[Amps] SB-220 and 50Hz and more

[R.Measures]

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