Hi, Tom

Thanks for the hint about gas in large glass tubes.
Also thanks for the link to your web site, plenty of wonderful info!

One question please - I have 8 type 3-500Z which
have not been used for 20 years. You mentioned operating
them at low plate voltage and positive grid voltage,
so that the plates show color (for gettering action).

What range of plate voltage/ grid voltage do you recommend,
and what current limiting resistor would you suggest
in the plate circuit?
Do you use a current limiting resistor in the grid circuit also?
I plan a test fixture to recussitate
the 3-500Z tubes before risking them in an amplifier.

73,
Ed Knobloch

wrote:
snip
As for cooling, seal cooling is critical in glass tubes. So is periodic
operation to keep the tubes gettered.

http://www.w8ji.com/vacuum_tubes_and...e_failures.htm

73 Tom

Edward Knobloch wrote:

What range of plate voltage/ grid voltage do you recommend,
and what current limiting resistor would you suggest
in the plate circuit?
Do you use a current limiting resistor in the grid circuit also?

Ed,

I use lots of airflow and a variable voltage grid supply. Since the
grid voltage is dc, you can use a simple Ig times Vg (to cathode) to
calculate dissipation. In a operating amplifier you would have to use a
Fourier (harmonic) analysis to calculate grid dissipation, but not at
dc.

Set the grid just below rated dissipation. When Eimac initially
gettered the tubes, they heated the grids well into color. I'm not that
bold, although they told me they could get almost white hot without
problems! So I stay at or below rated dissipation.

I use about 500-1000 volts of anode voltage, whatever it takes to show
good anode color. I use an old AL80B transformer for grid bias and HV,
except I don't voltage double the HV winding. A variac would be better,
I switch taps.

73 Tom