I recently got the urge to design a really low-powered home amp using 6AU6's in push-pull. For those unfamiliar with the device, they're a small 3W pentode, generally used as a voltage amplifier in radios. I noticed that I had a lot of them, so I thought "why not?"
Most small tube practice amps are single-ended (just one power tube), so they clip a little differently from the push-pull pair used in larger amps. Even my 5-watt 5F2A is too much at times, so here was my chance to build something even smaller, but with a very different character. Since I've already completed the thing and done most of the debugging, I'll try to leave the less important stuff out.Here's the loadline. To make things a little easier for myself, I designed it for class-A operation (neither tube going into cutoff), which means each tube sees half of the output transformer primary impedance. The blue line is what I was shooting for:
If you do the math, that's a 25kΩ load (50kΩ for both tubes)! To make it work, I used a Hammond 125A transformer, which can be wired for a primary impedance of anywhere between 1.2kΩ to 25kΩ. I wired it for 22kΩ primary/4Ω secondary, then attached an 8Ω speaker to double the primary to 44kΩ. Still with me?
The blue line is where my actual voltages (and 44kΩ load) put me. Here's the full schematic. Since I initially believed the current draw would vary wildly, I designed the amp with fixed bias, probably one of the smallest tube amps in history to do so!
And here it is being tested on my bench. I used a Jensen Neo 10" speaker so I could hear tube amp distortion without much coloration.
At first, the amp was very hummy, so I added an artificial center tap for the heaters using 100Ω resistors, which helped a lot. Because the Hammond 269JX transformer put out a lot more voltage than I expected, I had to juggle some of the power supply resistors to get things where I wanted them. So how does it sound? Well, not bad! At the lower end of the dial, there are some nice clean-ish tones. Bright enough to be clear without being piercing. Thanks to the limited voltage swing and lack of negative feedback, the tone gets grubby in a hurry. The two 12AX7 gain stages are enough to drive the output stage into mild oscillation if I'm not careful. Still need a little tweaking, but it's stable for the most part. I'm not entirely sure how to compute the clean power output, but I estimate it at around 1 watt.
I think this kind of amp holds a lot of promise for home practicing, with a little refinement.
The current draw actually turns out to be pretty stable, so I may convert it to cathode bias. Haven't decided yet.Here's another view:
Take care,
- Scott
P.S. More of the gory details here: http://www.tdpri.com/forum/shock-brothe ... rbose.html
And a tube rectifier too! That variac looks just like one I used to have!!!
