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Re: [Amps] SB220 6 M

To: <amps@contesting.com>
Subject: Re: [Amps] SB220 6 M
From: "Robb Urie" <rurie@bajabb.com>
Date: Sat, 6 Jun 2020 10:35:01 -0600
List-post: <mailto:amps@contesting.com>

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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