Just to keep the reflector updated with info...
I received a few private emails regarding using a chain of hi mu GG tubes in
cascade. The general consensus is that it was not a very good idea. One
reason is that there is grid flow in a GG amplifier producing a varying
load. My example may have been an exception because the drivers were all in
class A, small signal config producing no variation, but, the final still
was GG, AB2 with grid current. In contrast, a tetrode final in AB1 will
produce a stable load for the drivers.
Another thought was this lashup was sorta Rube Goldberg with all these
stages... :-) HA!
Plus a lot of class A power waste vs: two smaller class A tetrodes described
Another fella thought that a 6146 was a more stable and a better tube than
the 6550 for RF. A 6550 is better for audio he mentioned.
So, I am back to using a class A, grid driven, tetrode chain with negative
feedback as follows: The 5mw tap from the FT-1000D, into the OPA-695 op amp
[500 mW out], the 6146 or 6550 class A, [20V out] into the 4CX-250/
conduction cooled version class A [300V out] driving a 4CX-XXXX final.
The final will have negative feedback back to an early stage. The
neutralization and feedback may be tough on the higher bands, but maybe with
some swamping and good layout I can pull it off.
Case closed. It's nice to have this good advice and save lots of wasted
Just wanted to add these notes for the archives.
73, and the best of holidays!
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