has anyone converted a tube vhf (2 meter) fm transmitter to work on
lower freq.? maybe 50+ mhz ?

SEF-

For that particular equipment, you need to know the approximate crystal
frequency and multipliers used in subsequent stages to get to Two Meters.

For example, suppose it has a 6 MHz crystal (Output frequency divided by 24).
If the oscillator stage is followed by a doubler to 12 MHz, followed by a
doubler to 24 MHz, followed by a doubler to 48 MHz, followed by a tripler to
144 MHz, then you might remove the last tripler and connect 48 directly to the
output stage and retune. In this case, the crystal would be changed to output
frequency divided by 8.

Another multiplier chain scheme that may be easy to change uses an 8 MHz
crystal, 8-24-48-144. In some cases the last multiplier might need to be
converted to an amplifier to provide sufficient drive to the output stage.

There are other multiplier chains that would be more difficult to change, such
as 6-18-36-72-144, but you should see what is needed. You would need to change
coils or capacitors of stages that had to be moved to a different frequency
range.

Most equipment has test-points to be used for alignment, where you use a volt
meter to tune previous stages for maximum output. When you change a circuit's
range, it would be a good idea to use some kind of absorption wave meter such
as a Grid Dip Meter in absorption mode, to be sure you have the right multiple.

Sometimes a counter will respond to harmonics, so you wouldn't be sure what the
actual frequency is until the conversion is complete and output stage's filters
are tuned.

Another thing to consider is that FM modulation is affected by the frequency
multiplication ratio. If you changed from 144 to 50, you would end up with one
third the deviation, so you would need to add audio amplification to boost it.

Good luck.

73, Fred, K4DII