Date: Thu, 12 Jul 2018 16:24:39 -0600
From: "Jim Garland" <4cx250b@miamioh.edu>
To: "Amps group" <amps@contesting.com>
Subject: [Amps] parasitic suppressor for six meters
<I've used the amp for years, with about 750W output, but recently ran into a
<power supply problem and had to tear into it. In doing so, I noticed the
parasitic suppressor was burned. I believe it's probably the original
suppressor, and it consists of 1.5 turns of 12AWG tinned wire shunted by
two 51 ohm two watt resistors, one of which was burned in half. The amp has
never had parasitic oscillations to my knowledge, so I'm guessing there was
probably a bit too much inductance in the parasitic inductor. I'm thinking
about replacing the two burned resistors with a single 22 ohm 5 Watt metal
film resistor, and trimming the inductor to a single turn. Does this seem a
reasonable change? Does the tube require any parasitic suppressor at all?
73,
Jim W8ZR
## That tube does not require a suppressor on HF..or 6m.
The 3CX-800A7 only has .05 pf from anode to cathode.
## The 3CX-3000A7 has .6 pf from anode to cathode..and it too
does not require a suppressor on HF or 6M.
## A 8877 has .1 pf from anode to cathode. Never seen a 8877 6m
amp with a suppressor. Several friends have 8877 on 6m, none with
suppressors.
You dont require a suppressor on HF for the 8877.
## On the 2 x GS35B 6m amp,... GG triode, it too has no suppressor.
## 3x6, 3x10, 3x15, 3x20 dont require suppressors at HF or 6M.
## The 3CX-800A7 has the least stray C from anode to cathode of the bunch.
The last thing u need is a suppressor. Take it out, toss it, replace with
silver plated
cu strap, and be done with it.
## You could also use 1-3 small diam turns of cu strap, or heavy gauge wire,
but no resistors.
Then the 6.1 pf between anode to grid... + the tiny bit of uh from the 1-3
turns, will form
a step down L network. Then the plate load Z can be reduced by quite a bit.
Then the Z as
seen by the PI net... or PI-L net will be lower. Then you get to reduce
the tank Q .
Then u end up with less circulating current. The step down L network will
increase harmonic
suppression a bit. Then its called an .... L-PI.
## That 6.1 pf from anode to grid..is what u will see with an LCR meter..on a
wooden bench. Once
into the socket, it will rise a bit, due to proximity of the lower anode
fins..to chassis below them.
## considering the required uh between the tune and load cap is miniscule,
your .1875 tubing could
be replaced with a flat strap wound coil, like .375 wide. .375 wide strap =
.261 inch OD tubing.
With flat strap wound coils, the turns spacing can be quite close, since
adjacent turns are like knife
edges facing each other. No C between turns.
## designing the PI net for the lower plate load Z is easy. You will end up
using slightly more tune and load C,
and slightly less main tank coil. The GM3SEK PI net software makes
designing the PI net..or PI-L
or the L-PI..... or the L-PI-L a snap. But you need to measure the stray
C from anode to chassis, then you can
enter the numbers. That tiny bit of uh between plate block cap and main C1
tune cap can also be factored in.
It can also be located between anode and input to plate block cap. I juggle
the loaded Q, and uh between anode and
tune cap, such that its tunable, and pf left in the tune cap. If done right,
you can lower the loaded Q by quite a bit.
Then its more broad banded as well.
## Or just toss the burned out suppressor. On less thing you dont require.
Later... Jim VE7RF
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