Hello,
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.
73,
Robb NØRU
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