Not in that amp.
Carl
KM1H
----- Original Message -----
From: "Gary Schafer" <garyschafer@comcast.net>
To: "'Paul Elliott'" <paab@valornet.com>; <amps@contesting.com>
Sent: Thursday, February 16, 2012 12:42 PM
Subject: Re: [Amps] 3CX800A7 Grid Current Problem
> Often the plate choke is designed with less than optimum inductance on 160
> and sometimes 80 meters. This is done to avoid choke resonance on the
> higher
> bands.
> With the smaller choke it must be resonant on 160 and that is accomplished
> by using part of the plate tune capacitance to resonate the choke. This
> adds
> more current in the choke bypass capacitor. If that capacitor is too small
> or gets warm it will drift and you will get the results that you observed.
>
> 73
> Gary K4FMX
>
>> -----Original Message-----
>> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
>> On Behalf Of Paul Elliott
>> Sent: Monday, February 13, 2012 11:10 PM
>> To: amps@contesting.com
>> Subject: Re: [Amps] 3CX800A7 Grid Current Problem
>>
>> I really didn't know what was the cause of the grid current changing,
>> creeping or rising rapidly, depending on loading, only on 160 m, but it
>> did
>> seem that something was heating up and changing with time when the key
>> was
>> held down. There was no visual sign of anything wrong in the amplifier
>> RF
>> output tank section. I decided to change the plate blocking capacitors
>> and
>> the B+ bypass cap at the cold end of the plate choke. (relatively easy
>> to
>> do).
>>
>>
>>
>> I replaced the two paralleled 2500 pf plate blocking caps. Nothing
>> changed.
>> I then replaced the 2500 pf B+ bypass capacitor with a 4700 pf (thanks,
>> Carl) capacitor. Now, on 160 m, the amplifier tunes smoothly, and there
>> is
>> no change in grid current with a key down 600 watts for three minutes
>> (into
>> a dummy load).
>>
>>
>>
>> I still don't know exactly what was going on-but am glad the amp is now
>> doing well on all bands.
>>
>>
>>
>> My thanks to all who responded. Your kindness and knowledge are both
>> appreciated very much.
>>
>>
>>
>> 73 Paul W5DM
>>
>>
>>
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>
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