Re: [Amps] SB220

[Pete Smith <n4zr@contesting.com>]

Mike Hyder's comments are very thorough and right on.  I would just add 
four points:

-- The Harbach parasitic supressor mods are based on Rich Measures' 
work.  I took them out of my amplifier years ago and experienced no loss of 
stability.  I should mention, in fairness, that my amp was very carefully 
built by an old-timer who did it all very well -- others might not be so 
good, and stability could suffer as a result.

-- The instant-on capability of the 3-500Zs is a great feature, 
particularly for contesting and DXing.

-- These tubes also seem to have a lot more "headroom" in the grid current 
department than most of the "more modern" ceramic triodes.  I have read 
recently that some users of modern amps find themselves having constant 
problems with switching among antennas that are not all 50 ohms, or darned 
close.  Switch antennas, transmit, and the grid current protection will 
trip because the loading setting for one antenna is not the same as that 
for another.  By contrast, my SB-220 has almost 100 ma of grid current 
margin in normal operation.  I can switch antennas on a given band with no 
fear of exceeding its limits.

-- The wide impedance matching range of the SB-220's output circuit has 
been a happy surprise.  For example, I run a dipole parasitic array on 80 
meters, which is cut for the CW band.  Up in the 75 meter DX window the SWR 
is around 3:1, and yet the SB-220 will tune and match it correctly.  The 
"active antenna tuner" also works with my 40-meter yagi up in the phone 
band, and in other non-optimum situations.

73, Pete N4ZR

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