I'm not sure. I see at 450 MHz it is 46% efficiency
But typical operation at 150 MHz is listed as:
3CX800A7 – Typical Operation (Measured Data to 150 MHz)
Class AB2, Cathode Driven
Parameter Value
Plate Voltage 2200 Vdc
Cathode Bias Voltage +8.2 Vdc
Zero-Signal Plate Current 150 mAdc
CW Plate Current 500 mAdc
CW Power Input 1100 W
Peak Envelope Power Input 1100 W
Two-Tone Plate Current 313 mAdc
CW Grid Current 16 mAdc
Two-Tone Grid Current 16 mAdc
Peak RF Cathode Voltage 64 V
Peak Driving Power 70 W
Useful Power Output (CW) 750 W
Useful Power Output, PEP **1100 W
Cathode Input Impedance 54 Ohms
Resonant Load Impedance 2700 Ohms
Intermodulation Distortion (IMD)
– 3rd Order Products –36 dB
– 5th Order Products –32 dB
> On 12/10/2025 7:01 PM EST Douglas L <sparks06524@yahoo.com> wrote:
>
>
> Checking the specs, the 3CX800U7 is rated at 350 watts output with an
> efficiency of only 33%. That’s less than half the rated output power of the
> 3CX800A7.
> I wonder if it was derated for typical UHF service?
> If not, it’s a poor substitute for a 3CX800A7.
> Doug/WA1TUT
>
>
> > On Dec 10, 2025, at 1:01 PM, Paul Decker via Amps <amps@contesting.com>
> > wrote:
> >
> >
> >>
> >>> Has anyone created a work around to replace the 3CX800A7 tubes in the
> >>> AL-800, the AL800H, Alpha, Ten-Tec and other amps that used this tube?
> >
> > The 3cx400U7 and 3cx800U7 are usually much cheaper alternatives and in many
> > cases can be NOS. The datasheet lists the working B+ as lower, but I
> > suspect that is simply due to the higher frequency rating of the U7
> >
> > The only issue is changing out the base which seems daunting at first.
> > There is an "RCA like" plug connector that fits the base really well so
> > that only the grid ring needs to be figured out with some simple finger
> > stock and a metal plate.
> >
> > Someone with talent could design an adapter board that would work well too!
> >
> > Paul
> > KG7HF
> >
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
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