[Moon-net] 70cm 4cx250b amp question.
w2drz at madbbs.com
Fri Nov 7 22:55:12 CET 2008
It has been many years since have used 4X??? series of tubes,
But here is what I recall to be the way to operate those tubes.
NUMBER ONE IS: be sure the amp is neutralized.
these tubes have about 30 to 35 db gain, that makes it critical to be
completely neutralized to be at least 35 to 40+ reverse isolation to forward
If a high efficient input circuit will drive tubes full output with 1/2 (50
MHz) up to 5 watts (432 MHz) per tube.
Best way I have found is to reverse drive the tubes to do the
input a signal,
"(signal generator, +10 milliwatt or so) or exciter" of a watt or two into
and measure at the input circuit grid circuit coax connector (use a
receiver/transverter) or rf meter for loss of at least 40 db isolation thru
This is with the filament and G1 Grid voltage bias "only applied" ( AB1
linier) G1 voltage.
"NO" screen or HV on plate circuit,
NOW BE CAREFUL as the rf voltage in the PA rank circuit will be extremely
can have "rf burn" if touched by body parts with a watt or more applied
into tank circuit while adjusting neutralizing adjustment's.
If it is found to not be at least 35 db isolation all bets are off on how
the amp will perform, as unit will be unstable to operate or tune.
MUST achieve at least 35 db isolation in the amp.
Now to the G1 and G2 requirements.
G2 current will swing positive to negative normally during tune up and
operation on the air using the 4X??? tubes.
What I have found is for each tube in use, is to allow for 20 mills of
negative screen current and 25 mills positive screen current.
Use 25 or 50 watt zeners (G2 voltage) in series for proper screen voltage,
AB1, AB2 or class "C".
Adjust the (G2 supply) zener current to be (two tubes = 50 mills) 25 mills
per tube allowing for negative current flow up to neg 25 mills per tube,
screen supply now will supply a solid 25 mills positive per tube as
adjusted no problem.
As to using a screen series resistor that is a option,
I used a 10 ohm 1/2 watt carbon for tube flash over (PLATE TO SCREEN)
this is to protect the screen supply regulation parts while allowing the
screen source voltage to be stiff for G2 needs.
These tube are very sensitive to any variation of the G2 screen voltage
resulting in poor operation,
IE: splatter and or distortion along with power output being unstable.
IF too large of a resistor value,
will allow the voltage rise by the screen to exceed the screen bypass
voltage rating, usually it is a max of 450 volts,
(BLOWN/SHORTED SOCKET BYPASS cap.) is the result of a large ohm value in
This resistor can be at the tube socket if using a screen series resistor.
----- Original Message -----
From: "Leif Asbrink" <leif at sm5bsz.com>
To: <moon-net at list-serv.davidv.net>
Sent: Friday, November 07, 2008 1:44 PM
Subject: Re: [Moon-net] 70cm 4cx250b amp question.
> Hi Jordan,
>> It looks like I'll need a big resistor in the grid cavity... I didn't
>> a wire-wound power resistor would be a friendly thing in there, but at
>> dissipation, do I have an option...? I could parallel some 2w carbons,
>> that would also end up being a sizeable lot of 5 resistors....
> The purpose of the resistor is to prevent the screen grid voltage
> from going above 360V in case the current from the screen grid
> has the wrong sign.
> When the plate RF swing is low while the plate current is high, the number
> of electrons that hit the screen grid will create an even higher number
> of secondary electrons that will travel to the always much more positive
> plate. The inflow of these extra electrons create the reversed sign.
> Under full load, the RF swing will bring the plate voltage below
> the screen grid periodically which makes G2 the most positive electrode.
> The current pulses thus created are averaged by the G2 capacitor and
> constitute the normal screen grid current in these tubes.
> There should be a VERY low RF voltage on the screen grid. Capacitors
> should be used (possibly one with a series resonance at the frequency
> of operation). The resistor should not see any RF voltage and since
> the bandwidth of your modulation envelope should be very small, a couple
> of kHz only, the variations in the screen grid current would not
> contain very high frequencies. The screen grid current could have
> frequency components up to 100kHz or so, not more, so there is no
> reason at all to mount it inside the grid cavity. You may even place
> the screen grid resistor in your power supply unit. Use wirewound
> if that is what you have at hand. Just make sure the resistor will
> always be connected to the screen grid whenever there is a plate
>> I'd like to have 10mA on idle, so with 360v it looks like 36K @ 10w will
> That will give plenty of safety margin:-)
>> I'll be sharing a power supply between amps, with separate filament
>> transformers for each amp.... The screen and bias V's can be handled with
>> rotary switch, but what is a practical method of switching the 2200V
> I would leave the 2200V connected all the time to both amplifiers.
> Possibly I would make the screen grid negative in rx mode.
> (Listen to the antenna output from the PA to see if it produces any
> noise in Rx mode. If it does, make G1 or G2 or both more negative.
> Never below -250V on G1.)
> Leif / SM5BSZ
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