Some questions for Heath tube masters

[oh8jep]

Hello!

I have some questions for Heath tube masters hi!

I have been wondering that strange thing,which appears with my homebrew 3-500ZG amplifier.

I could not to adjust with anyway 20 meter point on the tank coil,the tube was heavily red and overcharged
all the time when I tried,and does not pushed out than only 150W,taking at same time 450mA plate current
and 180mA grid current. The point on the tank was calculated correctly. Well,then I put metal plate (magnetic metal)
inner side of  the plate coil ceramic rod,and what happened,all works great GOOD,not any problems in 20m,and push out almost 900W with one tube. (YD1130,3-500ZG equivalent) That phenomen appeared only in 20m,without that metal plate,did not work correctly,any other bands works. 80m,40m,15m,10m.

Well can anyone tell me what should be that strange thing,caused that problem?  

Best 73 de Aatos OH8JEP

PS. maybe this is a little bit off topic,but could happen also with Heathkit,I think..

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