[Mondial]

There is really not much to the oscillator circuit, but it definitely seems not to be working. Since you tried different 6SA7's its most likely not the problem. One suspect would be the 50 pF coupling cap C9. If its open intermittently, the osc will quit. You can try jumping another cap 50 to 100 pF across it and see if it cures the problem

If the osc coil has an intermittent winding is would also be a cause along with an intermittent 25K screen grid resistor R2. The voltage on pin 4 is low which could be caused by R2, but the low voltage could also be a symptom of the tube not oscillating.  Try jumping a 22K resistor across R2 and see if it makes a difference.

Just a few things to try ... Good luck!