Steve, I like to adhere to an absolute minimum of 1". I've been in
arguments with others that voltage and RF will not arc down to 1/10" etc...
However as you get COILS closer to the cabinet wall stray capacitance values
skyrocket. 1/2 the coil distance would really cut down strays but really it
is not practical.
How do commercial ham amps get around strays? They tweak the coils until
they work, just like you would.
A couple things you need to watch. Keep the tube away from the cabinet wall
a ways. On a metal tube the anode capacitance to the cabinet stray could
get so high as you wouldn't be able to tune 10 meters.
With 10.5pF on an 8877 for instance a pair of them and 21pF is almost the
necessary 35pF or so to resonate. That makes a bread slicer not be able to
get the job done and vacuum pretty close to the minimum also.
Layout your chassis for an even flow from one side to another and front to
back.
Good layout is the KEY to whether an amp works or doesn't.
None of it is luck. Never just start bolting down stuff and stuffing it
into a box.
The components come first. They need to be laid out well and then the
chassis put around the tank.
For instance my prototype 77DD amplifier in 1982 ran as much power out on 10
meters as it did on 20 meters. The RF flowed nicely around the compartment,
leads were tolerable and I was very pleased with the results.
Enjoy, BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Steve Flood
Sent: Sunday, March 25, 2007 6:46 PM
To: amps@contesting.com
Subject: [Amps] Tank Coil spacing
As I begin to experiment with component layout on an amp ("playing
chassis-chess"), the biggest hurdle is trying to maintain the requisite
space around the pi and L coils. I wound them 3" diameter 10 AWG. My
reading has informed me to keep the coils 'half the coil diameter' away from
the RF compartment walls and tuning/loading capacitors. This really eats up
some space in the RF compartment! When I look at photos of commercial amps
or even other homebrew amps, I notice this spacing is not adhered to. I
could save sapce by using just a pi tank, but again my reading indicates
that the pi-L is better engineering pracitce. So now my questions...
1. How critical is this spacing guideline? How close can you actually
place the coils before Q is affected or other bad things happen? How do the
commercial amps get away with it?
2. It seems most of the homebrew and Handbook amps I see are Pi tanks. Is
the Pi-L really necessary?
Thanks,
Steve KK7UV
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