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5 Watt

Background (Updated Late 2013):

"What has been will be again,
  what has been done will be done again;
  there is nothing new under the sun."  - Ecclesiastes 1:9

The circuit below provides a golden renaissance for the venerable single-ended directly heated triode amplifier, bringing together technologies considered cutting edge for their era, but separated by nearly a century in time. 

Nothing built today can match the liquid smooth, silky sound of a well done DHT SE amplifier.  Anyone who has ever listened to one will agree that there is an in-tangible 'sweetness'  to the sound simply isn't present in any other audio reproduction media.  

It is for this reason that we tolerate the inherent faults of these fantastic devices as well - constantly tweaking hum balance pots, adjusting bias currents, trying to find a preamp driver that doesn't skew the sound palette, and always suppressing the internal desire for just a little more power output.

Not to go against scripture, but I have built and demonstrated a circuit that allows a SE DHT circuit to shine in all of its sonic glory, but with none of the aforementioned faults.

The biggest breakthrough with the Skylark circuit is that it enables any generic DHT (45, 2A3, 300B, et al) to be driven directly with line level input voltages, with no preamp whatsoever.  I'm not cheating and sticking in a small solid-state preamp like others have done in the past.   I am operating the tube in a manner that is fundamentally different from all other methods employed over the past century, utilizing a novel drive and using enabling new technologies that have come out in the past few years.
Another big plus is the fact that the entire powerband of the tube can be utilized - it can be driven 'roof to floor' without any issues.   With a generic 2A3 tube, clean power output is available all the way past 5 watts while staying close to 20 watts of plate dissipation.  The same power output can be achieved at a more conservative idle point that doesn't exceed the 15 watt rating of older tubes, but distortion goes up slightly.

I am intentionally keeping the circuitry details to myself at this point, but here are the deliverables, as measured to date:

-    2A3 DHT directly driven (no preamp).  Generic Chinese ShuGuang dual plate.
-    5.0 watts class A output @ < 0.3% THD (measured at 1 KHz).  2nd harmonic @ <- 60dB
-    Frequency response with EDCOR  2.5KΩ primary output transformer at rated 5 Watt output into an 8 ohm     
     resistive load:  12Hz to 22.5Khz @ +0/-3dB,  17.5Hz to 21.0Khz @ +0/-1dB.
-    Maximum instantaneous plate dissipation:  24.5 watts
-    Plate dissipation at idle: 22.5 Watts (300V / 75mA)
-    Input impedance:  600 ohms
-    Input signal voltage required for rated output:  385mV RMS (~.5VPP / 200mW power).
-    AC filaments.
-    No Measurable hum (>-80dB noise floor)

1)    Factory datasheets for the 2A3 specify 3.5 watts output @ 5% THD using a 2.5KΩ primary impedance.   With that same load impedance, I am able to achieve over 5 watts of output, at an order of magnitude lower distortion. 

Listening Impressions:

Up to this point I have been listening to the prototype in mono driving a Dayton Audio P220-8 in an infinite baffle enclosure, using an OPPO DV-980H source and my KVX interconnects.  Even with the admittedly messy wiring, the amp is dead silent at idle - no hum, no hiss, no nothing. 

The sound is unlike anything I have experienced before;  More an emotional immersion than a listening session.  Vocals jump out of the background with a clarity and intensity that almost seems separate from the rest of the medium.  Listening to Johnny Cash's cover of Hurt, you can feel an anguish in his voice that simply isn't there with other amps.  Nuances in the material are more pronounced with less homogenization - individual instruments, even backup vocals are bright enough that they can be isolated from the whole, but are never overpowering to the scene.  Silent passages in music simply fall off into inky darkness.

Why does it sound so unique?  Because there is only one gain stage.  All you hear is your source material aggrandized by the triode, the output iron, and the speaker. That's it - nothing else.  No feedback, no coupling caps, no attenuation or propagation errors associated with an interstage, no signal inversion. 
Update 1/13/2014:

The Stereo Prototype is complete!  The circuit was grafted from the mono bench setup into the amp largely unchanged.  The only difference is that the bias point was shifted slightly to reduce static plate dissipation.  The power supply is configured as a simple CLC circuit.  There is a 22uF film capacitor directly off the rectifier. From there it splits such that  each channel has its own 2 Hy choke followed by another 40 uF film capacitor.  No electrolytics are utilized. 

Rated power output prior to the onset of clipping has dropped slightly due to the more conservative biasing.  As shown below, it  measures at 4.85 watts using a 1 Khz sine wave into an 8 ohm resistive load.  Initial THD assessment was performed at 4.50 watts continuous sine output into an 8 ohm resistive load.  At 1 KHz, it measures ~ 0.22%.  Other operating points will be made available here as time is available for testing. 

Frequency response bandwidth measured at 4.50 watts output is +/- 1dB from 19.9 Hz to 26.8 KHz.

Input signal requirements are un-changed from before; only 400mV RMS is required to achieve full power output.  I can easily drive the amp into clipping using only the headphone output on my IPod.