[Amps] Re: SB220

[Tony Wanschura]

Hi Morrie,

I bought an SB220 a couple of years ago.  Here are my thoughts.

You said the amp has the original tubes. If it has had heavy use, the tubes will probably be "soft". If not, the amp may have seen extended periods of non-use, in which case the HV filter caps could be in bad shape. My amp had been on the shelf for years, and the HV caps blew when I flipped the mode switch on the amp to SSB. I replaced the entire HV cap assembly with a new one from Harbach Electronics (see below).

The SB220's T/R switching runs on 120V dc. Modern transceivers can't handle it. You need to rig an interposing relay or install the Soft Key module from Harbach.

The SB220 uses an old-style, slow, open-frame relay for T/R switching. Modern transceivers switch much faster. This means that your transceiver will be pumping RF through the amp relay contacts before they have had time to close. At the very least, this results in a momentary spike of reflected power, which will cause your exciter output to momentarily  "fold back" before coming back to full output.  Maybe not serious if you are an SSB ragchewer, but an intolerable situation for a CW contester or DXer. QSK is out of the question, unless you install a good vacuum relay QSK system like the one sold by Rich Measures.  Use of an interposing relay will exacerbate the problem.

Because of the 220's smart power supply design, filament inrush current is not really as big a problem for the SB220 as it was for other amps of the same time period.  The damaging effect is mitigated if you start up the amp in the CW position (lower plate voltage).  Harbach sells a mod that cures the problem entirely.

SB220 mods by Harbach can be found at: http://www.harbachelectronics.com/.  It seems that he has suspended business temporarily, but will return soon. These are excellent mods, and well worth the money.

I love my 220. It just keeps cranking along. A fair price, I think, would be no more than $400 USD.

Good luck!

Tony
KM0O