The 6AS5 µ-Power UL

A Micro-Power Powerhouse! 

Here is a nice little amp that started out as an exercise in the inexpensive. 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, at least on the data sheet, is 2.2w into the output transformer with 10% THD. I was determined to improve on this distortion performance even though I knew it would cost me some power. When ever possible I selected surplus components (or at least those I already had on hand).

The Electrical Design

The amp uses a very straight forward design. The small base 6AS5 tubes are self biased at about 8.5v and run in UL mode with about 150v on the transformer. The driver is a 6CG7/6FG7 (a small base version of 6SN7) with a 50kΩ plate load biased at a little under 3v. Here is the schematic.

There is no volume control in this amp. It was concieved as a small amp to be driven by something like an iPod so the volume control would have been redundant. I also wanted not just good, but GREAT channel seperation on this amp. This meant seperate power supply filters for the power stages and the driver stage as well. Here is the power supply schematic.

The Build

Now, looking at the two schematics and the picture of the amp, something quickly becomes apparent. This amp has a lot of stuff crammed in a very small chassis. Here is the way I handled the situation.

In this photo. the power supply portion is on the right side of the chassis, and the audio portion is on the left. In the power supply section we have one 8H choke (in yellow), two large primary filter caps (these are the light blue ones running left-right), and three individual channel caps mounted on the partition (you really only see one with the other two underneath). The large items on the amp side are the two Edcor output transformers. The remainder of the components fit right in. Now because of all this iron, this amp is fairly heavy for it’s size. But the weight lends it a nice stability and solid feel when you pick it up. Overall I don’t mind at all.

Also note that due to the use of solid state rectification, I included a stand-by switch. The main power switch is on the back next to the fuse and power cord. When on, this lights the green LED on the top of the amp. The stand by switch is on top of the amp in front of the power transformer. When high power is supplied via this switch, the red LED lights. This allows me to let the tubes heat up prior to slapping the plates with high voltage and also gives a clear indication of the amp’s power state.

Here is a shot of the amp when under test.

It’s feeding resistive loads instead of speakers but you can see from the scope that the output is fairly clean. Shortly after I took this shot (everything checked out fine) I fired up the amp with it’s first real music. Here is the amp playing “Gabriel’s Oboe” from Chris Botti’s album “Italia”(driven quite nicely by an iPod Touch).

The amp sounds really good in spite of it’s low total power output. Here sensitive speakers are a must; at least 90dB.

The Results

The question here is about how to sum up this build. This amp has a very interesting look. The chassis is American dark walnut with an inlaid band of Australian Lacewood all toped with an oil finish.

This makes for an eclectic look that I think comes together well. The amp sounds good. The highs are crisp and clear, mids are smooth, and the bass end is well balanced. Overall I would call this an excellent performer, especially in the light jazz and clasical arena.