Any possibility that a HV MosFet might be substituted ?
LND150 Mosfets have been substituted for a different tube ( the 12ax7 ; which is Not the 12au6 discussed herein for the Tek 575)
See:
And likely, one might have to modify the circuit or bias somewhat, depending on the particular tube circuit.
I have successfully directly replaced triodes and dual-triodes using HV Fets in some old WWII military surplus equipment ( such as all of the triodes in the Rx section in the URC-4 , etc. ) with good results and no frequency shift nor distortion. Crazy but it worked very well. Also had success doing such with the BC-1335 Rx section which has a large compliment of tubes.
With such a modern diverse selection of IRF type HV MosFets there could be some interesting experimental work to be tried...
