Thanks to all who offered their assistance regarding the arcing 2100B. The
amp has been stabilised.
Dave, W8NF and Rich, AG6K were instrumental in curing the problem.
I checked the 33 Ohm grid resistors and they were a bit higher than spec.
They were not the original items. I changed them for some 30 Ohm units and
added around 1000pF to the existing 200pF grid capacitors.
As well, I made up some Nichrome suppressors from a spare kit I had lying
around. With these and the grid mods, the amp has become unconditionally
stable when keyed up with open circuit input and output terminals.
Before, I added the nichrome suppressors, I installed a couple of good
suppressors from a previously modded 2100Z. With these installed, with the
grid mods, the amp still produced a purple arc between the plates of the
I carried out tests for arcing after each step described above. It wasn't
until all the mods were done that the amp started to behave.
I hasten to point out that the amplifier seemed totally ok before any mods
were done with a radio on the input and a load on the output. It was only
by chance that I had the amp on the bench with nothing connected to it.
Other 2100B owners might care to experiment along the same lines with their
Amps mailing list