Re: Non-Linear Preamps

Patrick Turner wrote:
....
A 6U7 ( octal ) or 6BA6 ( mini7 ) remote cut off pentode would do the
job well, giving large amounts of
good sounding 2H but without hard clipping when you miss-bias a 12AX7 or
other linear triode.
The idea with a pentode is that you have to operate it at a fairly high
output voltage of about 50Vrms
to generate the THD you want, and then use an attenuated amount of it to
be applied to the power amp.

Nevertheless, the 6L7 / 6BE6 allows you to have the signal input to g1,
and a gain controlling bias applied to
g3. These tubes were meant for tiny signal levels in radio mixer stages
and the bias applied to g1
altered the gm which altered the gain but without causing very much THD
because at low levels below
0.1Vrms the distortion is low regardless of the bias.

Patrick Turner.

Thanks for all the suggestions (I'll be looking into them later).

Being curious about the SRPP circuit design,
I spiced up a SRPP (Shunt Regulated Push Pull) amplifier
today, using a pair of 12AX7 tubes (since that is what I had
for modelling). The circuit is interesting to me, because
designers focus so much on making the circuit linear. This
makes it sound ideal for non-linear use.
So I tweaked the circuit to provide fairly clean
amplification for 300mv signal (about 1/2 guitar
pickup strength).

Then simulated a 600mv signal input (@1khz). With a
cathode resistance of approx 1.2k, and plate resistance
of 12k, the bottom half of the signal output gets
slightly rounded. Doing a FFT plot,
reveals what I hoped for, 1,2,3...9th order harmonics,
evenly distributed, in decreasing strength. I have
temporarily made the plot available here:

http://rapidshare.de/files/18840184/300mv.bmp.html

(you'll need some patience to get the download ticket etc.)

The top plot is the 1khz signal @ 300mv, with very
little distortion (apologies for the bad X axis on
that chart). The bottom chart shows the same signal
at 600mv. The blue line is the output, which has all
the nice extra harmonics in it.

This won't be no death-metal sound (which is not what
I want). I want something that gracefully overloads
with colour on full chords or full pickup volume.
That magic sound is such an obsession..

Well, it all looks good on paper. I'm going to mock
up this circuit on a piece of wood to see if the
sound is any good (too bad LTspice won't let you
listen to an audio signal on your sound card). I
should also play around with the plate voltages --
I forgot about that today! These simulations were all
done with a B+ supply of 300 volts. Lower voltage
operation could introduce even more harmonic energy.

I haven't looked for them yet, but has anyone spice
modelled any of the mixer tubes mentioned here? I've
picked up the triode set from Duncan Amps already.

Warren.
.