Flipper said:
> 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.
Is that one mirror or two? I have wondered here in the past
whether it's best to use one of the valves as a reference,
or to use a seperate SS reference and two mirrors.
> 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.
How does that work out SPL-wise? If you switch from speakers
to 'phones is the apparent volume similar?
> 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 ****ted 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.
Circuit would be nice too. Don't you have any web space?
Easier for us.
Thanks, Ian
>
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