Lonestar Special dies just after the breaker goes..

[gordwait]

I decided to live blog my debug on google plus, if anyone's interested:

https://plus.google.com/u/0/103864209986771135372/posts/YEK1oaZWXiY

In a nutshell, it went like this:
 1) during soundcheck the breaker went. (No two different outlets on the same breaker is not "two different circuits" Mr. Landlord..)
 2) Power back on, the amp works for 5 minutes, then shuts itself off, and the drummer says "I smell something burning.."
 3) Manage to gig with just my RP1000 pedal (grrr.. I bought a tube amp for a reason - but thanks to the RP1000 pedal for seeing me thru)
 4) Pull it apart the next day and find what looks to be a shorted 68 ohm resistor. (Mistook it for a 0.68 ohm due to the "5 band" resistor code on it until I started hunting the web for 68 ohm and Lonestar)

Next up is to test the power tubes and the rectifier tube, and replace the resistor. Also people keep mentioning a couple resistors I should pull if I have the
serial number that needs the mod to protect the rectifier tube..

Anything I should be checking while I'm at it - or does it sound like I've got it covered?

Did anyone track down a schematic for the Lonestar Special by the way?

[John]

http://www.el34world.com/Forum/index.php?topic=10762.0

No schematic, but talks about the resistors you should clip out to protect the rect. tube depending on what power tubes you have. HTH.

[six el six]

I've repaired one.

It was the rectifier tube. I put in a JAN nos and it had no problems.

I don't think I did anything else to it.

A mesa employee/tech told me they don't give out schematics on current production amps

hope this helps.

six.

[gordwait]

Thanks for the tips!
Definitely going to test the tubes then, and check for those troublesome resistors..
I got this one used at the local Long & McQuade for a pretty ridiculous deal, and I wondered if there was a reason.
Perhaps it's been eating rectifier tubes...

[gordwait]

Anyone know where to look for R101, R103?
Seems like none of the resistors on this PCB have a silk screen label..
Somewhere off the pins of the 5Y3 I'm guessing?

Also, seems like "NOS" 5Y3 are out of stock at the first few online shops I looked at. Any suggestions?

[gordwait]

Ah, I see the silk screen for the resistors is just well hidden..
I see a couple of 470's as well, near the ribbon cable from the power PCB to the main PCB.
My amp serial number is 2398 so it looks like this one is a 5Y3 destroyer..

I'll probably unsolder those 470's to check if they are the right ones, and live with a non NOS 5Y3,
although after reading about the NOS 5Y3 I'd sure like to try one out..

[John]

there's quite a few NOS 5Y3GT's on Ebay. But the only reason I used the older ones myself was to drop the voltage more. From here on out I'll simply use a 250 to 400 ohm resistor with a beefy wattage rating, probably drill a few small air holes in the chassis above it to help the heat escape.

[gordwait]

Final update - tested the power tubes at the local shop and found that the 5Y3 tube was fine.
Power tube #1 (I assume the one beside the 5Y3 is the one used for 56 Watt Class A mode) tested one notch above fail (51/100) and the other three tested OK
(75/100) so I presumed that just the resistor was at fault - perhaps damaged from some previous incident before I owned the amp.

Unfortunately the 12AX7 socket on the tester was so worn out (decades of guitar amp users testing?) that it didn't work, but the preamps all seem ok.

I bought a new quad set of tubes (Electro Harmonix - comments?) but realized when I got home I asked for the wrong tube - I need EL84, and I asked for EL34 - I keep mixing the two values up when I talk about this!
I also picked up a spare 5Y3.

I put a 2 Watt 68 ohm in place of the shorted 1 watt, put a new fuse in and put all the old tubes back in.
When I did the smoke test, power tube #1 went nuts with all sorts of loud noise in sync with a cool blue sparky light show inside the tube that reminded me of the scene where R2D2 gets zapped!

Ok, so there's the culprit.

Since this amp has a selector between 5 watt mode (one tube in class A), 15 watt (the first two in AB) and 30 watt (add the second pair in class AB) I pulled the dead tube, and it's "brother" out, and put the second pair in for the first for testing. Works great - in fact I do think the amp sounds much better than before - I suspect that bad tube may have been degrading the sound..

I'll get a new quad set and retube anyhow, and keep the three remaining old ones as backups for gigging..
I also decided to leave the 470 ohm resistors in place since there was no issue with the 5Y3. I'd like to know what the 470's are doing for the amp before I clip them out (perhaps loading down the 5Y3 to get extra sag?)  If the amp starts eating 5Y3 tubes then I'll carefully remove the 470 ohms to see if it helps. I've only had this amp since last fall so I don't know if it has a history of eating tubes..

[John]

Nothing wrong with EH tubes. All brands have a few bad ones in the lot, but I think most guys here use all brands.

And if you don't have one already, build a light bulb limiter. It will save you countless fuses and extra work. I use it almost anytime I do anything to my amp except maybe messing with the tone stack. I've made enough dumb mistakes that I don't consider anything "routine". 

[gordwait]

A light bulb limiter?
I'm game, but not sure what it is..
I'd guess you use a lightbulb in series with the fuse to limit the current?
Sounds interesting!

[Willabe]

Look in here, with thanks to sluckey;

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


                            Brad