[Amps] sb-220 tuning question - closing in

[Dan Sawyer]

All,

I am closing in on 'tuning' an sb-220. To make a long story short I was 
tuning at low power and running it at a higher power. Now I am tuning up 
incrementally in 3 steps from 10 watts to 100 watts CW. This works 
smoothly on both 80 and 40.

However on 20 when I finished the a 3rd tuning cycle of optimizing 
'tune' and then 'load' I turned off the exciter drive. Upon re-engaging 
the exciter the amp went ballistic; the sound was a loud continuous 
spitting. It was continuous, not a single snap or pop. I backed the 
power off and re-tuned following the same steps and after turning the 
exciter off and then on it did the same thing. I waited about a second 
looked around the side of the amp. The tube plates were glowing dull 
red. I was able to prevent this by carefully de-tuning. However this was 
more by feel then by a plan.

What is most likely happening??

How can it be prevented??

Thanks,
Dan