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.