That's the latest project I finished, a little 'PC Speaker' Tube
amplifier using, per channel, a pair of 6AW8s PP into 20k OPTs. Does
between 3 and 4 Watts Po per channel.
Similar to my mini guitar amp it's powered off a 12VAC (40VA in this
case) wallwart with a reverse 20VA tranny inside generating about 240V
(loaded) for B+. Screens are at 200V.
PS filtering is 33uF --> OPT common --> 4.7k 33uF ---> screens --->
10k 33uF ---> Preamp. Hum is only detectable if you stick your ear on
the cones but I'm thinking about increasing the primary filter to 47uF
or 100 uF.
Idle is 10mA and, as has become my common practice with cathode bias,
I use a bypassed current mirror to keep a tight DC balance through the
OPT.
Preamp uses a common Rk PFB setup between the gain triode and
concertina phase splitter for a gain of about 140. The amp is then
closed with 18dB of.global NFB.
I used a 1/4 inch jack for the speaker connection to load switch
internal 10W sand block dummies, so the amp is never unloaded, and,
more to the point, the dummies provide a load when using headphones.
Might not be a monster amp but it's cute, compact, and fun as all get
out. The whole ball of wax, sans the wallwart of course, fits in a
5"x7" chassis.
For speakers I worked up a 300cu ported enclosure wrapped around the
MCM 1853 4 inch driver. They seem to have a response peak around 2kHz
but rather then smooth that out with passives in the cab I just use
the PC's equalizer and they sound amazingly good for 14 buck cheapies.
Darndest thing I ever saw, not that I've seen 'everything', but one of
the 6AW8s I got in a batch has a heater short. Not to cathode, or any
other pin, the heater itself. She pulls heavy current but won't light,
as if there's solid wire between the pins. Can't complain too much, I
guess, what with that batch being 50 cents each. Just wish he'd have
had more of them.
I'll post pics in alt.binaries.schematics.electronic if folks would
like to see it.
|
