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Alan Douglas
 
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Hi,

Sounds as if your EL34 has grid emission, and it takes a few
minutes for the grid to heat up. It is contaminated with material
from the cathode.


The means the Mullard el34 tube is bad can not work on the Amp?


Yes. If the tube has grid emission, it will work for a few minutes
but then the plate will glow red, as the grid bias goes positive and
the tube draws too much plate current.

Alan