[vacuum]

We an operational Model 89!

Here is the short version:
*Exhibited classic oscillator instability when powered again for first time in 6 years after rebuild. 
* First, replaced the #36 oscillator tube & socket with #77.  Result: same unstable behavior, lots of microphonics when tapping on #75.
* Second, re-wound the feedback coil of cellulose acetate insulated oscillator transformer T-14. Result: same.
* Reduced 7k3 Ohm resistor between T-14 (pin 2) and ground to 6k6 Ohm. Result: osc starts, otherwise same.
* At Ron's suggestion, re-soldered all pins and grid cap on 75.  Result, no change.  Conclude that 75 is remaining culprit, but the intermittent is a bad spot weld inside the 75.
* Drove to Elliott Electronics and found the only #75 in a bin of about 200 used tubes--5$. What is the probability finding one? And finding it to be good?   Changed tube: EUREKA! ... a stable, beautifully sounding, smoothly tuning Model 89
 
Conclusion: More than one thing had gone south since this unit was first refurbished--as a minimum: #75, feedback winding, and incorrect bias resistor.
 
Even though it is 108F today, I am feeling pretty cool for the sheer dumb luck of having found a 75 in that bin of old tubes, thereby solving problem quickly. Next to the bin, an RCA-67A signal generator for $17; now to find a manual!
 
To this Philco we now say: “stay tuned.”
 
Thanks all, your help was great.
John