[Amps] Initial capacitor settings 8877

[Chris Gare]

Mike has it right. Just set the caps to centre and tune for max. Start with
low drive and gradually increase drive and you should be OK.

Never had any issues with several 8877 amps I've built.

Chris G3WOS

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Mike Tubby
Sent: 19 February 2013 23:37
To: Drax Felton
Cc: amps at contesting.com
Subject: Re: [Amps] Initial capacitor settings 8877

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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