sb220

Modified SB-220

I bought this highly modified SB-220 in Dayton several years ago. With the amplifier itself came an external parallel HV transformer. When I initially turned on the power switch, the 20 amp circuit breaker to my 220 line blew. I removed the external transformer, and fired it up again. This time the amplifier turned on and showed acceptable HV. The filaments were heating up. When I started to tune up the amp into a dummy load, closing the relay showed 120 ma plate current. However, after adding some power input (20-30 w and more), changing the load and tune settings did not result in any relative power change--it stayed at 0, andthere was no grid current.

Fig 1. Just above the output connector on the right, you can see the connectors for the outboard accessory power transformer. The sockets of the tubes have been shielded (cover is removed), and several modifications are made to the connections. The relays have been changed. The "tube" in the upper part of the picture is an Amperite relay.

Fig 2. Here you can see the modifications on the load capacitor. The ALC circuitry and RF choke have been moved here from under the chassis to make room for the shielding around the tube bases and the modified antenna relay.

Fig. 3. Shows the PC board, the zener, and the connections. My earlier sb220 had the zener mounted on the chassis divider on the left behind the PC board. The rusty little thing at the top left of the PC board is a pot replacing the 5.6K resistor.
The zener in this unit was originally located in the assigned place, mounted on the chassis divider between the PC board and the chassis interlock. It was moved to the PC board, and a heat sink applied.

Before I start disconnecting lots of stuff on this amplifier, I'm wondering if anybody recognizes any of the modifications. Also, if anybody has suggestions about some simple approach that I can take to start troubleshooting the amplifier, I would listen to anything. I do not plan to use the accessory transformer. BTW--the tubes are ok. I checked them in my other SB-220. As I work on this unit, I'll add updates and pictures of the progress.

SB-220 Schematic-- web page or pdf

I originally posted these web pages beginning in August 2005 as I was seeking help with a highly modified Heathkit SB-220. The work on the amplifier has been completed, and it now works well, giving about 1100 watts out on CW with 100 watts drive. The following pages detail my (painfully slow) decision making as I tried to get the piece into a functional state.
During the original postings, I was given many useful suggestions and much encouragement by subscribers to qrz.com and the amps list on contesting.com, as well as several on-the-air hams. I am lucky that I have another, relatively unmodified, SB-220 available for visual comparison.

Page 1--posted 30 Aug 2005

Fig 4. Less modified amp. Underside of chassis. Right side up.

Page 2----posted 31 Aug 05

Fig 1. (Repeated for convenience.) Extensively modified amp. Underside of chassis. Right side up.

Fig 4 shows a relatively unmodified SB-220. (The only mod in this view is the Harbach Soft Key positioned just above the fan opening.) Figure 1 is repeated alongside for comparison. Note that the shielding enclosure around the tube sockets has had the lower cover removed to show the wiring of the sockets as well as the bifilar choke. The wiring for the outboard accessory HV transformer is more easily appreciated in this side-by-side view.
The shield virtually eliminates ventilation of the tube sockets. The plan is to remove it to improve socket cooling.
Another point of interest is seen when one compares Figs 1 and 4 above. The original position of the ALC circuitry was taken by the antenna relay mod. Fig 2 shows that this wiring was moved to the upper chassis compartment onto the load capacitor.

Fig 5. Closeup of shielding enclosure

Fig 6. Shielding enclosure removed.

Comparing Figs 5 and 6 demonstrates that even after removal of the shield, the socket is a mess compared to Fig 4. My plan is to move the bifilar choke back to the original position and duplicate the original circuitry around the tube bases.
The fan of the original kit has been replaced by a more powerful one, and when the xmtr is keyed, the fan increases rpm. Unfortunately the housing of the replacement obstructs flow to the underside of the chassis. Because of this, I will replace this fan with one of the original design.
I am planning to leave the antenna relay circuitry in place until I know whether the unit works satisfactorily with this modification left in place.

Fig 7. Shows the bifilar choke moved back to its original position. I've been given advice to eliminate the RF chokes connecting the grids of the tubes to ground. I am waiting to get capacitors to replace those removed from the modification.

Fig 8. Shows the top of the modified amplifier after the finals have been removed. There is an extra, perforated, baffle between the fan and the finals. This will be removed, and the fan will be replaced by one similar to the original.

Page 3--posted 15 Sept 05

After getting the replacement capacitors, I decided to wire the tube bases as in the original Heathkit design, since my other SB-220 has worked well with that design.

Fig 9. Shows replacement of fan with one similar to the original. Purchased from Harbach Elect. Notice that the baffle evident in Fig 8 (Page 3) has been removed. The parasitic chokes are obvious.

Fig 10. Shows replacement Harbach PC board. Compare with Fig 3. (Page 1)

Fig 11. Wiring of the underside of the chassis is now hopefully complete. The RF output cable has been rerouted to avoid interfering with the fan. The tube sockets have been rewired as in the original design--including the 1mH chokes. Just above the fan cutout, you can see the small Harbach Soft Key PC board which was added to this unit.

of minutes after switching to the antenna (a Gap 5 band vertical with 1.3:1 swr), however, the plate current became very anemic, down to only about 75 ma. Grid current was also very low. After switching back to the dummy load, no improvement was noted. At no time did any smoke or fire escape from the enclosure. (Joy!) I figure that the culprit is the antenna relay, so I will be replacing it with one similar to the original. I am also going to put parasitic suppressors on both the amplifiers. The next decision will be whether to move or disconnect the ALC circuitry.

After a careful check of the wiring, I fired up the amp without the tubes, and everything looked good. Relays worked, and HV was good. I then replaced the finals, and put it back into the enclosure. Tuning it into a dummy load, it seemed to be working well, showing appropriate plate and grid current response to increase in excitor drive. (An interesting observation was that the exciter SWR into the amplifier was very high when the amp was turned off, but low when it was switched on.) Good power output was noted into the dummy load. A couple

Page 4--posted 7 Oct 05

Please email me with comments or suggestions!

Thanks for looking!!
73--
John
w4kv

I got the fan, PC replacements, soft key from Harbach Electronics. Jeff at Harbach sometimes is somewhat back ordered, but it's worth the wait. I obtained the small components from Mouser Electronics. Mouser is very prompt and reasonable. The parasitic suppressors were ordered from Richard Measures. Rich gave much helpful advice.

Page 5--posted 10 Mar 06

Other projects and waiting for parts have delayed completion of this project. Over a few weeks, I have replaced the antenna relay modification with original relay circuitry. The components on the tune capacitor were moved back to their original positions under the chassis. It seemed better to restore the output coax to its original location, so that was done. I installed the Harbach soft-key kit, since the amplifier will mostly be used with solid state exciters, and they don't like 110 volts on the relay switch. I installed the parasitic suppressors to the plate circuits. The pictures below show views of the completed amplifier. The work would have been much quicker and simpler if I had originally followed the advice suggesting simply returning it to original Heathkit specs.

Fig 12. Note removal of the components from the top of the tuning capacitor on the left. The photo is not ideal, but the parasitic suppressors can be identified attached to the plate caps. They have replaced the parasitic chokes. The new fan is also obvious.

Fig 13. This view of the underside of the chassis of the completed restoration shows the new antenna relay which occupies its original position. The output coax was replaced and now follows the original route. The tube bases are wired as in the original schematic.

A few final notes:
1. The new fan is much more powerful than the original. During my work on the unmodified SB-220, I broke a fan blade, so I replaced the fan in it, too. The finals should like it.
2. The new antenna relay is much quieter than the original relay. I do not plan to replace the loud old one in the unmodified amp since I may do a QSK modification in it sometime.
3. Making the parasitic suppressors from the kits supplied by Rich Measures was a piece of cake. I have a second set for the other amplifier, but I'm tired of running barefoot, and I will save that for later.
4. This project has been great fun, and a learning experience. I got a lot of help from many people, and I thank everyone for their input.

The suggestions given ranged from "take everything down, and return the amplifier to the original Heathkit design" to "the workmanship looks pretty good, so look for something simple, like a blown diode." There was near unanimity that the shielding around the base of the finals would cause overheating of the tube electrodes and sockets.