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A lot of the new folks may not realize that the original SB-220
circuit has +120 volts on the keying line and connecting this to a
modern transceiver is likely to damage the radio.

When the SB-220  is turned on,  the relay contacts in the SB-220 are closed putting + 120 volts on the line. The line that feeds directly to you radio! The issue with solid-state transceivers is that companies are using smaller components (small relays or transistor switching) to make a smaller box!  These components are not rated to handle the high voltages presented from the SB-220 and some other older amplifiers.

 I originally avoided this problem by using a foot pedal to key the amp, (rather than
the radio), which has larger contacts capable of handling the load. Luckily I found out about this before making the cable interface for my radio to do it. This is old news to many, but it may
save those who do not know about it.

Here are some solutions for avoiding the voltage problem:

1. Modify the Heathkit SB-200 to present low voltages and current for modern radios.  
Harbach Electronics sells the SK-220 "soft-key". The SK-220 is an interface module which allows your transceiver to key the SB-220 seeing only 0.7 volts at 1.5 mA.
http://www.harbachelectronics.com/
 

I went with the Harbach Soft Key for my SB-220 and found it very easy to install even for those of us with limited abilities and resources.

Here is a PDF of my INSTALL complete with detailed photos.

2. BUILD or construct an external relay in an enclosure.  

K0BX design / solution:
http://www.qsl.net/k0bx/amp.html

KK5DR suggestions and design (for ICOM but could be adapted)
http://www.kk5dr.com/IcomKeyInterface.html

3. Buy a commercial solution:
There are numerous interface cables on eBay and the internet that use small relays within connectors. (W2ENY, N3ATS,cw-man on eBay, etc.) --  Google is you friend.
Make sure the cable has a protective relay built in and not just a cable.

AmpKeyer - by WB8VGE/Mike Bryce :
http://www.theheathkitshop.com/_ampkeyer.html

Ameritron External Box :
http://www.ameritron.com/products.php?prodid=ARB-704

Here are INSTRUCTIONS for building an interface cable for the Kenwood TS-2000 to use with the SB-220. ONLY USE THIS CABLE IF YOU HAVE CORRECTED THE VOLTAGE ISSUE!
Disclaimer: Instructions are a guide only and the author holds no responsibility to any damage you may cause to you equipment or self. Author recommends having an experienced
Elmer there to help you if you are not familiar with the procedures.

Special thanks to W9QB and N6YG for pointing me in the right direction ...
via QRZ's Questions & Answers Forum