PR machine gun

[Kenneth LaFleur <lafleur@pivot.net>]

I own an RT-3 with two PR tone cabinets, bought by the original owner 
in 1971. It's in great shape, but every year it develops a summer 
peculiarity: as the humidity increases (I live in the far North, so 
this is rather brief) one of the cabinets, a PR 40, begins to hum -- 
sometimes loudly. In the past I have lived with it, since it passes 
with the coming of cooler weather. But this year that PR 40 has 
learned a new trick. After being on for about 15 minutes, it begins to 
stutter out machine-gun like explosions of sound. It got so bad I had 
to disconnect it, and rely solely on the PR 20.

Can you technical experts give me a clue? Is a sick tube the most 
likely cause? An output tube? Or is an aging resistor or capacitor 
more likely?

Many thanks for any insights -- two cabinets sound better than one!

Ken LaFleur