[Amps] SB-220 and 50Hz and more

[W2RU - Bud Hippisley]

Just to conclude the power line frequency discussion, I have my SB-220 and original manual from 1977, when I built the kit.    There are two references to power line frequency in the manual:
     1.  [page 3] Introduction.  "...The Amplifier can be operated from either 120 VAC or 240 VAC 50/60 Hz lines and can be easily changed from one to the other.  Operation from a 240 volt line is recommended."  (Understatement of the century....)
     2.  [page 80] Specifications.  "Power Required:  120 VAC, 50/60 Hz, at 20 amperes maximum.  240 VAC, 50/60 Hz, at 10 amperes maximum."

The manual also mentions that the cooling fan always runs from 120 VAC regardless of whether you choose a 120 or 240 line input, but makes no mention of any concerns relating to possible reduction in cooling air flow as a result of a 50 Hz mains frequency.  The manual _does_ note that if you have been transmitting with the SB-220, you should let the cooling fan run for a number of minutes before turning off the power to the amplifier -- a practice I have followed with _any_ of my amplifiers.

Even though no one asked me, I have a few additional comments about the SB-220:

1.  Of the "affordable" amplifiers available in 1977, it had the lowest fan noise and generated the least TVI -- in _my_ shack, at least.  I remember it was noticeably superior to the Drake L-4B in the latter respect.

2.  Since 1977 I have used this amplifier as my primary HF amplifier at four different locations.  Although it lacks 160 meter coverage, I would keep it in preference to anything else I've ever owned, including some early Alphas I used to own and the Amp Supply LK-550 I currently use on 160.  I have gone decades without having to replace the finals.  Its reliability and simple circuitry (especially compared to the Alphas) are a joy for this owner.

3.  I operate 99% CW, and have added a 25 - 28 volt (50 watt!) Zener in series with the stock 5.1 volt operating bias Zener when I am on CW.  I originally did this because I run full break-in on CW, and use an electronic TR switch on the output of the amplifier, so I needed to cut off all plate current to eliminate "hash" in the receiver.  Secondary benefits of this are that I have no power dissipated in key-up idling current and my key-down efficiency is probably slightly higher than it would be with the stock class B biasing.  This system replaced an earlier system wherein I "lifted" the grids of the finals and brought them (after by-passing at the tube sockets, of course) to a new RCA phono jack on the rear panel, where I keyed them with a homebrew time-sequence keying circuit that turned the amplifier on before the exciter key line was activated and kept it on for many milliseconds after key-up, to avoid creating any key clicks in the amplifier.  I subsequently determined with on-the-air and oscilloscope testing that my "soft" biasing off of the finals was equally effective at avoiding introduction of key clicks, so I retired the extra circuit box.

4.  The SB-220 was designed and sold when the USA power limits were specified differently than they are today.  In particular, in a stock SB-220, the "CW/TUNE -- SSB" rocker switch changes the primary taps on the transformer to accommodate a 1 KW CW input limit that was then in effect.  When the present 1.5 KW output rule came into being, I modified my SB-220 power transformer wiring to run full (i.e., "SSB") plate voltage on CW as well, and used the now freed-up "CW/TUNE -- SSB" switch to switch my CW zener diode in and out when switching between modes.  These mods have worked flawlessly for as long as the 1500-watt limit has been in effect in the USA -- two decades, perhaps?

In my opinion, the Heath SB-220 was one of the greatest pieces of ham electronics ever designed and sold.  With the addition of certain of Measures' mods, it is very nearly the ideal amplifier.  (If only I could figure out how to slide the case back over the lip of the front panel without using a couch and two or three pillows....)

Bud, W2RU