An even better arrangement for the LFO is to use a
ring-counter generator for making the LFO signal. This
uses a digital oscillator to clock a several-stages digital
counter, and adding together the outputs with mixing
resistors makes the output waveform. You can get
good-purity sine waves, triangle waves, quadrature
sines, sawtooths, and exponential peaking waves just by
how you clock the ring and by diddling the values of
the mixing resistors. The digital oscillator can have a
*huge* range compared to a linear phase shift
oscillator, and the mixing circuit always mixes to what
you wanted for an output LFO waveform in a
frequency independent way. You can easily make the
waveform be unusually shaped any way you like.

I just put an LFO article up at GEO describing the
basics. If you make the logic out of CMOS, it lives
quite naturally inside a tube amp. The power needed
for CMOS is so low that a simple resistor/zener/capacitor
combination can supply all the current that the CMOS 
needs from B+. The output of the LFO can be buffered 
or amplified by high voltage MOS transistors, or it can 
be used to drive an LED or lamp source for modulating 
the rest of the amp.