Bad things to my Lonestar....HELP!

[njm137]

Guys,
I blew a few fuses a couple months ago in my Lonestar classic. I retubed everything (with JJs) and it seemed to work fine for another two months. Early this week, I turned the amp on and it just died. The fuse was blown with excessive 'splatter'. (Earlier when the fuse blew, it was just a few of the windings that had burnt). I see no visible or smellable damage to the amp.

Today, I pulled ALL of the tubes, put the amp into standby mode, and put it in Diode rectifier mode. Turned it on...heard a quick hum...and fuse immediately blew.

Please confirm my suspicion that one of the transformers is bad, the rectifier circuit is bad, or something is being shorted. The amp is far out of warrantee, and I'd like to get this fixed ASAP...any suggestions??

Thank you

 

[njm137]

OK...latest news, the 470 ohm screen resistor on the outer left tube is clearly cracked in half. I ohmed it out, and sure enough, 0 ohms. It seems like an easy resistor to replace, but I want to make sure there is not another component that is damaged. Why would this resistor crack?? Bad tube???

 

[plan-x]

Maybe old, defective?

 

[djw]

I've never heard of pulling all the tubes out of an amp as a diagnostic technique. Really? You can do that? I'm not a tech, so I don't know.

I do know I've had fuses blow because of a cracked tube; I assume the fuses blew due to current getting "backed up" (?) at the point where it would normally run through the tube in question. I also know now that if I hear a loud buzz/hum coming from the amp when it's on Standby, to power the amp OFF immediately or I'll blow a fuse.

 

[Charles Reeder]

about that resistor:

I was playing in a place that was notorious for inadequate voltage to the stage. I was using the 50-watt setting. I started blowing fuses. Replaced them twice and suddenly got a 'big-stink'...everyone smelled it and we all thought I had blown my Lonestar.
I took the amp home and dissasembled it. Just as in your case one of the 470-ohm screen resistors was literally blown in half and partially burned.
I ordered about a half-dozen. When they arrived I replaced the visibly destroyed one. But since the tubes and thus the resistors work in pairs...I went ahead and replaced the opposite resistor for the other 6L6 used on the 50-watt setting. When I removed it...it became apparent that it too was damaged. Although no damage was noticeable before I removed it...it literally fell apart in my hands once removed.

So, plan on replacing the burnt one AND its' opposite mate. If by any chance you were using the 100-watt setting...I would advise replacing all 4 of them.

Hopefully; you will discover as I did...that absolutely no other damage has been done to the amp.

Regards: Charles

 

[njm137]

Charles,
Thanks so much for the reassurance!! I will be sure to replace all 4.
However, since the amp is blowing the fuse even without the tubes in it, does that sound like a rectifier or transformer problem to you?

 

[Charles Reeder]

Well...
I don't know enough to say for sure...but if you have blown screen resistors...the amp will probably keep blowing fuses until they are replaced...whether or not there are tubes in the amp or not.

As an addition to my previous post about replacing the screen resistors:
The value variance in resistors can be between 5% and 10% (plus or minus)...in other words a resistor which is rated 470-ohms can actually be as low as 423-ohms and as high as 517-ohms. It is unlikely to vary THAT much...but it is possible.

So, buy more resistors than you will actually need and 'match' them in pairs using a digital-multimeter. If for example you used a 457-ohm resistor and a 480-ohm resistor your tubes would be out of balance and not matched. It's OK to use slightly higher or lower value resistors...but you definitely want them to 'match-each-other' in pairs. Both of the inner pair of resistors on my Lonestar (which didn't blow) measured 453-ohms (if I remember correctly). I replaced the outer pair with a matched pair measuring 462-ohms each. Amp sounds fine now.

Charles

 

[njm137]

I took the Lonestar to my amp guy, and it turned out that the rectifier diodes were indeed the culprit. However, he checked my OLD tubes, and one of them was shorted, which probably caused the first fuse to blow, and which eventually caused the screen resistor to crack, which in turn caused my rectifier circuit to fail.

So, learn from my mistakes:
1) If a fuse blows, do not ignore it!! Your tubes are probably bad. Have them checked.
2) Maybe invest in a tube checker.
3) Listen to Charles Reeder...smart man.

Thanks to everyone for the help!

 

[fuel37]

Man, This sounds so much like what happened to me this month, I could have wrote the original post.

I had my amp to rehearsal about a month ago and was playing along and amp sort of sputtered and shut off.... got home and pulled the fuse and it was blown, not blackened, just a small section of the "coil" was gone, hard to tell it was even blown. Replaced the fuse and the amp worked fine for me for the last three weeks or so at home at low levels. Took it to rehearsal again last night, and about three songs in, it sputtered and shut off, only this time, the fuse was blackened inside and there was a bit of a burnt smell. I didn't get to it in time to see if any of the tubes looked abnormal.

When I got home today I pulled the chassis to see if I could see anything obvious (I know very little about amp internals). The first screen resistor looks to be cracked.

Here's the problem... I live on Prince Edward Island up here in Canada, there are only a couple of guys here that do amp work and neither I would trust to work on this thing. I have a call in to Mesa to see what I should do but haven't heard back as of yet (about 4 hours since I called).

This is my fourth Mesa.... first problem I've had with any of them, first time I've had to call.... fingers crossed it works out well and doesn't cost me an arm and a leg to get running again. I trust Mesa and wouldn't hesitate to buy another at all but, i know this is going to be in the back of my mind every time I play a gig with it from here on out. Especially knowing there are at least two more guys out there that had the same problem.

Another interesting thing I noticed when I had the chassis pulled that there is a 1/4 inch jack right beside the reverb tank connections, anyone know what this is for?

 

[djw]

Another interesting thing I noticed when I had the chassis pulled that there is a 1/4 inch jack right beside the reverb tank connections, anyone know what this is for?

Reverb.

Look around this thread, I believe you'll see some advice from Charles Reeder about how to deal with a cracked screen resistor. It doesn't look too difficult to deal with. You could probably do it yourself.

 

[plan-x]

I've got mine (Sreen resistors) on order. Thanks for the pics, now I know what to look for. Haven't cracked mine open yet.

 

[fuel37]

Just curious where you ordered the replacements from? I checked on small bear but didn't see anything that fit the bill. Anyone know the wattage I should be looking for? 470 Ohm, 1W?

Thanks guys!

 

[plan-x]

Mouser electronics. 470 ohm, metal oxide, 1watt, 5% tolerance.