SB-221 (SB220) amplifier rebuild
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- Опубликовано: 17 сен 2024
- A quick overview of the 1978-era SB221 amplifier brought back to life in May 2016 after being on a shelf for more than a decade.
Details about the project are at:
va7st.ca/2016/0...
Work done to this amplifier included:
- replaced filter caps (with 330uf 470V)
- new 2016-design Harbach RM-221 meter board
- new soft-start inrush current board Harbach SS-220
- new high-flow fan blade
- new AG6K parasitic suppression kit (incl. 10-ohm "glitch" resistor in place of RFC-2 on B+ line)
- lots of rewiring to fix many issues found inside
- directly grounded grids
On the front panel, you'll see a small toggle switch. That's a standby switch with a red LED beside it (lights up when keying line is on), and an ALC adjustment knob.
Previous owner changed the old 110V T/R and bias relay to a 12VDC relay, so no soft-key mod needed for modern transceivers -- my FT-2000 keys the 14.5VDC line very nicely.
The amplifier will be used in CW and RTTY contesting (some SSB work, too), and DX chasing at VA7ST.
If you like radios and ham radio contesting, check out the Zone Zero podcast at zone.va7st.ca.
Years ago I built a Heathkit SB220 amplifier. Things went well during the kit building.
When I got finished with it, it was time to fire it up and test it out into a Heathkit Cantenna dummy load.
I got plenty of RF through bands 80-15. On 10 meters I got only the power the Kenwood 820 put out and nothing more.
I checked and re-checked all the wiring but gave up and took it to the local Heathkit store.
The store manager told me to bring it back to the test bench for testing. The technician set it to 10 meters with a wattmeter to a dummy load. Instead of tuning up the radio in steps as you did in those days he cranked the radio up to 100 watts and started twisting the tune and load controls on the ampflier.
The two 3-500Z tubes started to get red hot and then white hot, then the parasitic chokes on top started to smoke!
I told the manager to have him stop as he was burning up the ampflier tubes.
At that time they were $ 180 each!
He told me the repairman knows what they are doing and we sell the kits in front and that is all that counts.
After several letters and phone calls I got two new tubes and somehow they found what was wrong with the amplifier. When asked they did not know.
I had build a number of Heath Kits and about half of them had mis-labeled capacitors or errors. It’s no wonder some people call them Greif Kits.
I am still running the amplifier to this day and that was 37 years ago.
Very nice job...
The poster who said to run the 220 with the" filaments only" for a while to getter the tubes is mistaken. The anode is the only element in the tube that gets hot enough to activate the gettering agent (Zirconium oxide), & that is the silver colored stuff you see on the anode. The tube has to draw enough plate current to cause the anode to run red to orange in color, Several internet sources describe a set up for doing this with the tubes out of the chassis. Metal ceramic tubes will getter with filaments only because because the filaments heat the cathode & the cathode is the hottest element in metal ceramic tubes.
I'm a new ham and found a sb221 on ebay that was working. I installed the Harbach soft start module, rectifier/metering board, and capacitor block w/board. The amp short circuited to ground when I turned it on to test it. The short circuit was caused by one of the metal coils on one of the parasitic choke (100k resistors/metal coils strings) feeding the 3-500z tubes. The metal coil was touching the top chassis plate. When I turned the amp on the high voltage meter needle pegged out, immediately dropped to zero and the amp was dead. After a thorough visual inspection of all circuitry and testing the newly installed upgrade boards and capacitor block with a multi-meter (All the diodes, resistors and the eight uf 200 capacitors checked out Ok.) I discovered the two 10amp/10watt ceramic resistors on the soft start module had failed. I don't have a 48vdc power supply to test the two SPDT relays on the soft start module so I'm unable to test them. I'm waiting for another new soft start module to come in the mail. I'm hoping the failed resistors are the only damaged components. Can you tell me if there is anything else I should check for damage? I'm hoping the two failed resistors saved the day. Thanks much for the video.
tubes are going to do fine on the shelf..it is how they are handled and used, that kills them. Often prudent to run them fils on only for a while to let the getter do it's thing. Heathkit is excellent design wise in running the fils at the correct voltage.. others kill them with too high fil voltage. Good old Heathkit !
Capt. Larry : All glass envelope power grid tubes suffer air ingression through the Kovar seals around the pins. Running the 3-500 on filaments will not "getter" the tubes because the gettering agent is Zirconium oxide & it is applied to the plate of the tube, because the plate is the only tube element that gets hot enough to activate the gettering agent. The "silver" appearing color of the anode is the Zirconium oxide. Your comments regarding Heath filament voltage in the SB-220 is correct.
Eimac stopped making 3-500z tubes in about 1996 so your 1995 tubes were just about the last ones made. The last few years were the worst ever made so I am not surprised one bit that they went bad on you. The older ones are much better and many are still running strong 50 years later. I suggest you directly ground the grids and build new parasitic suppressors just like the old ones using similar resistors. I DO NOT suggest ANY mods by AG6K, especially the suppressors and do not install all that garbage on the grids. Best to directly ground them with short copper braid or flashing close to ground. The .82 ohm resistor is actually the grid shunt resistor which calibrates the meter to read grid current correctly. The original resistor is a special one. It is NOT a plain old resistor. It is a "fuse resistor" and is made to blow like a fuse if the grid current goes too high in the case of a tube arc or short. They are sacrificial and should be replaced with a similar one if they blow.
On your contesting amp, where did you get the square replacement meters. I can't find a place to get replacement meters anywhere. Thanks, Buddy, K4VYX
The square replacement meters were installed by a previous owner. I believe they were once available from Harbach Electronics, but are no longer available. Meters for these amplifiers are rare today, and it appears they mostly come from retired amps that are being parted out.
I forgot to mention that the previous owner had done the K1 relay modification to 12 vdc.
Any idea where I can find a replacement Plate Voltage meter (the one on the right)? I'm helping a friend to repair his SB-220. R3 was blown and took the meter with it (switched to Grid metering when it blew).
Don't know why it blew yet. One of the tubes may be shorting or it had a bad parasitic oscillation. Wish I could test the tubes somehow.
Good luck finding a meter for these Heathkit amps -- they're considered "unobtanium" -- The only means of getting one that I've heard about in years is to scavenge from another SB amp. Doesn't look like any aftermarket manufacturers make them any more.
What would cause the tune cap to arc.?
You show but don't mention the change in the parasitic supressors, which is extremely important.. and the cause of the arc and associated meter problems.
does this amp work on 10 and 11 meter band? or can it be modified? I seen one for sale the other day.
The SB-220 had 10M as stock, and the SB-221 could be modified with Heathkit parts to operate on 10M. My SB-221 amps have the 10M addition.
Those old elco's are much better qualoty then new one. If they mesure good-dont replace them.. Its simple..
I need knobs so bad