I have an SB-221 who's history is as follows:
1) Bought used in 1991 with original Eimac tubes (1982). Well constructed.
1,350 watts out. Nice, no problems.
2) 1996. Joined USAF MARS. Found SB-221 worked well on 4.5 MHz.
3) 1997. Listening to MARS net while watching SB-221. Plate current shoots
to 500 ma or so. Grid current slams against the peg. Still listening to MARS
net and wondering "what's going on" as lazy wafts of smoke drift back and
forth in the convection currents above the amp. Jumped out of chair and hit
the power switch. Never could find any heat damaged components.
4) Installed Harbrich's suppressor kits (Nichrome/resistors on both tubes,
parallel nichrome wire/coil to blocking cap, R&C in series balanced drive to
filament, etc. which I presume are based upon Rich's work.). Amp tunes up
much nicer. No instability problems, except that operation on 10, 12 meters
was not possible (gain = 1 w/normal plate current).
5) Concluded that the surpressor to the blocking cap was too good. Physical
observation showed some sagging and discoloration of the straight wire. Added
a 16 gauge Cu wire in parallel with existing nichrome wires. Normal operation
restored on 10 meters (G=10) with somewhat more sensitive tuning on other
bands.
I am planing on replacing the tubes as output has been steadily declining
since "the event." Power is now at about 750 watts with one tube hogging most
of the current.
Any suggestions with respect to precautions that should be taken or tests I
should perform prior to installing the new tubes?
Should MARS frequencies be avoided? Although the input SWR is okay, I didn't
have problems until I started operating outside the normal Ham bands.
Does the 16 Ga. wire seem like an adequate solution for operation on 10
meters?
73's
Chris, KF0FX
KF0FX@aol.com
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