The basis for this amp was a stack of 6AS5 power pentodes that I got for $2 each. This is a little power tube initially developed for automobile radios. As such, it's not very powerful and runs off of fairly low B+ (150v max). Typical operation is 2.2w into the output transformer with 10% THD. I was determined to improve on this performance.
Now why, you may ask, do I call this "The Amp that Never Ends". Well, I started this project about 14 months ago. It was initially going to be a directly line powered little amp with solid state rectification. I wanted to keep the cost minimal. So I put together a nice little chassis and went to work. My initial design didn't go well. You can read about it here
. So I ordered a power transformer from Edcor and then tried again. Of course, in the mean time I redesigned the amp power supply to clean it up. I also prototyped the amp and decided to make some changes in the design. To make a long story short, I'm on my third power supply design and my fourth amplifier design. But I think I finally got it right.
Take a look.
The chassis is black walnut with a strip of lacewood inlay. The power switch is on the back and the standby switch is on top. I used a 6CG7 (just a 9 pin version of the 6SN7) for the driver tube. I decided that to tackle some of the distortion I would run the tube in pure UL. In spite of what the spec sheet says I was sure that I could do this and it actually paid off much better than I could have expected. The amp sounds very nice.
Here is a picture playing it's first music.
However, there is a reason that I called this a Micro-power UL amp. Peak output power is just 1.0W per channel into 8Ω. Input sensitivity is just a little over 1.0v peak for full power out so this can be driven by almost any line level input. I had to sacrifice power to get performance out of this little tube, but it was worth it. It's a neat little amp.
The schematic is fairly straight forward. It's a simple class A operation with a nice clean driver and no NFB except for the unbypassed driver tube cathode.
The power supply is a little more involved with separated filters for the two power tubes and the driver stage. This approach really helped give this little amp a really wide and deep sound stage. I was pretty impressed with the performance in this respect. Here is the power supply schematic.
I do however seem to be cursed with making the same mistakes over and over again. Once again I have produced a design with a lot of components to stuff inside a fairly small chassis. Take a look at the insides.
This amp has been a long drawn out effort. But perseverance finally payed off. The amp is low power, but with some sensitive speakers like the Fostex FN126En I think it will make a nice background amp. Maybe even for my office at work.