[Heathkit] SB 220 on AM

[WA3GIN]

I have a slightly different approach.  Take the plate dissapation of the 
tubes and don't exceed that wattage or you'll be wondering why you can't get 
more than a couple hundred watts out of the amp on SSB the day after you ran 
AM for 20 minutes. If it were my amp and the total plate dissapation was 250 
watts I'd run not more than 185 watts input by the numbers, i.e. DC Plate 
voltage x input current.

Have Fun,
dave
wa3gin
----- 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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