Hi all.

I'm trying to find a good circuit for a Class C 4CX250B linear.

I've found a few for multiple valves in Class A, but I'm after just a simple
circuit that shows a 4CX250B linear for 2m with Class C biasing.

Idea is to use it for a little project where I want to run about 100W, but
run it cleanly without all the RF hash that transistor amps give.


Can anyone point me in the right direction?

Also, how would you go about varying the RF power output on a 4CX250B linear
in class C? Is it by varying the 1.2kV supply voltage, which would make
sense, or is there some other recommended way?


Sam

"Samuel Hunt" wrote in message
...
Hi all.

I'm trying to find a good circuit for a Class C 4CX250B linear.

I've found a few for multiple valves in Class A, but I'm after just a
simple
circuit that shows a 4CX250B linear for 2m with Class C biasing.

Idea is to use it for a little project where I want to run about 100W, but
run it cleanly without all the RF hash that transistor amps give.


Can anyone point me in the right direction?

Also, how would you go about varying the RF power output on a 4CX250B
linear
in class C? Is it by varying the 1.2kV supply voltage, which would make
sense, or is there some other recommended way?


There is not really any difference in a class A and Class C circuit. Just
adjust the screen and control grid voltage to the class of operation you
want. Should be in any of the tube manuals.

So basically take off the bias?

I assume then that I can just use the tank circuit to tune it nicely?

And I might have to increase the drive by 4 times in order to get the
potential 4 times increase in output?


Sounds simple enough to me!

Sam


"Ralph Mowery" wrote in message
link.net...

"Samuel Hunt" wrote in message
...
Hi all.

I'm trying to find a good circuit for a Class C 4CX250B linear.

I've found a few for multiple valves in Class A, but I'm after just a
simple
circuit that shows a 4CX250B linear for 2m with Class C biasing.

Idea is to use it for a little project where I want to run about 100W,
but
run it cleanly without all the RF hash that transistor amps give.


Can anyone point me in the right direction?

Also, how would you go about varying the RF power output on a 4CX250B
linear
in class C? Is it by varying the 1.2kV supply voltage, which would make
sense, or is there some other recommended way?


There is not really any difference in a class A and Class C circuit. Just
adjust the screen and control grid voltage to the class of operation you
want. Should be in any of the tube manuals.

On Sun, 12 Mar 2006 11:30:43 +0000, Samuel Hunt wrote:

Hi all.

I'm trying to find a good circuit for a Class C 4CX250B linear.

There are a few designs in the old RSGB VHF/UHF Manual. If you
are searching the web for info, leave out the word 'linear'.
A class C amp is usually not linear.

I use a single 4CX250B on 2M. The G1 bias control is on
the front panel so that I can adjust it to class AB for
SSB and class B or C for CW,JT65 or FM.

http://www.qsl.net/la0by/2m-hpa.htm
http://www.nitehawk.com/sm5bsz/power.htm

73, Ed. EI9GQ.


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On Sun, 12 Mar 2006 11:30:43 +0000 (UTC), "Samuel Hunt"
wrote:

Hi all.

I'm trying to find a good circuit for a Class C 4CX250B linear.

Good luck. "Linear" and "Class C" are mutually exclusive.


I've found a few for multiple valves in Class A, but I'm after just a simple
circuit that shows a 4CX250B linear for 2m with Class C biasing.

Idea is to use it for a little project where I want to run about 100W, but
run it cleanly without all the RF hash that transistor amps give.

Then you want to run it class AB1, which is much easier on the fragile
grids, BTW.


Can anyone point me in the right direction?

Also, how would you go about varying the RF power output on a 4CX250B linear
in class C? Is it by varying the 1.2kV supply voltage, which would make
sense, or is there some other recommended way?

If you would just forget the Class C part, you would simply vary the
input (drive) power and get a linear change in output power. Varying
the drive power to a Class C amplifier will change the output power
too, but in a "non-linear" way. Changing the anode voltage on a
tetrode to vary (DC) input power is fraught with trouble and doesn't
make sense.

If you insist on Class C bias, (grid cut off with no drive) and you
can't vary the rf drive level for some reason, with excess drive
power, you can bias the grid further into cutoff. This will increase
the harmonic content, but the plate capacitance of these tubes
(valves) is high enough that the Q of the plate tank will be
necessarily high so this may not be an issue.

You might consider changing the screen voltage. Be advised, however,
the screen grids in the tubes are very delicate and you will have to
insure that dissipation limits are not exceeded.

"Samuel Hunt" wrote in message
...
So basically take off the bias?


You don't take it off like you would a transistor. Tubes work sort of
backwards from transistors. YOu increase the voltage to the recommended
value of plate current. Somewhere around -50 volts for AB and -100 volts
for C. That would give around 60 ma of plate current for AB and very low
current for C with no input signal.
You will need the tube spec sheet for closer values. Also the screen
voltage should be changed for the mode if operation is for CW only. Think
it is about 250 Volts for CW and 350 for AB but may have it backwards.

Samuel Hunt wrote:
Hi all.

I'm trying to find a good circuit for a Class C 4CX250B linear.

I've found a few for multiple valves in Class A, but I'm after just a simple
circuit that shows a 4CX250B linear for 2m with Class C biasing.

Idea is to use it for a little project where I want to run about 100W, but
run it cleanly without all the RF hash that transistor amps give.


Can anyone point me in the right direction?

Also, how would you go about varying the RF power output on a 4CX250B linear
in class C? Is it by varying the 1.2kV supply voltage, which would make
sense, or is there some other recommended way?


Sam


Oxymoron. Class C is NOT linear!
Now if you want to amplify CW or FM signals, class
C is fine. For AM or SSB signals you want class A, AB or B.
For this tube the grid #1 bias, grid #2 voltage and grid #1
drive (signal input) are different as you change class of
operation. (Mostly the screen and plate voltages can remain
the same and just change the resting plate current with the
bias and don't apply more than a specific signal level so
your max plate current isn't exceeded.)

Samuel Hunt wrote:

Hi all.

I'm trying to find a good circuit for a Class C 4CX250B linear.

I've found a few for multiple valves in Class A, but I'm after just a
simple circuit that shows a 4CX250B linear for 2m with Class C biasing.

Idea is to use it for a little project where I want to run about 100W, but
run it cleanly without all the RF hash that transistor amps give.


Can anyone point me in the right direction?

Also, how would you go about varying the RF power output on a 4CX250B
linear in class C? Is it by varying the 1.2kV supply voltage, which would
make sense, or is there some other recommended way?


Sam


Sam:

If you want to run an RF amp in a linear manner then one cannot bias the amp
in class C. Class C is not a linear mode of operation and will give all
kinds of trash if running in SSB. Now if you are running either CW or FM,
then you can run a class C amplifier.

The Most linear class of amplifier operation in the classic modes is class A
mode. This mode means the active device is always conducting even when the
input is not being fed any signal. In an ideal world efficiency of class A
is only going to be 25 percent.

Most amateur class amplifiers run in a mode that is either ABone or ABtwo.
I can't recall the differences between the two at the moment but they are
clean under normal operation and don't put out a lot of noise.

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Ken Scharf kirjoitti:




Oxymoron. Class C is NOT linear!

True
Now if you want to amplify CW or FM signals, class
C is fine. For AM or SSB signals you want class A, AB or B.
For this tube the grid #1 bias, grid #2 voltage and grid #1
drive (signal input) are different as you change class of
operation. (Mostly the screen and plate voltages can remain
the same and just change the resting plate current with the
bias and don't apply more than a specific signal level so
your max plate current isn't exceeded.)

Hi

Class C isn't fine.
Class C is producing a lot of harmonics
Specially odd ones are bad.
I don't recomment class C elsewhere but for transistor FM use in VHF and
UHF
Maybe also for KISS QRP in HF CW

73 , Risto OH2BT

Hi Samuel and all

I'm trying to find a good circuit for a Class C 4CX250B linear.

*cough* *cough* *choke*

A Class C is *not* linear.

I think the usage of "linear" for "an external amplifier to boost
the power going to the antenna" comes from the CB crowd, who *do*
need a linear amplifier (for SSB).

OK, you want to boost FM, and for that you can use Class C. The
only ARRL Handbook I have here (1992, I'm not counting the '48 :-)
has two very similar schematics, one for a 3CX800 and the other
for a 3CX1500, but I do seem to recall that a different edition had
a 4CX250 schematic. Or maybe it's the RSGB handbook.

I've found a few for multiple valves in Class A, but I'm after just a
simple circuit that shows a 4CX250B linear for 2m with Class C
biasing.

Don't search for "linear", search for "amplifier" :-)

W