[Heathkit] SB 220 on AM

Sat Sep 2 23:29:10 EDT 2006

Suck it up James! The Heathkit SB-220 is a decent HF amplifier, a real work 
horse in fact. So unless the former owner/owners assembled her incorrectly 
you should be as happy doing eons worth of DX bank shots off the D layer of 
the ionosphere as Ben Franklin was after that solid Brass key he was holding 
between his wet lips evolved into the dance step of the century that 
historic electron Cha Cha, Proton puddle piddle and neurotic Neutron 
necromancy when that inspired bolt of wisdom from the great blue beyond 
ignighted his mustache, abundance of 12 gauge nose hairs and bushy eyebrows 
transforming his bifocal parking place into what put one in mind of a 
seriously char burned super sized marshmellow! So Jim should a 'Headless 
Horseman' pull up in a horseless carriage and deliver into your trembling 
hands an envelope bearing your Colonial family monogram. Easily verified as 
an authentic manual, bold sprawling script listing slightly to starboard 
from Peter The Scholar of Schematics, the Master of manuals and Lord 
Laffayette the only guy in Benton Harbor, Michigan who could seamlessly 
solder up a sword shank withouta cold shoulder joint!

James, this final tidbit that I trust will help you distinguish the properly 
assembled Heathkit from one that really should be used to anchor a boat! If 
when you remove the bottom plate that covered the inside of the chassis, 
turn it back rightside up and observe what gifts befall you. The General 
class rule of thumb is, assuming you still have said digit, having niether 
burned it off due to poor soldering iron habits or slipped with a razor 
blade knife and whittled flesh and bone into a living toothpick, if you can 
fill the Anthrancite Coal car of a 1952 Tom Thumb 1/50 scale model with 
loose solder, don't buy it! Because some kit clutz kid, or Bozo The Clown 
drop out, lived in a joint with no heat and a lot of cold solder!

If you should need them Jim, I still retain copies of the original assembly 
and service information manuals. A great amp, 'provided' you DO NOT 
overdrive it. If you let her run about 800 watts keydown and watch the grid 
current, the 3-500Z's can last twenty years. According to a note from Jim 
H., he was still using the original finals he put in his SB-220 in the early 
seventies.

If you need "good" quality original, or reproduction, manuals then utilize 
WA2CWA, Pete or NI4Q, Al Bernard. Excellent detail, sharp contrast, well 
bound, reasonable prices and prompt shipping. Two gentlemen! There are 
others who also do nice work, but Pete and Al get the old blind dude's 
"thumb and a third" up! (LOL!)

Duane Fischer, W8DBF

 ----- Original Message ----- 
From: "Jim Brannigan" <jbrannig at optonline.net>
To: "Robert & Linda McGraw (K4TAX)" <RMcGraw at Blomand.Net>; "David Thompson" 
<thompson at mindspring.com>; "David Rose" <w4dja at hotmail.com>; 
<heathkit at mailman.qth.net>
Sent: Saturday, September 02, 2006 8:11 PM
Subject: Re: [Heathkit] SB 220 on AM

> Thanks for the tips. I've been chugging along with a Ranger on AM and have 
> been a bit "chicken" about using the SB-220 for all the reasons noted.
> I have a schematic for a Ranger attenuator, time to get the parts and 
> build it!!!
>
> BTW: The Harbach(sp?)  fan is just a bit noisier, but it moves a lot more 
> air.
>
> Jim
>
>
>> Agreed, the SB-220 will do OK on AM although I consider it a wee bit 
>> light in the power supply and cooling areas.  Keep in mind that carrier 
>> level modulated 100% will then be a PEP value equal to carrier times 4. 
>> Thus 350 watts of carrier equals 1400 watts PEP.  Legally one can run 375 
>> watts of carrier modulated 100% for 1500 watts PEP.  Also, keep in mind 
>> the duty cycle of the SB-220.  I seem to recall it to be something on the 
>> order of 50% or less.  Thus the transmit time must not exceed the 
>> non-transmit time.
>>
>> Regarding tune-up.  With the FT-990 in tune mode drive the SB-220 to max 
>> output, watch the grid current which is the most critical parameter to 
>> observe during tune-up.  Don't exceed 260 ma, as I recall, for 2 tubes 
>> and adjust tuning and loading on the SB-220 for max output.  Maximum 
>> plate current should not exceed 800 ma for two tubes.  Increase the drive 
>> as necessary but not so much as to exceed the grid current rating of the 
>> tubes. This can be done in the SSB mode for the SB-220.  Switch the 
>> FT-990 to AM and adjust the drive for about 250 to 300 watts output.  Do 
>> not change the tuning or loading of the SB-220.  Remember, it should be 
>> tuned for some 1200 watts output PEP although the un-modulated carrier is 
>> only 250 to 300 watts. Ideally you have and use a monitor scope to look 
>> at the waveform to assure that the negative peaks do not exceed 100% and 
>> the positive peaks are not flattened.
>>
>> I've run everything from a Heath DX-60 with control carrier modulation, 
>> then converted it to classic screen modulation and used it to drive a 
>> pair of 3-500's to legal AM limit of 375 watts output (1500 watts PEP). 
>> At present I use a Heath TX-1 Apache at 150 watts output (600 watts PEP) 
>> but with high level modulation.  For this application I don't use an 
>> amplifier.  Then there is the retired BC transmitter that weighs in at 
>> 3.7 lbs per carrier watt or 1400 lbs.
>>
>> Good luck on AM
>>
>> 73
>> Bob, K4TAX
>> 1885 KHz and 3885 KHz
>
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