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.