[Heathkit] SB 220 on AM

Sat Sep 2 18:13:25 EDT 2006

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

> ----- Original Message ----- 
> From: "David Rose" <w4dja at hotmail.com>
> To: <heathkit at mailman.qth.net>
> Sent: Saturday, September 02, 2006 10:15 AM
> Subject: [Heathkit] SB 220 on AM
>
>
>>
>>    I  have a Yaesu Ft 990 and I want to do some AM work just for fun. The
>>    990  puts out 25 watts on am. Can an SB 220 be used on AM? I was going
>>    to  buy  an AM rig at the Shelby Hamfest today, but they are very high
>>    priced  ( I think) and I would have to purchase lots of other parts to
>>    make a station operative. Maybe with more power and a higher antenna I
>>    cud work some AM with what I have.
>>    Thanks
>>    Dave