[Amps] 8877 negative grid current (MU 4CX250B)

[Chris Hays]

I read somewhere that this can happen in a tube with a lot of hours and 
possibly some abuse. Over time, the grid can pick up contaminants from 
the cathode, which may let the grid get warm and start acting as an 
emitter (cathode).

I have also found that tubes with very light grid structures, like the 
4CX1000, will do this without much provocation.

Depending on the circuit, the tube may operate fine, but clearly this is 
abnormal and if it gets high enough can destroy the grid (just like any 
other over-current situation on the grid).

Chris, AB6QK

original message:

From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Conrad PA5Y
Sent: Saturday, July 14, 2018 2:21 AM
To: amps at contesting.com
Subject: [Amps] 8877 negative grid current

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