[Tech-Assist] Convert Harris FM5K to SS IPA

[RichardBJohnson at comcast.net]

Using a solid-state driver for direct drive to a final amplifier tube requires only some simple design. The tuned-circuit between the previous tube driver and the grid of a tetrode final amplifier stage can be used for parts as a matching stage.

Use the 'L' and 'C' of the existing drive plate circuit to resonate the final amplifier grid. Take care of the required grid bias connections. Connect a 10 to 32 pf piston trimmer beween the center of a type 'N' connector and the grid. Use the piston trimmer for "loading" and the grid tuning (ex driver plate tuning) for "tuning." Juggle these for best VSWR. If things are two "squirly," connect a 2k non-inductive resistor across the grid coil for damping. 

When adjusting for minimum reflected power, note that this kind of circuit will reflect lots of harmonics back into the driver. These reflected harmoncs will make the VSWR look worse than it is.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Tom Taggart" <tpt at literock93r.com>
On the 10K, the IPA assembly is mechanically joined to the
PA cavity. If the 5K is like the 10K, suggest stocking up on
whatever it uses for the IPA tube. Haven't heard of anyone
successfully inserting a solid state IPA stage in a 10K. The
10K uses a 4CX300a which is an obsolete tube type.  However,
RF Parts has ICAS type 4cx300A's in stock, have one in my
10K which seems to be happy, and several on the shelf.