[Boatanchors] HP-606 tube failure

[Ian Wilson]

No apologies for including a signal generator in this list since it
weighs more than most of the radios I work on using my trusty HP-606 :)

Anyway. I have a repeating failure. One side of the 12AT7 that is used
as a cathode follower in the level-stabilizing output loop dies. This
effectively
turns off the output (the unregulated low-level output is unaffected).

The fix is easy - replace the tube - but doing this requires removing the
internal RF cage. This is a straightforward but tedious process.

Looking at the cathode follower part of the circuit, I wonder whether
the heater-cathode voltage rating is being stressed when the power is
applied. If so, perhaps this could be protected by a <whisper> zener
diode or something?

I also wonder whether, if this was a common problem, whether there
was a known HP service patch for it...

Thanks for any suggestions.

73, ian K3IMW