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.

Prototyping

Since designing an amp requires lots of iterative suck-it-and-see (as the British say), I decided I needed a quick and easy way to make circuit changes.  I considered using a bread board, but those aren't usually rated for more than 120 volts and mine will be running about 300 volts.  My father often used barrier strips for experimenting and I happened to have about 10 of his old bakelite strips from the 50's.  I mounted those to a piece of Masonite, added 4 feet and I had my prototyping board.  The screw heads make it fairly easy to swap components.  Here's my first working single channel amp showing the power conditioning (top), a 6SN7 driver (large tube) and a EL84 power tube (smaller tube).

That prototype was a bit low powered and had quite a bit of midrange distortion.  I soon was on a quest for a set of tubes which would suit my needs better.

Power Amp Design Goals

To avoid analysis paralysis and getting stuck trying to design an amp that's all things to all people, I realized I needed to formalize my design goals.  

• Class A, Single Ended design
• Output: > 3 Watts per channel 
• Overall power consumption: < 50 Watts
• Heater power consumption: < 20 Watts (6B4G: 2 x 6.3 = 12.6 Watts)
• THD (total harmonic distortion): < 5% (ideally < 3%)
• Power tube cost: < $70 each (rules out 300B, 45, etc.)
• And of course, amazing sound 

50 Watts of input power is rather low for most power amps, especially a tube amp, let alone a Class A single ended one.  My power consumption goals are not just to produce a "greener" amp.  It seems silly to consume power just to turn it into heat.  I've seen many amps with huge power resistors (10 to 25 watt) for biasing the power tube or in a DC heater circuit to trim the voltage gained via the rectification.  I believe, if you design for the right voltages, you generally shouldn't need to use big honkin' power burning resistors to trim it down.

Also, I'm building at least one of these amps for my friend who will be living off the grid via photovoltaic solar in New Mexico.  I'd like this amp to make as little of a dent in his daily collection of power as possible. 

Previous Projects: My Headphone Amp

This past May, my best friend of 35 odd years asked me what I would recommend for a headphone amp.  I told him "One that glows in the dark" - A vacuum tube based amp.  I was excited by the idea of a new tube project.  I researched dozens of designs and settled on an open source design (http://gilmore2.chem.northwestern.edu/projects/cavalli2_prj.php) mainly due to it's use of a medium high voltage supply (65v).  Most other designs ran the tube on 24v or less.  Yeah, that might still amplify the signal, but starving the plate of a tube will always result in unnecessary distortion.

Anyway, I found a dude in China who sold to me a couple of boards for the SOHA headphone amp.  I sourced the parts and my friend and I each built one over a few weekends.  Once I fired up the first build, I noticed a low rumble in the headphones during power up.  With a volt meter, I confirmed that it was passing almost 4 volts DC to the headphones until the tube reaches it's operational temperature.  This was not acceptable and could definitely damage a set of headphones.  Thus my main mod was to add a dual pole "standby" switch to disengage the headphones during power up and power down.  Other than that, my part selection was mostly as the SOHA project suggested.  I used all Vishay Resistors, Nichicon Electrolytic Caps and 0.22uF Musicap coupling caps (the yellow ones in the photos).

Drilling the rectangular hole for the power entry connector:

Drilling the 1-1/8" hole for the 12au7 tube:

Previous Projects: My Guitar Amp

Back in 2003, I found ax84.com, an opensource community for designing and building tube guitar amps.   My goal was to build low wattage single-ended amp with wide tone/gain options and reverb.  ax84 had a couple of schematics for low wattage single ended amps but none with reverb or tone variability beyond a typical bass/mid/treble tone stack.  I settled on the Hi-Octane (http://ax84.com/hioctane.html) design for the basis of my design.  It is based on the EL84 pentode power tube with 2 12AX7 preamp tubes and a tube rectifier power supply (4 tubes total).  I wanted to have the ability to switch between two "channels" (rhythm/lead, clean/dirty, etc.).  I decided I could achieve that by adding a switch between the first and second gain stages to allow bypassing the second stage and a pot to control it's gain.  I then found a circuit for a Fender reverb driver/recovery and inserted that with some impedance matching between the tone stack and the power tube. That added two extra tubes (a 12AT7 and 12AX7 and a audio transformer).  I added an adjustable bias circuit and my final mod was to add a switch on the EL84 screen to allow selection between pentode mode and triode mode for a more vintage/gritty sound.  Many of the parts I used we're old parts from my late fathers part stash, including 4 switches, the jacks, the lamp/jewel and the brown turret boards.
Here's the some amp porn:

Here's a closeup of the tone stack, and reverb driver/recovery circuit:

Why a tube amp?

While analyzing the circuit and testing my first tube/hybrid headphone amp, I rekindled my love for tube based audio.  Sure tubes distort more than transistors, but that's also the main reason they sound more natural and pleasing than transistors.  Tube distortion is mainly of the even harmonics (for a 1kHz signal, 2kHz, 4kHz, etc.), transistors distort mainly the odd harmonics (3kHz, 5kHz, etc.).  Even harmonics are musical intervals of the fundamental and of each other.  Odd harmonics aren't musically related so they sound more harsh and unnatural.  So transistor hi-fi has adopted the theory that they just need to apply tons of negative feedback to reduce all distortion to near unmeasurable levels.  Ok, now you have a completely sterile amp which does it's main function of amplifying, but has no character of it's own.  Fine for some listeners, but I really like the dynamic and organic sound of tubes, including a splash of second order distortion.

Once we had our headphone amps, both my friend and I both started thinking about how cool it would be to create a tube based hi-fi power amp.  I started reading everything I could about tubes, tube theory and amp designs.  I purchased some software to model the driver stage, the output stage and the power supply.  Soon, I was obsessed with designing, prototyping and building my own.

I decided I "needed" a good amp of a similar design as a reference to know how my prototype compared.  I found a great deal on a Cary SE-1.  It can use either 2A3 power tubes or 300B.  Both sound amazing yet have their own distinct character.  The 2A3 is probably a better all around sound, very detailed and great imaging.  The 300B has a darker, warmer feel.  Now, I'm really hooked on tube audio and look forward to chance I get to go into my man cave and rediscover classic old albums.

Why a blog?

I've been designing and building a vacuum tube based power amp for a few months now.  I finally have a working prototype and now it's tweak time.  To log my daily progress and results, a blog seems to be a natural tool.  If you find it interesting - cool!  If not, well, go back to Facebook.