Vacuum Tube Theremin

This page is a log of building the Keppinger Tube Theremin.

At the moment it’s not working correctly. Once it’s fixed I will supplement this page with information on the construction.

d) Check the plate voltages of V1, V3 and V5: They should be identical now. Post values here.
V1 Plate : 74.0V DC
V3 plate : 78.3V DC
V5 Plate : 75.1V DC

e) Check the grid voltages of V1, V3 and V5: They should be identical now.Post values here.
V1 Grid : -0.006 V DC
V3 Grid : -0.6 V DC
V5 Grid : -0.3 V DC

Now let them oscillate under free-run conditions without external
influence of the antennae:

f) Disconnect L6 from the tank circuit, shorten L3
so that the non-plate side of R2 is directly connected to
+150V. Put back C4, C7 and C12

g) Now all three oscillators should work under identical
conditions at about the same amplitude on the cathodes
(post values here), both pitch oscillators at about the same
frequency (around 180 – 200kHz if I remember well) and the
volume oscillator around 400kHz. Post values here.
Tell us if this are clean sine waves.

I seem to be only getting oscillation from the variable pitch oscillator – the output from the fixed oscillator goes away when I stop the variable oscillator from running. The volume is not oscillating at all.

One of the identical Pitch oscillator coils - 0.64 mH

The volume oscillator coil - 0.4mH end to end

The 6H6 (stubby) and 6SN7GT tube used in the mixer/VCA

The output from the Pitch (Variable) Ocillator

The output from the Pitch (Fixed) Oscillator (I suspect this to be a shadow of the Variable Osc)

...and the Volume (Non) oscillator