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Re: [Amps] 8877 negative grid current

To: Conrad PA5Y <>
Subject: Re: [Amps] 8877 negative grid current
From: MU 4CX250B <>
Date: Sun, 15 Jul 2018 07:18:40 -0700
List-post: <>
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.
Jim w8zr

Sent from my iPhone

> On Jul 14, 2018, at 1:20 AM, Conrad PA5Y <> 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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