> For the JA tube naming, the first figure means the plate dissipation.
> 
>   4: 100 W and up to below 200 W
>   5: 200 W and up to below 500 W
>   6: 500 W and up to below 1000 W
>   7: 1000 W and up to below 5000 W
>   8: 5000W and up to 50 KW
>   9: 50 KW and above
> 
> The second letter means the number of electrodes.  The third figure means
> the individual name, usually the year in which
> the tube was issued (sometimes JA Emperial year was used).
> 
>  D: diode
>  T: triode
>  F: tetrode    
>  P: pentode
>
>  R: means radiation cooling
>
>  For eg. 7F71R means a tetrode of 4500 W plate dissipation,
>          radiation cooling, and issued in 1971.
>  For eg. 4P60 means a pentode of 125 W plate dissipation,
>          maybe issued in 1960.
>