[Amps] 8877 negative grid current

[Conrad PA5Y]

Hello all.

I have a question on behalf of an Italian friend of mine.

He has 8877 amplifier for 2m and what sounds like some tired tubes. His 
'normal' tube is a 3CPX1500. Recently he has noticed that when he tunes 
for maximum during tuning his grid current meter goes negative. I 
personally have never seen this on either of the two 8877 amplifiers 
that I previously ran. He has tried 2 other tubes which do not have this 
behaviour although they do produce less output than the tube that 
exhibits negative grid current. So I suppose the simple question is what 
causes negative grid current on high Mu triode like a 8877? Back 
bombardment?

73

Conrad PA5Y