blew my amp up

[kb3ouk]

Tried tuning up on 10 meters and as I was tuning the plate cap I noticed the needle on the meter was really flopping around. Must've hit the sweet spot on the tuning-BANG, lights go out. Tripped the breaker. Went down in the basement and flipped it back on, the amp came back on, but it wouldn't put out any power. Fuse isn't blown, but I noticed a nice burnt smell coming from the Ep/Ip meter. I think I might've burnt it out, when it came back on, the needle was pinned clear to the right, I tapped it and it just flopped back to zero, even though the power was on. I don't think I blew the rectifiers, if I would've it shouldn't came back on without tripping the breaker or blowing a fuse.

[kb3ouk]

Looking at the schematic, there's a 5 ohm 10 watt wirewound resistor in the power supply circuit that serves as overload protection. If I were to take a wild guess, I'd say when I tear it apart, I'll find out that resistor opened up. Wonder if that would have any connection to why the plate voltage was reading low too? It was reading 1200 volts on the meter, but it should be 2000 volt.

[Pete, WA2CWA]

It might be helpful to know the model number of the amp, or if it is homebrew, etc.

[kb3ouk]

oh yea, forgot that. It's a hunter 2000b. http://bama.edebris.com/manuals/hunter/2000b

[KM1H]

Are those Chinese 572B's and did you replace the parasitic suppressor resistors? Also see if the plate choke has fried.

Carl

[kb3ouk]

Wasn't hard to tell what was worng after I took the cover off. Looked down around the area of the power supply and noticed a rectangular shaped ceramic rod that was cracked. Got that flashlight out for a better look. Remember that 10 watt wirewound resistor I mentioned in the second post? Well, it did its job. Stuck the ohmmeter across it (after shorting both sides to ground) and sure enough, it was open. Tapped the side of it with my screwdriver and it disintegrated. The outer ceramic busted loose, the inner part was blackened. Now, the manual said it wasn't supposed to blow unless it had 1 amp through it for at least 30 seconds. I'm looking for either another one to replace it (it's a Sprague Koolohm, 10 watts 5 ohms) or wonder if I could find a small overload relay to put in there. One that will handle about 2500 volts and will trip when it goes over 1 amp. Suprisingly, nothing else in the amp is shot, except for that resistor.

[WBear2GCR]

I'd keep the resistor.
It worked, right? 

Maybe make it plug in...

                     _-_-bear

[kb3ouk]

It worked, the only problem is when they wired it, I don't think they realized that it would get that hot, so they ran some of the power supply wiring right past it, and when I removed the remains of it, the wires that were behind it had scorch marks on them and a few little melted spots. I might try the plug-in idea, where it goes, there are 2 insulators that the leads of the resistor wrap around, then the wires are soldered on. Maybe on those wires I can put some kind of terimals that will slip over the resistor leads.

[WD5JKO]

Looking at that circuit, I'd say the resistor popping is the symptom, and not what originally went wrong

Now, the manual said it wasn't supposed to blow unless it had 1 amp through it for at least 30 seconds.

Ohms law for power says I^2 * R = W, so one amp through 5 ohms dumps 5 watts heat, and a 10W rated resistor will do that forever. Since this is feeding a voltage doubler, the resistor will see much higher peaks than 1 amp when the plate current meter says 1 amp. Still to fry that part like it was must have taken a prolonged overload. Maybe that resistor had taken hits many times before, and this last one did it?

So when tuning that amp on 10m looking for the dip, did you have the drive reduced, or the power switch on low (there is a low-med-hi tap to the voltage)? Or was it on Hi, and full drive? Was the load a resistive 
50 ohms, or possibly a high SWR load?

Something went amiss, like the finals went off into a parasitic, or the power supply crapped out (shorted diode, or cap). The plate coupling cap, C5 could also have shorted out.

There is likely more wrong then that fried resistor.

my 2 cents

Jim
WD5JKO

[N8ETQ]

  I agree Jim, Horizontal 572's...  I found this in a SB200 a
year ago or so...

/Dan

Looking at that circuit, I'd say the resistor popping is the symptom, and not what originally went wrong

[kb3ouk]

Yea going by how much those wires were melted around it and the heat marks everywhere, I'd say that thing got pretty toasty a few times before I got it. That might explain why it came with no tubes in it, too. But as far as the tuning on 10, I was driving it with about half the amount of power I use on 75, which is 25 watts, so I was putting about 12 watts in. I always run it with the power switch on low, I've never even had it on the medium or high tap yet. I had it running into a dummy load. And after I blew it and had no voltage on the tubes, I keyed it and could tell that there was power still passing through the cathode to the plate and out through the tuning network. But the tuning was very flaky, bump the knob and the power that it did make went away completely. On 75, the tuning is very broad, but not up on 10.

[Opcom]

Hopefully R12 the current shunt is OK. and the meter.

Why not also add a high voltage fuse made of 3" or so of #30 wire between the plate supply caps and the RF section? There ought to be some space for a couple of standoffs, etc.

I took this advice some time ago and I am sure it has saved other costly parts. When the tube arced, there was no other damage.

[Bill, KD0HG]

Some of the Chinese 572s and 811s will short their grids out when used horizontally and blow stuff up. Bad design, flimsy grids. Do a Google search on them.

Better check out your tubes manufacturer.

I have a quad set of Svetlana 572Bs in my 30L-1 which have played fine for 10 yrs in a horizontal position. Stated by manufacturer and RF Parts good when used horizontally.

Bill

[kb3ouk]

They are Shuguang tubes. I'll check, but I don't think they are shorted grid to plate. The only damage I can find is that resistor, which I still think had been in failure mode for while before it finally crapped out. There's no way a momentary overload caused scorch marks on three different wires, melted a hole in one, and made scorch marks on a circuit board half an inch away. Plus I've never heard of a ceramic coated wirewound cracking before like that one did, when I found it, the outer layer of ceramic was cracked the whole way across the width of the resistor.

[N8ETQ]

  Just a suggestion, If it hasn't been done already
now may be the time to lash a couple back to back
diodes across the Ip meter.

/Dan

[kb3ouk]

Well, I'm half-tempted to replace it altogether, since that meter only works half the time anyway, I have a few meters to replace it with, the one in there now only reads after you knock it a few times, then it might work a few days till you have to knock on it again.

[W3GMS]

Well, I'm half-tempted to replace it altogether, since that meter only works half the time anyway, I have a few meters to replace it with, the one in there now only reads after you knock it a few times, then it might work a few days till you have to knock on it again.

Sounds like the tachometer in our 1973 Mach 1

You have received a lot of advice, but I would be leery of those imported tubes your using. In directly heated tubes, grid to filament shorts can cause big problems. Eimac had that problem when they first introduced the 3-500Z's with grid to filament shorts but was for the most part corrected later.  Since the tubes are horizontal in your application and considering the sag factor of the grid, especially if the element is not supported properly, this could result in the grid hitting the filament.  Then when the tube cools of, it may contract back and not cause a hard short.  I also know nothing about the Hunter amplifier but some of the review I have read are pretty good.  So it appears that parasitic's are not common in that amplifier.

I agree with others in that the resistor is not the root cause of the problem but rather a result based on something else going wrong. 
 
73,
Joe, W3GMS

[K5UJ]

if the resistor is in series with the h.v. between the power supply and the plate choke then it is a glitch resistor.  That's a common item in amps and is a sacrificial component, intended to blow if there is a major B+ fault like a short to the chassis.  I'd also suspect the tubes since they were horizontal.  I'd never run big RF power tubes that way even if they are supposedly designed for it.   

this might be a case where the amp suffered from AM operation, if it was made for SSB and CW.

Rob

[kb3ouk]

I think that's part of the problem is it was made to run SSB and CW. That resistor is only there for overload protection, but when it's being stressed with continuous carrier, that's probably helping to make it easier to blow it. Does the tube's orientation when it is horizontal make a difference as to whether or not it will sort from grid to plate? The tubes in this amp are aligned so that the grids of every tube are vertical, which I've always read as being the way they are supposed to be arranged when being used horizontally, so that the grid supports can still hold the grid up, if the grid was horizontal then it could sag towards the plate.

[W3GMS]

Looking at that circuit, I'd say the resistor popping is the symptom, and not what originally went wrong

Now, the manual said it wasn't supposed to blow unless it had 1 amp through it for at least 30 seconds.

Ohms law for power says I^2 * R = W, so one amp through 5 ohms dumps 5 watts heat, and a 10W rated resistor will do that forever. Since this is feeding a voltage doubler, the resistor will see much higher peaks than 1 amp when the plate current meter says 1 amp. Still to fry that part like it was must have taken a prolonged overload. Maybe that resistor had taken hits many times before, and this last one did it?

So when tuning that amp on 10m looking for the dip, did you have the drive reduced, or the power switch on low (there is a low-med-hi tap to the voltage)? Or was it on Hi, and full drive? Was the load a resistive 
50 ohms, or possibly a high SWR load?

Something went amiss, like the finals went off into a parasitic, or the power supply crapped out (shorted diode, or cap). The plate coupling cap, C5 could also have shorted out.

There is likely more wrong then that fried resistor.

my 2 cents

Jim
WD5JKO

Great analysis Jim.  That resistor should have never blown on any mode based on its rating.  Something else went wrong. 

Joe, W3GMS

[KM1H]

As I said at the beginning, check the parasitic suppressors before trying to tune up again. If the resistors are well out of spec it is easy to get the amp to take off on 10M and under driving it that much makes it even less stable. Ohmite OY series resistors at Mouser, etc make good substitutes for the carbon composition originals.

For R8 to basically explode something serious had to short. It could be diodes, filter caps or anything else in the HV line from the rectifier/filter section to the tube and even out thru C-5. It should be visible

Also check the grid bias resistors, R1 to R4. Those resistors dont provide a decent RF ground, especially for squirrely Chinese tubes. If they are still good bypass each tube with a .01uF  1000V disc. If one is bad replace all with one Ohmite OY 33 Ohm plus those caps.

R8 does nothing to protect from a tube arc. A 15 Ohm 20W real enamel WW should be installed as close to the bottom of the plate choke as possible with an additional bypass cap so there is one at each end. A 1000pf 6KV disc is a good choice for both and the RF doorknob, C-6, can be used for something else where it is better suited.

The Bandit 2000 is a nice amp, for an early 60's design. Ive operated them in a contest station when living in IL in the early 70's (where they did get touchy on 10M) and have worked on several over the decades, mosty for stability related failures.

Carl

[kb3ouk]

I'll check everything, but i'm not too concerned with 10 meters if I can't get it to work there. It only really needs to function on 75 and sometimes 40 for me, I don't have any antennas for anything higher.