Carl KM1H wrote:
> Sounds like a simple case of internal arcing in the tube.
> A 10-20 Ohm resistor in the HV lead would possibly have minimized
> the damage. The grid chokes are also way too large in current
> capacity. In extreme cases when the tube arcs or shorts you will
> also take out the Zener D-2, and Bias diode D-1 and caps C-3
> and C-26. If the amp failed while on 10/12M best to check PC-1
> and PC-2 R values also. Replace with metal oxide.
Good morning,
I am interested in the failure mode of the bias diode D1, because
this componect failed when my TL922 went *bang* with a new
pair of Eimac 3-500z (unused but 10 years old). In my case the
amp was on standby and the bang was at switch-on time. So I did not
suspect parasitics were involved.
My theory is that V2 suffered a grid-filament short, which took the
cathode (has 110v shut-off bias) to ground via the grid circuit,
fusing the grid choke and burning the 4.7K grid resistor. (The grid
RFC and resistor for V1 were not affected). Because the 2x 15A fuses
did not blow, I assumed that an anode short was not involved.
After repair, I checked the faulty 3-500z with my Fluke 77, but
no DC short shows up at a low voltage check. I cannot make a HV check.
Is it possible the failure mode is that a plasma starts, and this
enables the cathode voltage to discharge through the grid circuit?
Regards,
Martin ZL1ANJ
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