Sorry, I hit send too soon.
This is in the CV-2108 cavity. I'm fairly certain the 3CX800A7 is not measured
at 850 MHz.
Typical Operation – Class AB2 Cathode Driven (850 MHz)
FM / CW
Heater Voltage (warmup or standby) . . . . . . . . 13.5 / 13.5 Vac
Heater Voltage (during operation) . . . . . . . . . 12.0 / 13.5 Vac
Heater Current (at 13.5 volts)* . . . . . . . . . 1.5 / 1.5 Aac
Plate Voltage . . . . . . . . . . . . . . . . . . . . 1500 / 2000 Vdc
Cathode Bias Voltage* . . . . . . . . . . . . . . +10.0 / +8.2 Vdc
Plate Current* . . . . . . . . . . . . . . . . . . . 500 / 500 mAdc
Grid Current* . . . . . . . . . . . . . . . . . . . . 25 / 20 mAdc
Driving Power* . . . . . . . . . . . . . . . . . . . 25 / 25 W
Useful Power Output* . . . . . . . . . . . . . . 260 / 325 W
Power Gain* . . . . . . . . . . . . . . . . . . . . 10.9 / 11.2 dB
> On 12/10/2025 7:26 PM EST Paul Decker <kg7hf@comcast.net> wrote:
>
>
> 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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