A buddy is building the 250w SS HF amp you see in the last few arrl books.
Its almost finished. It only requires aprx 7-10 watts of drive to the pair of
VRF-151G finals.
He also ran a series of tests on his K3....and found that min IMD occurs with
30 w pep out. It increases above or below 30 w pep. Its very clean
at the 30 watt level. The original plan was to use a 5-6db pad between the K3
and the hb 250w SS amp. Upon close examination of the 250w ss amp,
it occurred to me that if the NFB was increased by 5-6 db, the 5-6db pad
between k3 and SS amp could be eliminated.
I think the base resistor values on the SS amp can be increased to do this.
Resulting IMD from the 250w SS amp...on paper, should drop a bunch.
The 250w SS amp has pretty good IMD as is, at 250w pep out...and a lot better
at 225w..and better still at 200w. The bias on the SS amp could also be
tweaked a bit to improve it some more.
OK, for every 1 db increase in NFB on the SS amp, how many db will the
IMD3-5-7-9-11-13 drop ?? I have no info on this anywhere.
Is it going to be a 1 to 1 deal, or does the IMD drop several db for each 1 db
increase in NFB ? What about a tube amp, does it follow
the same pattern as a SS amp re: NFB vs IMD. ?
On paper it appears this scheme should work. What we are after is a squeaky
clean 180-250 watts pep out. The K3 only puts out
100w pep.....and the IMD at 100w pep out is nothing to brag about....its lousy.
The thought here was we could kill about 3 birds at once. Increasing idle
current on the K3 final stage plus its mating driver is not appealing
due to heat issues, plus minimal IMD improvement. Ditto with the 250 W SS
amp. Now whether the 250w SS amp will handle the 30w of drive,
and no 5-6 db pad....and just relying on a 5-6 db increase in NFB is another
issue. Any thoughts would be much appreciated.
later........ Jim VE7RF
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