On 04/16/2011 10:20 AM, COLIN LAMB wrote:
One thing that has not been mentioned is that an amp should not be driven
with grid current and no plate current. If you see no plate current, the
full power of the transmitter is absorbed in the grids, which will destroy
them. As soon as you lose plate current, shut down and find out why.

Ceramic resistors are not used simply because when they explode, you have
nasty particles of ceramic all over. Somewhat like a slightly docile
grenade. Wire wound resistors do not do that - generally.

73, Colin K7FM


The 3-500Z is a fairly robust tube and can take a bit of abuse like that
before it is destroyed. (At least the Eimac built tubes could, I don't
know about the Chinese clones). OTOH ceramic metal external anode tubes
usually DON'T have a grid structure that can tolerate such abuse and
WILL suffer 'meltdown' if not treated correctly. This is why the 4cx250
series CANNOT be used in grounded grid operation. They CAN be used as
cathode driven tetrodes in AB1 (NO GRID CURRENT) however.