Megawatt Bi Polar Transistor Upgrade

Both devices have a fairly low gain. Whether or not the difference will make a difference depends on how the devices are used and the other circuitry associated with them.

If you have the schematic with device part numbers you can use a program like PSpice to run a simulation to determine if the substitute part will work. There used to be free versions available that are fully functional but restricted to smaller circuits and with fewer parts models.

digital was in its infancy when i went to school for this stuff

i miss going to the supply house and getting the new GE RCA Sylvania cross reference catalogs

and the big thick GE handbook with all the specs, and typical use schematic circuits

i learned all my digital going to OEm machine mfg schools for CNC tools service
then migrated to PC's

Here

An analog boy in a digital wilderness...

I don't knwo, analog is not my cup of tea. Sorry I can't be of more assistance, the bulk of my experience is in the digital realm.

The voltage comes "automatically" with the gain in current carrying. 500 PIV is plenty. Upgrading the I tried to keep gain differential as small as possible. Needs more study on my part. Trying to keep this upgrade rational. Paralleling the Bi-polars means a major design change of the board, same for adding a darlington to aid the driver Q1

Upgrading the 2SC3320 is not a cut & dried process. But a definite unknown is whether the FJL6920TU is the wisest alternative.

I am trying to increase the I capacity of the Q2's. Obviously 15-amperes is insufficient. The gain difference troubles me. It sure would be nice to find a single component upgrade. Having a 40+ ampere adjustable power supply that can live for several thousands of hours maxxed-out appeals to me.

But then a person can spend free time popping Budweiser's and watching NASCAR, the movie, or Dukes of Hazzard reruns. I prefer to work as a relaxation.

This may change after a neurosurgeon fixes my disc. The idea of returning to trouble$hooting cruising $ailboat electrical$ has great appeal. For now, I must limit hobbies to what I can physically do.

Why do you need such a high voltage, isn't the megawatt a low voltage supply?

For more current, have you considered paralleling transistors? It appears a small value "ballast" resistor is required on each emitter of the transistors in the parallel, to stop one from trying to eat all the current on its own.

Parallel bjt
scroll to the section "Thermal mismatch (problem with paralleling transistors)"

Alternately, your new high current transistors might benefit from a lower current bjt acting to boost the gain, a Darlington "pair" made from discrete transistors.

Darlington

The studio audience laugh-sign lights up...

After the magic smoke is released.

Hmmmmm...

Sounds like a Steve Martin comedy routine.