[Amps] 3cx800a7 Grid Current Problem

[Carl]

Stoutness isnt the issue as the input RF isnt much compared to the chokes 
ability. That plus the usual .01 to .05 caps at the cold end to ground 
provide sufficient bypassing.
What does happen with inadequate inductance is that some portion of the 
choke becomes part of the input network but that can be tuned out by the C2 
of the inputs pi network.

In amps with just a T or L network that doesnt work well and amps without an 
input network, such as the Clipperton L, are simply out of luck. OTOH the 
Clipperton L can be made as efficient on 160 and 80 as on say 40 with 
minimal work and expense.

Carl
KM1H

----- Original Message ----- 
From: "Rob Atkinson" <ranchorobbo at gmail.com>
To: "Vic K2VCO" <k2vco.vic at gmail.com>
Cc: <amps at contesting.com>
Sent: Thursday, January 26, 2012 1:47 PM
Subject: Re: [Amps] 3cx800a7 Grid Current Problem


I have heard that on 160, some of those amp's filament chokes or other
input components (caps) are not as stout against RF as they are on the
higher bands.  the matching circuit may be okay but some of the drive
is going somewhere else after it gets past the input matching network.
 This may be hokum but it is an interesting possibilty.

73

rob
K5UJ

On Thu, Jan 26, 2012 at 10:37 AM, Vic K2VCO <k2vco.vic at gmail.com> wrote:
> 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/
>
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