[Amps] SB-200 80 Meter dilemma

[Ka7lpo at aol.com]

I own the SB-200 amp. It operates fine on 40-10 meters. I have two well worn 
tubes. But the amp does not want to function properly on 80 meters. On 80 
meters the amp indicates proper plate current but very low grid current. On 80 
meters the amp puts out a minimal 150-200 watts as indicated on my watt meter and 
then drifts down while exciter power is applied. Any thoughts on diagnosing 
the problem or leads I might pursue? 

Help,
Don