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
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