DH>>     I was once told that very high plate voltages will tend to
DH>>     choke off the high harmonics.  Zat so, do you know?
DH>It minimises the electron cloud.  The number of electrons which come
DH>off the cathode are pretty much dependent on the temperature of the
DH>filament, not the voltages applied.
DH>If you run under low plate voltage, you don't attract all of them away
DH>and you will get a cloud forming between the cathode and the first
DH>grid.  This is also called self-biasing.  If the grid is negativly
DH>charged, it will block the flow of electrons to the plate and this
DH>cloud tends to drive the grid negative...  Non-linear and you then get
DH>the distinctive tube distortion - even order harmonics, very musical.
Ooh!  Ooh!  Ooh yeah!!!  Thank you!!!  You just solved a puzzle I've
been puzzling about for years!  I built an amp very similar to a
1956 Fender Champ (mainly, lower plate voltages) that sounded _much_
better than the modern stuff.  I had a suspicion that lower voltages
(about 110V on the 12AX7) had something to do with it.  But it sounds
to me as though running the tube at 45V pushes it off the steep end of some
linearity curve, and accentuates the musical-type distortion.
This solves a problem I've been mulling over for some years.
I built a larger experimental guitar amp with the passive tone
controls removed.  Yet, the amp still has the tube stage that
recovers the signal loss from the tone controls -- so the preamp
gain was way too hot.  Without that gain stage, the gain was much
too low, but with it the gain was too hot.  I fixed it with a 3 meg
resistor, but it didn't sound right.  Now it's obvious that the thing
to do is reduce the plate voltages drastically, so the overall gain
is nominal but I'll get this effect you've mentioned.  Gotta try it!
DH>If you have a tube with just the filament running, you can usually
DH>measure about a volt negative from the first grid to the cathode.
DH>This is from the electron cloud.
Questions:
Fender-type amps are set up for a gain of 40 in preamp stages.
Can I assume this would not need to be changed?
If you started with a single 12AX7 preamp stage with standard
voltages (100 - 150), and replaced it with two parallel 12AX7
stages at much lower voltages, so that the voltage gain was
the same, can you predict any behaviors or problems that might
come up?  It seems obvious that the current would be higher.
In 12AX7 preamp stages, I'm assuming that the self-bias
created by the electron cloud is fundamentally different
(much more non-linear) than that created by the cathode bias.
Could you therefore reduce the cathode bias and get the same voltage
gain with more warmth, or would that be a quick way to make a
stupid mistake?
Power tubes (pentodes) can be switched to triode operation,
resulting in a lot less ouutput and a lot more nice greasy
50's distortion.  IIRC, this is because the two extra grids
are involved in reducing the electron cloud.  Is it therefore
possible that you could incrementally add this effect by
reducing the voltage to the suppressor grid rather than just
switching it out?  IOW, if the plate is at 300V and the suppressor
grid is at 275V, what might you get if you reduced the suppressor
voltage to 200V?
Sorry to be a pest with all these questions.  But one of the
things I learned in working with tube guitar amps is that
unlike hi-fi tube amps, linearity is not a design goal; rather,
if guitar players like it, you got it right.
Apparently Leo Fender understood this.  When he built a prototype
amp, he didn't go by what the o-scope said.  He would send it out
on the road with some of his favorite professional players, and
their opinions told him whether or not to put in production.
 * SLMR 2.1a *      Forgive your enemies.  They HATE that!
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