2.5" strips.  Perfect!  I even justified a new carbide tipped ripping blade which did an amazing clean cut.

I wanted to finally try my hand at some nice half blind dove tail joints.  I've had a jig to do those for years.  I spent a many hours one weekend learning how to set it up and use it.  There's about a dozen adjustments and it's imprecise enough that the only way to really get a tight joint is by iterative trial an error.  Keeping everything aligned and square is the key.  Also logging measurements for each trial.  I had some hemlock blanks which had very similar dimensions to my new Ipe blanks.  I used those to hone my dovetailing skills and made a few small frames.  Here's a "half size" frame about 7" x 5" with a kerf (slot) for the aluminum top panel to slide into:

Driver Design and Fun with Distortion

My original *mono* prototypes had the 6B4G output tube biased using "Grounded Cathode biasing" or "Auto Biasing".  They included a 6SL7 driver tube configured in SRPP (Series Regulated Push-Pull).  The driver preformed pretty well: decent gain and distortion, but because SRPP has the cathode of one triode at half the HT voltage (125v in my case), I had to drive it's heater from a supply separate from the heaters of the output tubes or else I'd exceed the maximum heater-to-cathode voltage of the driver tube.  In the end, my power transformer had 2 heater windings, so I probably could have made it work by using one for the power tubes and one for the driver tubes, but I had already set off into the land of fixed bias for my output tubes with a simple 6SL7 common cathode driver (see my previous post).  This reduced my tube count to three (2 output tubes plus the 6SL7 with one triode to driver each channel). 

My first test with that configuration in October resulted in a much better overall sound.  The details were nice, gain was lower than expected but the distortion profile was generally more pleasing (~4.5% THD@1kHz, -27dB 2nd order, -35dB 3rd - see red spectrum graph below).

Still not outstanding, but better.  However, that was just one channel.  Unfortunately, duplicating the circuit to the second channel resulted in almost double the distortion, but more gain.  WTF!  Both channels were using identical components.  The only difference was the cathode bypass capacitor on the driver triode.  I had used a junk box generic 47uF cap on the original channel (the one with the good distortion) and a decent Nichicon VX series 47uF on the second channel.  Sure enough, swapping the caps pretty much swapped their distortion and gain profiles. So the crappy generic cap had some magic mojo for reducing distortion...

Well, kindof.  Turns out, it's value measured by my multimeter was about 0.02uF.  For a bypass cap, that's essentially not bypassing much other than very high frequencies.  It wasn't a magic cap, it was a blown cap which wasn't far from no cap at all.   Many designers believe that with a proper design, a bypass cap shouldn't really be necessary.  It increases gain, but also increases distortion.  It's used most everywhere because we generally don't want to "throw away" gain.  So to not include a bypass cap means that the lower gain has to be designed into the whole amp.  Fortunately, the 6SL7 driver I'm using has a high transconductance and yields about 25x gain even without a bypass cap.  Thus with an input signal of 1V RMS, I could expect about 25V RMS (~ 35V peak to peak) on the grid of the output tube which is just below the maximum for the operating point that I selected for my output tube.  Almost like the 6SL7 was born to drive the 6B4G!

Look Ma, no driver bypass caps!

Output Section Design

The output section includes the output tubes, the output transformers which drive the speakers, and the biasing circuitry.  The job of the output transformer is to take the high-voltage/low-current/high-impedance output of the output tube and convert it to a low-voltage/high-current/low-impedance output for driving the low impedance (4/8/16 ohms) speaker.  Early on, I settled on a pair of James output transformers, partly because they look awesome in their potted cans, but also because budget conscious Single Ended amp builders seemed to like them, they pair well to the 6B4G output tubes I chose and gave me an option of 3.5kOhm windings and 5k.

In my first mono prototypes, I biased the output tube the good old fashioned way, Grounded Cathode (aka Auto bias).   Because I didn't have enough 6.3v heater windings on my power transformer to heat two output tubes plus two driver tubes in a SRPP configuration, I decided I needed to be able to power both output tube heaters from one 6.3v winding.  That suggested that I should consider using Fixed biasing of the output tubes so that both cathodes could be tied to the same potential (ground).  Fixed biasing adds a negative DC potential to the grid of the output tubes.  That allows a positive AC voltage (the signal) to applied to the grid and still be at a lower potential than the cathode.  The main reason fixed biasing isn't often used is that it requires an extra power supply.  The supply draws almost nothing though (a small handful of microamps in my tests) so the only real design negative is the extra parts count and associated real-estate they will occupy.

I built a quick and dirty (yet remarkably clean) negative supply, reconfigured the output stage, and was amazed by my first listen.  The upper-mid distortion that had bugged me for weeks was almost gone.  When I looked at how other designs (like my Cary SE-1) implemented fixed bias, I was surprised that they usually use the same negative supply to bias both channels.  That seemed odd to me since that basically means they are tied together.  Granted there's 400k ohms or so of resistance between the two grids, but still the signal will take that path with a resulting crosstalk.  I tried shorting the Right channel of my SE-1 to ground and sure enough the Left channel bled to an audible level to the Right speaker.  Granted, it was maybe only 2 to 5% the volume of the Left channel, but still, that's substantial for "Hi-Fi".

I decided I could do better, and I did.  I searched online for hours for a small power transformer with dual primaries.   It's common on large trannies, but not small (5 to 12 volt) trannies.  My plan was to not connect the transformer to the main voltage, but to connect it backwards to a heater winding of my main power transformer keeping it as THE isolation unit and not having to worry about fusing two (or three) power trannies, etc.  So the voltage gets stepped down by the main power tranny from 110v to 5v, then back up to about 90v by the small biasing tranny.  Since it has dual primaries (which I'm using as secondaries), I get two -90v adjustable supplies, one for each channel - zero crosstalk introduced by the biasing.  This was the one idea in my whole design which was completely my own.  I'm sure someone has done it before, but still, it felt great to have that spark of a novel idea and avoid the need for a second transformer.  Below is what the final dual -90v supply looks like:

I needed about -90v so that I could slap a high resistance potentiometer on the end and have the resulting voltage divider produce my target biasing voltage (roughly -40v) somewhere in the middle of the potentiometer's range.  A fixed bias configuration really simplifies the output stage circuitry.  Below is my final output stage schematic with the bias supply applied where you see "-39 v".

Auspicious Day - 10/10/10

My First STEREO prototype...

... Not too pretty, but it sounds pretty darn good!

AC/DC - For Those About to Rock!

My original plan was to employ DC voltage for heating the tubes, but keep prototyping simple by using AC.  DC heater voltage is used mainly because it should reduce the amount of ripple induced into the DC rails and signal path, especially for Directly Heated Tubes like the 6B4G.  That should reduce the audible "hum".  Today I tried DC (added rectification and smoothing) for both the driver and output tubes.  I was surprised that for the driver tube, DC seemed to make absolutely no audible difference to the small hum I hear, and no measurable difference to the measured ripple.  DC on the output tubes, actually increased the hum.  I used one very one high quality 4500uF cap and a low EMF 15,000uF cap, so the DC was about as smooth as it could reasonably be.  With DC, one lead of the heater is grounded while the other is at 6.3v.  By contrast, using AC, both leads are connected to the transformer windings with the center tap at ground, so essentially, each heater lead is at 3.15v.  All I can think is that the symmetry of the AC is preferable even if it's oscillating.  The hum really isn't that bad, so for now, I'm gonna stick with AC heaters.

Power Supply Design

The power supply of a power amp is very much the heart of the system.  Instead of blood, it pumps electrons ("current") through the vacuum tube heaters allowing them to do their thang.  The pressure at which the current is supplied is called "voltage".  In addition to the low voltage heaters, tubes require a high voltage (aka "high tension" or "HT"), on their anode.  It's this HT voltage that allows a tube to amplify the voltage of the incoming audio signal from less than a volt to hundreds of volts.

A good portion of the circuitry, especially the big and heavy parts, within a tube amp is actually the power supply circuitry.  There are numerous different common power supply topologies, each providing the general functions of power-line isolation, rectification, smoothing and stabilization.  Isolation is usually performed by a mains transformer.  It keeps the amp circuit electrically separate from the power line and usually steps up the voltage from 110V to a tube friendly voltage (200V to 500V).  Rectification turns the AC voltage from the mains transformer to DC used by the tubes.  Rectification can be done by a rectifier tube or by silicon diodes.  The voltage output of either type of rectifier is not pure DC, it's all positive relative to ground but still ripples (oscillates at 120Hz) around the nominal DC voltage.  To smooth out the voltage, we smooth out the ripple using honker electrolytic capacitors.  Those are usually the largest caps in an amp.  Capacitors take time to charge and discharge so they slow the rate of the voltage ripple almost to the point that it becomes almost perfect DC.

Based on the 6B4G output tube that I selected, I have the following power supply requirements:

• High Voltage (HT): 225v to 325v @ 120mA
• Heater (LT): 6.3v @ 2.3 Amp (1A x 2 for the 6B4G output tubes, plus 0.3A x 1 for the driver tube)

Choosing a power transformer is probably the most critical part choice in a power amp.  It's operating parameters and performance influence most every other part of the design.  R-Core transformers are fairly new technology.  They have the following claimed advantages:

• Lower profile and smaller size.
• Lower stray EM field from the round cross-sectional area and the balanced windings on either side.
• Lower core losses from no cuts the in core and minimized distance between the core and the windings. 
• Lower temperature rise and noise form the round cross-sectional area and tapered slitting.

The main disadvantage is that core saturation is more likely if not careful.  Since efficiency, size and low noise are 3 of my design goals, an R-Core seemed a perfect fit.  I found some nice ones built in China which included a nice set of windings (3 sets of 6.3v windings, 230v and 260v HT windings).

I obsessed for weeks to select a power supply topology.  I've been tweaking it for months since.  The basic topo can be seen in the schematic below.

Silicon diodes for rectification, then a capacitor-choke-capacitor smoothing circuit.  It's a common design for amps which spurge for the extra cost/weight/size of a choke.  I chose a choke which was large enough to do a decent job resisting current spikes, but still small enough that it wasn't hard to justify.

The capacitors I chose were a spurge by most designers standards but well justified by mine.  ESR (equivalent series resistance) of smoothing capacitors has a dramatic effect on their ability to smooth voltage ripple.  A perfect capacitor would have a ESR of Zero ohms.  Capacitors made for high quality switch mode power supplies have much lower ESR values than typical caps (even super expensive Audiophile caps).  I found some made by Mallory, which cost about $35 for the pair, but my simulations showed that using those, I could achieve a ripple voltage of about 50mV (.02%).  That's very quiet and DAMN good!

My next tweak was to add snubber capacitors to the choke to help suppress high frequency voltage spikes and to add snubbers across each rectifier diode to reduce switching noise.  Here's what it looks like:

Eventually, I'll probably replace the standard 1N4007 diodes (hidden under the brown snubber caps) with super fast switching hexfreds.

Side Project: a Switched Attenuator

I use a PC oscilloscope for many AC measurements and for measuring distortion.  My audio interface connected via USB provides the analog inputs (2 channels).  Unfortunately, they have a maximum input of 2Volts.  I need to measure up to 500 volts.  For a while, I used a couple high wattage resistors as a voltage divider to attenuate the voltage down to 2v or less.  Sometimes I need to measure low voltages, sometimes high, and switching the resistors became a pain in the ass.  I decided to build a switched attenuator.  It's basically just a ladder divider.  Since I'm often measuring the output of the amp, I also added 8ohm thick film resistor which can be switched in to provide the speaker load without a speaker blasting out ear piercing test tones.  Here it is with one channel completed:

Choosing an Output Tube

Choice of an output tube dictates the power supply requirements and the type of output transformer which can be driven.  I've changed my idea of the perfect output tube 3 times now.  My first choice was an EL84 pentode because that's what I had used for my guitar amp that I built in 2003.  My first prototype of a hi-fi using an EL84 were somewhat disappointing though.  It at least made sound, but it included some upper mid distortion and wasn't very loud.

I decided a 6V6 might be a better choice: higher output and easier to find NOS (New Old Stock) tubes around.  That was a bit louder, but still not enough to "crank".  The more I read, the more I started to think the sound I was after was really a large bottle directly heated Triode instead of a Pentode like the EL84 and 6V6.

Now I was on a quest to find the perfect Triode.  I've always loved the 300B, created by Western Electric in 1937 to amplify telephone signals.  It can belt out 7 Watts and was used in most every movie theater before the takeover of the transistor in the 70's.  The problem with the 300B is that it has a 5volt heater supply (my power transformers have 6.3v windings) and even new production 300B's cost well over $100.  That led me to consider the 2A3 and better yet, it's sister tube, the 6B4G which has a 6.3volt heater, Octal base, and still puts out 3.5 watts.  It's specs fell within my main design goals.  I scored a "matched" pair of NOS (New Old Stock) Sylvania 6B4G on ebay for $67.

 Those will be the output tubes for my first version.  For my second round, I might design in a 5volt heater supply so I can run a pair of 300B's.