SB-200 & 572-B Problems - Need Help

[KI4AX]

I recently aquired an old Heathkit SB-200 amplifier which I do not know much history on. When I got the amp and pulled the covers off it was pretty caked up with cigarette smoke. I cleaned it up pretty good and performed the Harbach's Power Supply, Soft Start, and Soft Key mods. And in addition to the Harbach mods I also installed a Glitch Resistor and a Fuse in the HV B+ line. The Glitch Resistor is a 10 ohm 10 watt glass coated (I can supply specs if you want it) and the fuse was a 250v 1amp Buss type fuse. Most of this I did following information that I found on the internet.


After only tuning up a couple of times and having a short 40 meter QSO the fuse blew while I was attempting to tune up on 15 meters. I am using the tune up proceedure in the Heathkit Manual and I am driving it with a Kenwood TS-820. At the time I was trying to tune up I was at a pretty low driving power level (20 to 30 watts). When the fuse went I saw what appeared to be a blue camera flash on the wall near the amp. After that the Grid current was full scale on the meter. In other words the meter was slamming against the stop at the right when I keyed the amp with the meter set to Grid Current. When I pulled the fuse it was black inside of the glass. I did not hear any kind of pop or frying sound when the fuse went.... just the flash.


I can not tell you much about the 572-B tubes that are in it other than the guy I got the amp from claimed that the tubes were new and that the boxes that they came in are stamped "made in china." That is the only identifying marks on those tubes. He also gave me the old set that had been pulled out for what ever they are worth. The old tubes were Cetrons.

I noticed when I was trying to use the amp that one or both of the Chinese tubes would rattle. At first I thought the fan was hitting the tube but I checked to make sure and it was not. After the fuse blew I pulled the tubes out and notice that one of them rattles when I shake it. The other one does not rattle no matter how hard I shake it. There are no loose particles inside of the envelope of the one that rattles as far as I can see.

Is it normal for a 572-B to rattle both during use (powered up) and if I shake it (like a can of spray paint)? Do you think this tube is the source of the HV glitch? I have checked the old tubes and one of them rattles a little bit also. So I have one old and one new tube that do not rattle.... would you try the two that don't rattle? Is it ok to run a weak tube with a stronger newer one? I do not have a tube checker that will check this tube... is there any check that could be done with a DMM? Like checking for shorts etc?

I would appreciate any comments good or bad. I know what some people say about Heathkit and then others swear by them.

Thanks

Dan KI4AX

[WB2WIK]

You have a bad tube, or maybe both tubes.

Hopefully that didn't take out much other circuitry. If you had a cathode fuse and a glitch resistor, probably not.

Try unplugging both tubes, keep the plate cap connections away from everything (maybe tape them up and make sure they're suspended in mid-air not near any metal) and power up the amp again to see if the B+ is still okay.

[AG6K]

I recently aquired an old Heathkit SB-200 amplifier which I do not know much history on. When I got the amp and pulled the covers off it was pretty caked up with cigarette smoke. I cleaned it up pretty good and performed the Harbach's Power Supply, Soft Start, and Soft Key mods. And in addition to the Harbach mods I also installed a Glitch Resistor and a Fuse in the HV B+ line. The Glitch Resistor is a 10 ohm 10 watt glass coated (I can supply specs if you want it) and the fuse was a 250v 1amp Buss type fuse. ... ... ...
Dan KI4AX

 Using a 250V-rated fuse in a 2500v circuit is bananas because an exploding fuse does not limit peak-I.

[KE6YX]

I did the same glitch resistor mod on my FL2100B, and awhile ago when I was tired, I did a really dumb tuning move. I saw the same bright blue flash and was sure my amp was toast, but all that happend was the 250V 630ma fuse blew. I replaced the fuse, and all is working well.

I have to question how much of a voltage drop there really is across the fuse.

Anyway my amp is still doing fine, putting out full rated power using the orginal Cetron tubes, and I have made many more contacts with it and the 250V fuse.

David

[AF6LJ]

It is an Extremely Bad idea to use any fuse in the high voltage side of the power supply.
A properly selected glitch resistor will do the job.
The place to have the fuse is in the cathode return line of the tubes...
In this case between the center tap of the filament transformer and ground.

[WA6MHZ]

I can't find any fuses rated at 2500V, except those made for power Pole pigs.

[WA7PRC]

I can't find any fuses rated at 2500V, except those made for power Pole pigs.

They're available. They're neither small nor inexpensive.