I have an LA-1000NT that has some sort of dead short. Plug it in and it blows the fuse. It worked fine then one day just had a dead short.

It looks like this.

Looks like a sweep tube amp. First suspect would be the filter capacitor(s), next would be one or more of the tubes.
Manufacturers were notorious for pushing those tubes to the limit and beyond. After all, they were relatively inexpensive.

I've fixed quite a few but I have 2 more waiting in line to be repaired. I can only do a little bit at a time. These repair jobs take a long time. I only do this for friends, club members and only charge for parts.
There are several guys who do this for a living ( I don't). Look up "Mr. BBI amplifier repair". He does a lot of CB amps but works on ham, tube amps too.

Based on my experience, shorts in the HV circuits are often caused by failed tubes, failed DC blocking caps or failed HV caps in the power supply.
Most commercial amps have safety switches that short HV (high voltage) to ground when top cover is open. It must be temporarily deactivated when testing the amp with the cover open. Don't open the cover or attempt to repair the amps if you have no experience with tube amps. HV caps may have lethal voltages for some time after the power is disconnected.

Tubes may be hard to check properly but you can try checking for shorts with a multimeter. Check resistance values of the heaters. Also SLIGHTLY shake and rotate the tube while holding it close to your ear. If you hear something loose in the tube, it may be an indicator of a failure. Of course it's not a 100% reliable test. Tube types and designs vary greatly.

I recently lost an 811 in my Ameritron AL 811H.  The tube shorted and caused the fuses in the HV circuit to blow. A multimeter showed the bad tube.  $26.00 later and the amp is repaired.  If your amp uses 807 or 1625 type tubes, look there first.  Also, visually look for burnt components in the HV circuit. Be very careful when working on a tube type amp. Lethal voltages can be present for a while after the power is switched off.

If it worked fine and then all of a sudden it didn't and there was no smoke coming out, then it is likely just a bad tube with a grid short.

Of course it could be other things, a cap or a short in the transformer, but those things usually cause smoke. and or a burning smell.

It worked fine and was put on a shelf. I then put it in the back of my jeep and drove it about 50 miles to a friend's house where we were going to set it up on a table and do some field ops. We plugged it in and heard a hum and it blew the fuse. Replace fuse, same thing. Maybe the ride in the vehicle damaged one of the tubes.

We were outside but it was calm out so I would have thought I would have smelled smoke or seen it but I didn't notice any.

Bouncing tubes around can destroy them, most are fairly delicate

Makes sense. I should have put it in the front seat. I wish the corner drug store was still there with the tube tester I used to be fascinated with as a kid.

I had to use the one near my house when the family TV went on the fritz (pretty sure that's the official term! ) and dad sent me to check the tubes.

My one-eyed high school electronics teacher, Mr. Zmolek, had a portable tube tester built into a suitcase that he foolishly entrusted to a couple of students, and then turned his back.  He wasn't real happy when he looked up and saw a tube shining like a 75-watt lightbulb.  

We had a basic electronics class in high school. Most everything was dontated by the local Bendix King factory with top notch stuff. At least once a day the regular turds would fill the classroom power strips with capacitors and whatever else then turn them on for a fireworks show. I spent a good portion of the time playing music into the modulation input of a sig generator hidden under my desk so I could jam the overhead stereo and play my own since I hated country music.

you can do some basic testing of tubes with a multimeter.

You can check that the filiment isn't open and you can check no shorts to the grid.

I was able to do that with a 3-500Z tube, it had a short to the grid, I can't remember whether it was cathode or anode to grid, but it was one of them.

Yep. This is why glass tubes are always removed and transported separately in a padded box. Also, the problem can be in the HV DC circuit. Maybe something got loose during transport and is touching the ground. Most likely it's one of the tubes.

Thanks guys. This gives me some place to start. Solid state, no worries, I would have had it fixed years ago. I know nothing about tube stuff.

If you can diagnose and fix solid state, you can do all that on tube gear. You just have to read a little bit on how tubes work. The only things different that tube gear has that solid state gear does not is tubes instead of transistors and high voltage power supplies. Everything else is the same except big and most of the gear does not have circuit boards, it is all just connected with wires going everywhere.

After all, the solid state stuff was made to work very similar to the tubes they replaced. In many respects, tubes are easier to understand, they are simpler.

Unplug the amp, remove the cover, make sure the high voltage is shorted to ground, pull all the tubes, replace the cover THEN plug the amp in and turn it on.

If it still blows the fuse, then there is likely a short in the high voltage power supply.

If it DOESN'T blow the fuse, you have a plate to grid short in one or more of the tubes.

Get a pinout diagram of the specific tubes it uses and take a multimeter and check grid to plate for a short. If shorted, discard it....then replace the tube.

I'll give it a try. Thanks.

Embarrased I didn't think of doing that.

Attached File

The schematic symbols look all weired with lots of dashed lines and what not but I suppose in the end it's the same concept, sort of like a vacuum flourescent display as opposed to an LED display with grids and plates and all.