[Amps] SB220 6 M

Robb Urie rurie at bajabb.com
Sat Jun 6 12:35:01 EDT 2020


The QST article is a good starting point but I found with my conversion, I 
had to fabricate several tank coils until I hit the right amount of 
inductance.  Besides winding a new plate choke, I had to make new parasitic 
chokes also...I followed an old design in one of the Bill Orr handbooks that 
seemed to work better than the original ones.   Utilizing an MFJ-259 on the 
output and manually activating the relay, I could set the tune and load caps 
to a mid-point setting and adjust the tank coil to achieve the desired 
results (Tubes removed and the correct amount of resistance to ground).

My SB-220 was built in 1972 and still has the original tubes.  Normally I 
run 50 watts in and see 600 watts out on the low voltage and about 1 KW on 
high.   With full drive, I get a little over 1400 watts but really pushing 
the grid current at this level and rather just loaf along at a lower 
setting.   I did update the power supply with the Harbach kit along with the 
fan and the amp runs nice and cool.   It was a fun project as I pretty much 
gutted the entire amp and rebuilt it to include repainting the front panel 
black and the case a light gray.   Some pervious owner drilled a hole next 
to the Heathkit badge, why I have no idea.

Good luck and enjoy the project.



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