I have been intriqued for some time now with the idea of building an
output transformerless tube audio power amplifier. I have seen
schematics of two different versions of this, one in an old 1962 radio
book that I have and another on the web which was published in the
February 1990 issue of a magazine which I've never heard of call
"Glass Audio". One design uses 1682 triodes for the output and the
other uses 6AS7's. The 1682 is just a 26 volt filament version of the
6AS7 and so the heaters can be wired in series eliminating a filament
transformer. In fact this particular design uses no transformer for B+
either with AC neutral being connected to one side of the chassis. As
long as you polarize the plug everything should be ok the article goes
on to say, however I don't know how crazy about that scheme I am
though. Each design boasts 25 and 35 watts respectivly into 16 ohms
with .50 percent total distortion at full rated output. Frequency
response is not limited by the transformer and and so extends from
10HZ - 250KHZ on both. The circuit for the 35 watt design is very
interesting and can be seen at:
http://www.bonavolta.ch/hobby/en/audio/6as7_2.htm#top
If I'm going to the trouble to build something though, I would really
like to wind up with a unit that can do at least 70 watts or so. By
the looks of these designs, perhaps just adding more triodes in the
output and beefing up the power supply would do it but I was wondering
if anyone else had any further information on this type of project, or
had looked into this?
Thanks, Lenny Stein, Barlen Electronics.