Ignacio,
One problem with the TT amp is they have no positive
protection for excessive grid current.
A set of 3CX800's can be ruined in just seconds of operation
with excessive grid dissipation. Worse yet, the damage can
slowly accumulate over years of minor or very short
overloads. The overload LED warning light is really too
little much too late.
The failure mechanism is the gold from the grids migrates to
other areas of the tube. This happens from the kenitic
energy of the electrons hitting the grid, it is an
evaporation process without the necessity of the entire grid
overheating.
At this moment I have three TT amps here all with bad
3CX800's, and the failure in all three is the tubes arc. The
tubes only HV test to 2-3kV average and have very low
emission, and that is a clear sign of severe gold migration.
If it was my amp, I'd put some type of very fast electronic
overload cutout in it. That would be number one mod.
73 Tom
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