2M PA with 4cx250

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Kenneth Berg:
I want a bit more power on 2 meter...
What is the "best" way to bild one?
At the beginning I start with a singel one, and hope to get around 300 watts.
Pleas tell me about your exp. to build one...

(ore sell me one..)


Steve Katz:
A single 4CX250 will yield 250W carrier or 500W PEP output, provided it's connected to a good power supply providing at least 2.2kVdc to the anode.

However, a pair in push-pull will provide 2x that power, and a bit more, with relatively little additional work or cost.  The biggest "cost" increase in going to two tubes is in the power supply, since you'll need 2.2kV at 600-700mA.  The added load on the screen and grid bias supply is just about nothing, but of course you'll need twice the filament power at 6V for two tubes.

Besides the obvious power advantage, a pair of tubes in push-pull nulls the second (and all even) harmonic output and makes for a cleaner signal on the air.  Also, since a single 4CX250 in AB1-AB2 only requires a couple of Watts of drive power, a pair of them is still very, very easy to drive.  Most people have to "throw away" a lot of their drive power when running these tubes, since they have so darned much gain (power gain for a 4CX250B is about 250, e.g., 2W PEP at the grid will create 500W PEP at the anode when operated in the linear mode).

Dual-tube PP circuits, complete with schematics, assembly guidelines, photographs and parts lists, have appeared in most all VHF-UHF handbooks for about forty years.  The ARRL VHF-UHF Manual and RSGB VHF-UHF Handbook both contain them, and all the older ARRL Handbooks (from the 1950's through the 1980's) had them as well.

My own homebrew PP 2x4CX250B 2m amplifier is basically from one of those books and has delivered a clean kW output power for about 22 years now.  Still on its original tubes!


Larry J. Rolewic:
I hate to disagree with Steve, (WB2WIK) but he's being a bit liberal with the ratings.  The 4CX250B is only rated for 2000 V. plate, not 2200 V.  I doubt pushing the tube that extra 10% would yield an appreciable increase in output, anyway.  I do agree, that using two tubes would be a better way.

    But make note that the filament voltage for the 4CX250B is 6.0V, Not the usual 6.3 Volts; it also needs to be derated to 5.5 Volts above 400 MHz.
    Also note that while you can substitute a 4CX250B for the older style 4X150 od 4X250, these older style tubes may not always work in a circuit designed for a 4CX250B.  (Also verify the filament voltage; the 4CX250B has a 6.0 V. filament, but there's also a 26.5 volt filament version of this and the older version tubes.)

Steve Katz:
WA9SVD: The 26.5V filament version is a different part number, 4CX250F, so it's pretty easy to tell the difference at arm's length.

The up-to-date rating for the 4CX250B is 2500V for ICAS.   The 2000V rating is an old one, although I have never found any problem running even 1960-manufactured 4CX250Bs at 2500V.  

In reality, the higher the Ep, the better these tubes operate, right up to the point of flashing over.  Reason is, gain and linearity improve with higher Ep, while Ig2 is decreased (for any given set of conditions) with higher Ep, and the weak link in these tubes is the screen (G2).  If you run a PA with 2.0kV Ep and 5W drive applied and the Ig2 is 20mA, simply bumping up the Ep to 2.2kV normally decreases Ig2 to nearly zero, greatly prolonging tube life.  That's the reason to run Ep as high as possible -- the greater difference between Eg2 and Ep, the lower Ig2 is.


Larry J. Rolewic:
Hi Steve,

    I was going by the data sheets I have... And yes, they are a tad "old."

    As far as the 4CX250F (and the older 4X150D) are concerned, they do use a 26.5 volt filament, but I have several tubes that were marked only with ink, which has worn off.  So the only way to tell the difference is with an ohm meter.


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