I was bored, so I built a 12b4a single ended amplifier from my bottomless pit of unused electronics. Bias points for the outputs are roughly similar to Gaby Levinson's project listed at
Transformers are 5K / 8 ohm transformers from an old SE 6BQ5 console for the outputs and a hefty low DCR (less than 10 ohm) power transformer from a fisher 800C . Rectification is UF4007 SS diodes bypassed by 1000V mica caps (o.1uF, IIRC). For what it's worth 12B4A tubes have a neat inbuilt slow turn on feature, so turn on nasties are minimized. The 12b4a cathodes are at 20V (24 mA).
All other passives were generic off the shelf components from Tanner Electronics, here in Dallas and the chassis is a gutted Intel 2U form factor router that I picked up for $3 at a surplus sale.
I didn't pay too much attention to cosmetics, this being a budget project, but I suppose I'll eventually make an attempt to make this amp more presentable given how good it sounds.
The sound is amazingly good for the 1.5W or so that it puts out. Not too much low bass (I blame the cheap OPTs for this) with an excellent mid bass and mid range. Very liquid sound that just sounds right. Highs are smooth without being excessively rolled off (okay, maybe just a little bit, but definately not in a old-school vintage sound way). I haven't even bothered decoupling the driver and output stages, or bypassing the electrolytics and yet, this amplifier sounds so "RIGHT" . These speakers take the edge right off my fostex full rangers and fill in the mid bass very nicely. Hum is pretty much non - existant even with ear to speaker. I used twin CLC filters, one for each channel to reduce the current load on each choke .
Probably best with speakers of real life sensitivitiy above 95 - 97 dB/W/m. These sound "so - so with your typical "shoe-box" 2 - way monitor. Maybe okay for an office, but definately won't show their full potential unless paired with high efficiency speakers. I'd really like to compare these to a good DHT 45 SET .
Update: September 20, 2008
the story continues at http://zobsky.blogspot.com/2008/09/12b4a-amp-version-2.html