Everyone,

Some pointers to any circuits using, say, a 12AX7 for an extremely low power
audio (guitar) output stage would be very much appreciated. Both Class A
and self-split push-pull would be of interest. Most of the ones I have seen
to date are too high power (over 300 milliwatts) . I would like to get in
the area of 50 to 100 milliwatts into a speaker using a common, inexpensive
preamp tube as a power tube. At this power level the thought is that very
small output transformers and speakers could be used.

Thanks in advance for your assistance/comments,

-Charlie (charrich56_AT_hotmail.com)

"Fred Nachbaur" wrote in message
news:XSf3b.68621$K44.67825@edtnps84...
....
There's a point of diminishing returns with transformer sizes. Below a
certain size, it's impossible to get enough primary inductance to end up
with any kind of bass response.

Thanks Fred, point well taken. The idea would be to size the transformer and
power tube to get the true "power tube cooking iron" sound of a larger
guitar amp (say, a Marshall 50W head) being pushed very hard, but at a lot
lower volume level. Some bass rolloff is actually an advantage for
distorted guitar usage. I was looking at some miniature (50-100 mW)
transformers at Mouser and wondering if I could build a whole micro-amp in a
stompbox format, including speaker.

There are and have been a couple of commercial guitar amps like this -- the
Z. Vex MicroAmp and Gerhard Gilmore Junior would be two candidates, but they
are in the 1/2 watt class. Also, the new Vox Valvetronix ToneLab unit
purports to use a 12AX7 to emulate power tube distortion, along with some
electronic wizardry to model various Class A and AB guitar power stages. I
wanted to homebrew something a bit less expensive.

By the way, Fred, you have a ** GREAT** DIY tube amp Web site, one of the
best I have ever seen; it is a true Internet blessing.


The little transistor radios of yore used itty bitty transformers, and
did well if you got *anything* below around 300 Hz. And that's with a
(typically) 1200 ohm primary.
To get anything usable out of a 12AX7 you'd need at least an order of
magnitude higher primary impedance, preferably more like 50,000 ohms.
This worsens the bass response problem even more.

You could improve your odds by using a bunch of them in parallel, or
maybe use an SRPP approach to lower the output impedance. But even then,
it will be a challenge.

There's several low power tube headphone amp projects at www.headwize.com
that use SRPP, White cathode followers, etc. to go OTL. The direction I
would like to take is to use an output tranny which could be saturated with
the low drive from the tube, to get the "authentic" cranked amp tone.


I *can* think of one possible application for the 12AX7 as an output
stage -- driving a crystal earphone. You won't even need an output
transformer. If you can find one of the "real" ones that were once used
on crystal sets, that would be perfect. There is current production of
these with the same physical appearance, but they use the ceramic piezo
disks instead of a rochelle salt crystal, and sound absolutely awful --
even by crystal set standards. But -- you could probably get usable
output from a 12AX7 into one of these, using ordinary old RC coupling.

I wonder how a Les Paul would sound through one of those? 8-) I'm not
wedded to the idea of a 12AX7, but would like an inexpensive, widely
available tube to do the power tube job. I have seen some 6BQ5 circuits out
there and might go that way. I really wouldn't care about the exact way of
going about it as long as we ended up with the overdrive/distortion, dynamic
range compression, and "feel" of a much larger guitar amp, but very scaled
down.

The other way to go about this would be to go to a more conventional
approach and build a 1 to 3 Watt SE or PP amp, such as one of the ones on
your Web site, and use a reactive dummy speaker load (Weber makes these).
That might not be a bad way to go, especially given the great projects on
your site. (MiniBlok II or I.)

Come to think of it, one of the old-time radio high-impedance dynamic
earphones could probably work alright with a 12AX7, by using the
earphone directly as the plate load as did the radio set designs from
the '20s. Don't expect anything even remotely resembling fidelity,
however.

As long as the distortion sounded as cool as, or cooler than, a Marshall
stack when we connected a line-out to it .... 8-)

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

mebbe you can scavenge some old AM tube radio and make something like old
good Champ,with lower Ub (depending what power xformer you find in that
virtual radio set).
starved tone you can always have with "starved" triode stage-say that you
use for first or next (driver ) stage very low bias voltage,so that stage
goes easily in positive region; in that case tone will go wild.........
for volume issue-use whatever you find,but it's always possible to have gain
and volume control,and volume control is always possible to turn clockwise
)

--
Choky
Prodanovic Aleksandar
YU


"Charlie Richardson" wrote in message
...

"Charlie Richardson" wrote in message
...

"Fred Nachbaur" wrote in message
news:XSf3b.68621$K44.67825@edtnps84...
...
There's a point of diminishing returns with transformer sizes. Below a
certain size, it's impossible to get enough primary inductance to end up
with any kind of bass response.

Thanks Fred, point well taken. The idea would be to size the transformer
and
power tube to get the true "power tube cooking iron" sound of a larger
guitar amp (say, a Marshall 50W head) being pushed very hard, but at a
lot
lower volume level.


** Output transformers in Marshalls (or practically any other guitar
amp) DO NOT saturate in normal use, no matter how much overdrive is
ed - that idea is just another *stupid myth* put about by guitar amp
know nothings. The tubes "saturate'" since that is simple amplitude
limiting. Transformer iron saturation only happens at low frequencies - too
low for a guitar to even produce.




............. Phil

Steven,

Thanks very much for your schematic! It looks like it would be just the
ticket, especially if a preamp stage were added.

Take care,

-Charlie

"Steven Swift" wrote in message
...
(Steven Swift) writes:

Charlie Richardson wrote:
Everyone,

Some pointers to any circuits using, say, a 12AX7 for an extremely low
power
audio (guitar) output stage would be very much appreciated. Both
Class A
and self-split push-pull would be of interest. Most of the ones I
have seen
to date are too high power (over 300 milliwatts) . I would like to
get in
the area of 50 to 100 milliwatts into a speaker using a common,
inexpensive
preamp tube as a power tube. At this power level the thought is that
very
small output transformers and speakers could be used.

I built a small amp that puts out about 250mW using a pair of 6GH8 (the
cheapest tube ever made-- maybe). The Triodes are wired as a classA
diff pair with resistive load and the pentodes are transformer push-pull.

I can scare up a schematic and post it if you are interested.

Steve.

Okay, I found the schematic. Pretty cheesy, but I have dozens of these
tubes which were used in color tv sets. But the original poster didn't
seem to care about distortion. Increase the gain and add feedback. I'd
also convert the output to a long-tailed pair and ditch the caps.

Any take a look: http://novatech-instr.com/Ebay_Images/6gh8.pdf

I used this to amplify a pocket radio to a 4-inch speaker.

Steve.

--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA