[Heathkit] SB 220 on AM

[Jim Brannigan]

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