[Top] [All Lists]

Re: [Amps] GS35 QUESTION

Subject: Re: [Amps] GS35 QUESTION
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Sun, 12 Jun 2005 22:42:59 +0100
List-post: <>
ON4IQ wrote:
>I have a 2m amp running a gs35 tube and a G3SEK triode board in it.
>While runnning the anode at 3.2kv , i get good bias control and all is well.
>Raising the voltage to 4.2kv off load gives met correct Ia when in ptt, 
>but once
>the tube goes in cut off , the Ia drops to zero ( thats good) but grid 
>current goes
>completely negative. After changing tubes with brand new one still have 
>the same problem.
>I suspect there is something wrong in circuits or  power supply.
>I've checked power supply and find no leakage to ground.
>Would you have any idea where this negative grid current comes from 
>when tube is in cuttoff.
>Its abt 50ma.

What's happening is that with the higher B+, the tube needs a higher 
bias voltage to cut it off. This voltage may be greater than the turn-on 
voltage of the VDRs, so some current will flow through the VDRs.

If you trace the current path back to B-minus, you will find that it 
flows in the reverse direction through the grid current monitoring 
resistor - that is why you see the apparent reverse grid current. You 
will also probably find that the VDRs get slightly warm on standby 

If you know what is happening, this is actually not a big problem. I 
suggest that you do not increase the VDR voltage, because the VDRs are 
protecting the bias circuit from excess voltage.

73 from Ian GM3SEK
Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>