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