DH>>Some of the effects boxes which are designed to add "tube" sound will
DH>>run the tubes in a starved condition - the voltages fed to these tubes
DH>>will be several times less than what they normally use for operation.
DH>>A typical low-level amplifier ( 12AX7 ) runs at 150 to 300 volts  on
DH>>the plate.  The PAIA design for tube pre-amp uses 45 volts.  Lots and
DH>>lots of really nice warm tube sound!
>     I was once told that very high plate voltages will tend to
>     choke off the high harmonics.  Zat so, do you know?
It minimises the electron cloud.  The number of electrons which come 
off the cathode are pretty much dependent on the temperature of the 
filament, not the voltages applied.
If you run under low plate voltage, you don't attract all of them away 
and you will get a cloud forming between the cathode and the first 
grid.  This is also called self-biasing.  If the grid is negativly 
charged, it will block the flow of electrons to the plate and this 
cloud tends to drive the grid negative...  Non-linear and you then get 
the distinctive tube distortion - even order harmonics, very musical.
If you have a tube with just the filament running, you can usually 
measure about a volt negative from the first grid to the cathode.  
This is from the electron cloud.
>     Tube guitar amps tend to run 12AX7's (7025's) at 150-250 volts,
>     mainly to get high gains.  The old 50s amps ran them at 100-125
>     volts, and sound sloppier but defintiely warmer.
Right, and this preamp used 45 volts and sounded very sloppy and warm 
as buttered rum!
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