LoneStar Special Help!

[art burke]

Hi to all:

I've had my Lonestar Special for a little less than six months, and took it in the shop several days ago.
After powering it on, I plugged a guitar into it at low volume, and after giving me about 3 or 4 seconds of clean sound I heard a "pop", then saw some smoke come out the back, and the amp went silent but did not blow the fuse, and the power light was still on.
I shut the power off, looked at the tubes and couldn't see a thing, but smelled
like a burnt component.
After taking the power tubes out, I plugged in the one for the 5 watt setting, turned the 5 watt setting on and the amp sounded great. Then I plugged in the tube at the opposite end and tried the 15 watt setting, it sounded great as well. After plugging in the other two power tubes, the 5y3 rectifier tube turned blue and the fuse blew.

I then took it to the repair station. The tech checked out the amp and said that it was a bad power tube and of course a bad 5y3 rectifier. A Mesa tech (after I called them and explained the symptoms) said that a power tube had shorted and took out a screen grid resistor.

The techs at the repair shop that do warranty work for Mesa said that the screen grid resistors check out fine, did not exhibit any symptoms of being burnt, so insisted that the tubes were all that were wrong with the amp.
I'm picking it up tomorrow, and would like opinions on this.

I am an electronic tech as well, and would love to get my hands on a schematic to check these screen grid resistors myself.

I'm looking for opinions on that amp repair and any ideas as to where the smoke could have possibly come from. Could a shorted tube "smoke" after shorting? Could it be that the vacuum was destroyed and smoke came from inside the tube?


Thanks,

Art

 

[Brewski]

Hi Art,

I found an amp debugging page online: http://www.geofex.com/ampdbug/ampdebug.htm

Here's one excerpt but this doesn't sound right as you have a newer amp: Intermittent breakdown of output transformer or choke
The filter choke, if your amp has one, and output transformer are connected to the highest voltages in the amp. If they are old (can you say "vintage"? I thought you could) and if they get hot, the insulation on the wires inside can start being intermittent. Internal shorts that clear will cause popping. Shorts that don't clear will pop a fuse, usually. Sometimes it'll just cause smoke.

Here's what they stated under "smoke or burning smell:
There's good news and bad news. The good news is that it's easy to find the problem, at least the thing that is burning. Just unplug the amp, open up the chassis and look for what's burned, charred, or overheated looking. That's what's causing the smoke/smell.

The bad news is that in almost all cases, the part that is burning is a power handling component. These are ALL expensive. Worse, in some cases, the part that is burning is not what is causing the problem, and you still have to find what else is faulty.

In some ways, having smoke coming out of the amplifier is kind of a deviant, hard-headed version of having a fuse blow - something is eating too much power, it's just that the fuse for some reason is not blowing. This is especially suspect if the fuse blew, and you didn't have another of the right rating, so you stuck in a higher current rated fuse.

Possible causes are:

Failing/shorted output tube - this can overheat the output transformer and/or power tranformer. More rarely, it can also overheat the choke, but usually the transformers go first.
Improperly biased output tube.
Failing bias supply on fixed bias amplifiers
Failing cathode resistor bypass capacitor on cathode biased amps.
Failing/shorted rectifier tube (or solid state diodes - they do fail, if rarely) can overload the power transformer as well as killing the power filter capacitors by letting AC through. A failing filter cap or shorted output tube can pull so much current that it overloads the rectifier tube, too.
Failing power filter capacitor. These can sometimes get hot enough to literally explode or burn, as well as just quietly overloading the power supply and popping rectifiers and power transformers.
Failing power transformer.
Failing power filter capacitor
Choke with a "soft" short between winding and core
Failing output transformer

I hope that this is somewhat helpful, although it is difficult to armchair diagnose a specific problem without seeing what's going on with it. I hope that it was just the faulty power tube and rectifier. It'd be interesting to know what your tech says.

 

[art burke]

I got my amp back and talked extensively to the tech.
He replaced an EL84 and the 5Y3 rectifier tube.
He checked for anything burnt and couldn't find anything. He checked the
screen grid resistors and they were all good.

He had it running with music playing through it when I got there.
I've been burning it in for about 2 hours now and it sounds great.

I bought some 2amp slo blow fuses (what it's rated for) and 2 new
EL84 power tubes. I need to order a 5Y3 to have on hand as well.
I'm just wondering where the smoke came from. The tech gave me the bad EL84 (shorted screen grid to control grid). The 5Y3 had an open filament.
If you can, please email me at [email protected]

Thanks,

Art

 

[art burke]

Now I'm grasping for straws.

Is the dielectric constant of the tube socket for the power tubes higher than that of air?

I'm wondering if the tube was "cocked" out of the socket just a little, that one of the pins could arc to the chassis and short out the tube?

I've looked and looked at that board and even took close up pictures to re-examine until I'm blue in the face, looked at the screen grid resistors (at least where I think they are), and can't find a thing that is smoked.
The amp is working great, and has been "burned in" for probably 50 to 60 hours after the repair.
Are any of the resistors in inconspicuous places? (like under a board, etc...)

Thanks for the help in understanding the amp.

Art

 

[Brewski]

The LSS takes a 2 amp fuse. As far as the tube arcing, I haven't heard of that, but then again I'm not an engineer and have to defer the arcing question to someone else out there who has a more vast knowledge of the technical stuff. Maybe the tube itself had a flaw in it that wasn't immediately apparent or some foreign material was on the tube that made it smoke. Glad to hear that your up and running again.