[Amps] 8877 negative grid current

[MU 4CX250B]

That’s abnormal behavior for an 8877, Conrad, and it’s interesting
your friend hasn’t seen the negative grid current with other tubes.
Otherwise, i’d speculate the problem was a measurement artifact,
caused by a small leakage of the cathode current to chassis ground
(instead of all of it returning directly to the B- line.) When that
happens, it can cause a negative deflection of the grid current meter,
but that doesn’t reflect actual grid current.
73,
Jim w8zr

Sent from my iPhone

> On Jul 14, 2018, at 1:20 AM, Conrad PA5Y <g0ruz at g0ruz.com> wrote:
>
> 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
>
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