[Amps] Flashover and QSK board in TT Centurion

[Tom Rauch]

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