[Amps] 3cx800a7 Grid Current Problem

[Vic K2VCO]

The stock 922 apparently isn't optimal at the low end of 160 (or 80). I had to add some 
capacity across the loading capacitor to get a reasonable match on mine, but output is 
still low compared to other bands. I suspect that there isn't enough inductance and the Q 
is too high.

On 1/25/2012 11:14 PM, Mike Tubby wrote:
> On 26/01/2012 02:40, TexasRF at aol.com wrote:
>> About a month ago, JE1BMJ reported a similar problem with his SB1000. In
>> his case, the problem was found to be inadequate bypassing of the plate rf
>> choke. Apparently enough rf was reaching the bypass capacitor to heat it up
>> and  cause it to change value and that was enough to detune the plate circuit.
>>
>> 73,
>> Gerald K5GW
>>
> This appears to be not uncommon across several amp designs - even the
> venerable TL-922 shows lower output on 160m than it does on 80m/40m and
> no doubt if the input power is there then the output is going
> somewhere... okay in the case of the '922 it might not be causing
> de-tuning and grid current but I bet soemthing's getting warm ;-)
>
> Mikie G8TIC
>
>>
>>
>>
>> In a message dated 1/25/2012 7:20:22 P.M. Central Standard Time,
>> at6c at bellsouth.net writes:
>>
>> Carl/Paul,
>> There isn't a fix loading padder (Carl, I think you  are referring to a
>> capacitor pad for 160M correct me if I am wrong) in the  HF 1250.  There
>> is a tapped ferrite transformer/coil that is switched  in for 40-80-160.
>> That being the case the problem could be something  shorting out part of
>> that transformer in the 160M position.  Also the  band switch could be
>> bad.  Have you checked all the components in the  RF compartment for
>> visual damage/overheating/value?  The only  difference in the
>> tuning/loading circuit between 80M and 160M is the  position of the band
>> switch and which part of the above coil is in the  circuit.
>>
>> Another point to consider is that the LP100A has three  different
>> selections for measuring output  power(average/peak/?pulse?).  I find
>> that if you are tuning a twitchy  amplifier a needle type watt meter is
>> easier to use than the LP100 (maybe  it's just me).  I think if you are
>> not using a pecker tuner,   peak is recommended, check the manual.  Also
>> carefully follow the  instructions in the HF 1250 manual as to the way
>> you tune the amp.  I  found those instructions will prevent surprise grid
>> current  fluctuations.  Good  luck.
>>
>> 73
>> George
>> W2GS
>> _______________________________________________
>>

-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/