[Amps] Vacuum relay question

[Tim Duffy]

A few times in the past 30 years the output relay of my single band
amplifiers have failed. 

They are used pulls - HC1 and RJ1. Typically from MGS.

I am not upset that they fail - they have a finite life, but my question is
- how they fail.

 

RF output goes to zero - and the meters (plate and grid) continue to look
normal. That is just strange to me.

 

When the output relay fails - why doesn't the amplifier go nuts - drawing
excessive current, etc.?

 

I always have 25 mS of TX delay - so the RF is never "hot switched".

 

73

Tim K3LR