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.
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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