[Amps] henry amp problem

[Douglas Johnson via Amps]

 During the cw contest my henry 2-kd classic started to act up, no power output on 40-80 meters and after 3 or 4 seconds transmitting on 20 power drops to zero. 10 and are normal output . plate current, plate voltage and grid current remain ok on all the bands and input tuning is normal on all bands. all power goes thru bird wattmeters and bird coax switches which have functioned fine for 35 yrs. I eliminated all of them with 1 coax and wattmeter into the dummy load, same results. At no time did I observe any noise or smell from the amp to indicate trouble, this amp is going to be a pain to remove and open up for repair so I hope to learn something before that has to happen,  any comments?....Doug, w9iix