Hi Chris
In the matter of locating a replacement 8873 for an SB-230 you are quite
correct. As they have been out of production for a number of years your
friend's chances are slim at best.
If you want to change the type of tube that can be done but there isn't a
Drop In replacement available. Based upon what is the most practical your
friend has two choices.
1) Modify the amp for use of a 3CX-800A7/3CPX-800A7 This tube uses the
same socket as the 8873 (SK-1900). As you stated this option will require
some metal work relocating the socket and installing and cooling system.
The 3CX-800 also uses a different filament voltage 13.5 volts. The big
advantage to this tube is the increase in power dissipation to 800 watts.
Plus the fact that this tube is readily available. The 3CPX-800A7 is
manufactured on both sides of the Atlantic. Svetlana makes this tube also
but I've heard reports that requires a little more umphhh to drive it.
2) The second option is to use the 8560AS. This tube is similar but not
identical to the 4CX-250B. It is a conduction cooled tube so you can use
the existing cooling system. It also has the a 6.3 volt filament like the
8873 so that doesn't need to be changed. As it is a tetrode you will have
to make some changes there. I have seen some unorthodox circuits that
drives the cathode, grounds the control grid and forward biases the screen
grid. (Posted here on the Amps list some time ago) I have no idea how it
performs.
The 4CX-800A is also an option but requires substantially more work than
the previos two.
There are your choices Chris
73s
Bob K1TA
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