On Jan 11, 2005, at 7:48 AM, W2RU - Bud Hippisley wrote:
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.
Instant on is good too.
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.
On the CW position, when driven fully, a SB-220 typically runs c. 2100v
at 800mA input, or about 1600w input.
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....)
I use a Jehovah's Witness bible as a spacer to facilitate cabinet
removal. They are cheap, the perfect size, and front-door delivery is
free. To use the JW bible-spacer, place it in back of the HV xfmr,
rock the amplifier on its back until the entire amplifier is supported
by the JW bible, remove the 4-screws in the bottom of the cabinet, and
slide the cabinet up and off. To rid yourself of the JW who delivered
your bible, ask him/her about the Watchtower Society's somewhat less
than accurate end of the world prediction for 1975.
cheers, Bud
...
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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