[Amps] SB220 plate meter question

[Warren Volz]

All, 

My SB-220 is sick again. The plate meter works fine after start up but
then ~15-20 minutes later it starts to show lower and lower values.
Eventually it doesn't show any plate current, but the amp still produces
full output. It doesn't look like I have loose connections. I suppose it
could be a poor solder joint? Anyone have any other ideas? 

-Warren KD5YPH