Notes on the SB-220 HV board by AG6YJ

Background:

The combination of being retired and being a ham leads to a lot of tinkering. And as the owner of a Heathkit SB-220 which for some reason was aging more rapidly than myself, I set out to do a rebuild. The internet is full of advice, some good, some bad, and some absurd, so I selected a reasonable subset of information and did a respectable job of rebuilding it and getting it back on the air.

But along the way the tinkering spirit led me to lay out a couple of PCBs to clean up some of my work. So when the boards eventually came in, the chassis went back on the bench and went through another revision. And like so often happens, I realized that the boards could be a little better and set out to design another set.

Well, when you order printed circuit boards, you don’t get just one. Generally I get several dozen so if I am lucky enough to have a useful design, I can have spares to give a few to friends.

In any case, this time I elected to sell a few on eBay to further support my otherwise frivolous tinkering. There is no intent here to compete with the other boards (W7RY and others), they do a good job and provide a reliable source for much needed replacement parts.

I did use this board to upgrade 3 different amps.  Two SB-220s and one HL-2200.  Of course along the way I did a lot of other upgrading.

This is the dis-assembled version of the HL-2200.  One of the 220s is behind it.  I decided to re-do the capacitor bank as well.  Boards are on the way.  I also added a fan speed control to the 220.  Used a controller from ebay.  It works fine but is a little kludgey...

The HV diode board:  (this is from the kit that I put on Ebay)

For the most part, the diode rectifier board is a drop in replacement for the original board in the HeathKit SB-220/221.

However, it has a number of additional features not included on the original.

• Connectors to allow the front panel to be removed or extended during service. The original pads are still present and can be used if connectors to the front panel are not desired.
• A fuse and a 5w resistor in series with the high voltage to minimize damage in the event of a tube failure.
• A string of 1N5408 diodes for the voltage doubler (larger than the originals)
• A filter cap across the high voltage output.
• Diode protection for the plate meter.
• A string of 1N4007 diodes to replace the original Zener, including capacitors to filter the voltage.
• A string of 2meg resistors to divide the high voltage for the voltmeter.
• A limiting resistor to provide the current for LEDs on the panel meters if you decide to do that.

To use the connectors to allow the front panel to be removed, you must add a few new wires to the board. The Relative Power wire must first go to the board so that it can go through the connector to the front panel. Similarly, you also have to route the lamp voltage through the board. Being able to remove and protect the front panel is worth the extra effort.

If you elect to replace the original #47 lamps in the meters with the provided Led arrays you will have to come up with the appropriate dc voltage. The arrays are a string of Leds totaling nearly 12v, so to operate from the original filament voltage of 5vac you will need some sort of voltage multiplier.

One method is to build a voltage quadrupler (about 28vdc). A 150-200 ohm resistor on the board is about right in this case if the two arrays are placed in series.  Use 200uf to 500uf 16V caps and 1N4004 diodes or better.

It is fairly easy to put this little circuit on the front panel using a terminal strip.