On 19/02/2013 23:36, Mike Tubby wrote:
> On 19/02/2013 04:52, Drax Felton wrote:
>> I'm building an 8877 amp for 6 meters. What rough values should the input
>> and output capacitors be set to as an initial starting points? The tables
>> in the Radio Handbook stop at 10 meters. I want to avoid a totally wild
>> mistune first power on. Is there a procedure for a "cold tune up"?
>>
>> DE kb3x
>>
>>
> Hi,
>
> Firstly I suggest that you look at the G3WOS and GJ4ICD amplifier
> designs both use a 8877 on 6m - these will get you in the right ball
> park for anode tune (30pF vacuum variable), the main inductor (around 3
> 1/2 turns of 1/4" copper tube on a 1.25" form) and the output loading
> cap (around 300pF).
>
> See:
>
> http://www.gare.co.uk/gj4icd_8877_amplifier.htm
>
> http://www.gare.co.uk/amplifier/
>
> http://www.qsl.net/gm3woj/8877amp.htm
>
>
> Secondly to see if your anode circuit is working remove the tube and
> replace it with a 2200R carbon resistor from anode to ground - this will
> simulate the RL at the tube - then you can connect a network analyser or
> an antenna analyser to the output socket and tune-up the output and see
> if you can get a good match.
... forgot to say ... having removed the tube you are testing the output
tank in the RF deck "cold" - no supplies (no HT, no bias, no supplies)
> My GJ4ICD design wouldn't tune initially and it turned out that I had
> too much L between anode tune and loading - Steve G8GSQ used this
> technique to fix it...
>
> Mike G8TIC
>
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