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
>
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