Schematic of the DG-9 model:
http://www.curtcass.com/detectron/dg9-sch.jpg
Probably quite similar to the DG-7.

I came across that already, but it's quite different and therefore not much
help.


Replace it with what? Is silicon, the B+ is probably too high.

I tried putting a decade resistor in series with the diode, but even at
100k-ohms it still didn't make a difference. It still oscillates even with
the diode disconnected.

and testing the tubes.

How? With a tube tester? I sometimes work on ancient Motorola radios
with pencil tubes. I have to make my own adapter to work in the tube
tester.

Yes, I checked these with a tube tester, and even trying subbing them with
new-old-stock tubes.


Nope. Probably a neon lamp used as a regulator. Photo?

Here's a photo of the circuit:

http://s282.photobucket.com/albums/k...=detectron.jpg