Most of the "parasitic tank circuits" consist of 'accidental' capacitance and
inductance
and vary greatly depending on construction practice. For example, one "tank
coil" consists
of the inductance in the path from the plate through the blocking capacitor,
then through
the frame of the pi input capacitor and back to the cathode. The capacity of
the input
capacitor is unimportant and just serves to reduce the effective inductance of
the path.
In the grounded-cathode 2 x 813 160-10m amplifier I'm building now, I've cut a
copper
sheet to fit around the bottoms of the tube sockets, which are mounted about
3/4" below
the chassis. The sheet has a wide tab which passes through the hole in the
chassis and is
bolted to the top. Everything that needs to be bypassed or grounded goes
directly to the
sheet with shortest possible leads, and the pi-net tuning capacitor is grounded
on top of
the chassis right next to it. Screen and filament bypassing is done with
parallel 0.01 and
0.001 uf discs.
All of this is intended to reduce the inductance in this parasitic tank
circuit. If I'm
right, it should allow the use of less inductance in the suppressors, which
will result in
less wasted power in the resistors on 10 meters.
Once I have it working I intend to follow the procedures for checking for
parasitics (LF,
HF and VHF) described in the ARRL handbooks and use the minimum inductance
necessary.
Maybe I won't need suppressors at all.
In practical terms it's probably good enough to just copy the suppressors used
in
generations of 813 amplifiers :-) but part of the fun of building is the
tinkering!
On 7/16/2010 5:20 AM, Dave White wrote:
> Maybe another reason that we don't see too many worked-through rigorous
> calculations is
> that there are so many variables in play and since parasitics result from
> positive
> feedback the smallest variation in the values of the large numbers of
> seemingly
> insignificant factors can cause huge variations in the system behaviour down
> the line.
> The sheer randomness of many factors also comes into play.
>
> Do a bit of Googling about Chaos Theory versus deterministic predictability
> and you'll
> see what I mean. Chaos is a fundamentaly different way of looking at a
> problem. It
> has many applications from meteorology to Agile software development,
> business planning
> and ..... who knows .... maybe parasitic suppressors :-)
>
> 'In theory, theory and practice are the same thing. In practice they're not'
>
> Discuss.....
>
> Dave G0OIL
>
>
> -----Original Message----- From: "Bill, W6WRT"<dezrat1242@yahoo.com> To: "Amps
> reflector"<amps@contesting.com> Sent: 16/07/2010 04:03 Subject: Re: [Amps]
> parasitic
> suppressor voodoo
>
> ORIGINAL MESSAGE:
>
> On Fri, 16 Jul 2010 06:29:33 +0800, Alek Petkovic<vk6apk@bigpond.com> wrote:
>
>>
>> Don't tell me stuff like "four turns and 50 ohms worked for me." I am
>> talking science
>> and that sort of approach is NOT scientific. Gimme calculations. Somebody
>> must know
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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