Lone Star blown tubes

[Maintain]

I recently turned on my Mesa Boogie Lone Star Special and had an immediate burning smell. I looked at the back and was able to witness both the 5Y3 and one EL-84 go out (glow went out). The burnt smell was not accompanied by any smoke. The 30 watt setting still works but both 5 and 15 do not. I read of the grid resistor being an issue so I pulled the chassis. I was able to locate one resistor that appeared to be shorted. It is a 1 watt, 68 ohm resistor. There appears to be no other component that has had the smoke let out of it. Any ideas

[Steve Conner]

There have just been several threads here about tubes blowing up in Lone Star Specials. I think we concluded that they run all of the tubes quite hard and they don't last particularly long. If one of the EL84s shorted out, it could take out the 5Y3 with it and that resistor too.

So, try replacing at least the EL84 that died (preferably all of them), the 5Y3, and the burnt resistor. Hopefully the amp should work again.

I'm not sure, but I suspect the 5Y3 tube might be bypassed in the 30 watt setting, and the burnt resistor also.

[danmagruder]

I have talked to the top managers of Mesa and a few years ago they had a bad batch of 5Y3's. when they go out the amp only works in the 30W mode. In the 30W mode the power goes through silicon diodes for rectification but in the 5W and 15W modes the 5Y3 comes into service. It usually takes out some resistors on the power tube pcb which is under the back panel connector pcb. Good luck.

[Jared Purdy]

Ditto on mine, though I won't know for sure what the problem is until I take it into the repair shop at Steve's Music in Toronto. They don't send their amps out for repair, unless it is to Mesa. They do allrepairs on site. BTW, how old is your amp, and what is the serial number??

[danmagruder]

I work repairing gear for the US Navy Music program. We bought 12 of the Lonestars and a couple of them had this issue. It is pretty straight forward. If you get lusky the recto tube can sometimes go out and not take anything with it. That's rare though.

[MWJB]

"I was able to locate one resistor that appeared to be shorted. It is a 1 watt, 68 ohm resistor." If this is the cathode resistor for the EL84 is 5/15W mode, it looks to be on the small side regarding Wattage rating, I wouldn't put a 1W back in there if a 4W will fit (possibly a 3W) elevated off the board, with a little spare lead length to allow of expansion. Check the condition of any electrolytic cap that bypasses this resistor too & up the voltage rating & move it away from the resistor body if it sits very close.

[danmagruder]

I look at resistors like fuses. If there was a 1 W there I put a 1W back. If there is a resistor of greater wattage is there it might not burn if there is an issue, which could cause a bigger catastrophic failure.

[Jared Purdy]

I look at resistors like fuses. If there was a 1 W there I put a 1W back. If there is a resistor of greater wattage is there it might not burn if there is an issue, which could cause a bigger catastrophic failure.

Did you happen to notice if this problem occurred on both channels, or just one? When my fuse and rectifier went last night, I was on Ch.1 in the 5 watt mode.

[danmagruder]

It should be with both channels because the issue starts in the power supply and manifests itself in the resistors attached to the power tubes. something on one channel or the other would be a preamp issue. The issue we've been discussing is in the poweer amp/ 5y3 rectifier tube.

[MWJB]

"I look at resistors like fuses. If there was a 1 W there I put a 1W back. If there is a resistor of greater wattage is there it might not burn if there is an issue, which could cause a bigger catastrophic failure

OK Dan, if that 68ohm resistor is a cathode resistor (seems most likely?) then it will have 9 or 10 volts on it, square that up then divide by ohms & you get 1.5W...dissipated by a 1W resistor. Manufacturers often use underrated resistors these days. Fuses make better fuses, probably best not to overrate virtual centre tap resistor wattage though.

[pdf64]

'probably best not to overrate virtual centre tap resistor wattage though'
Mark, what's your reasoning behind this? My thinking is that it may be beneficial to keep the heaters firmly referenced to ground, even under fault conditions; eg if there was a tube fault that resulted in B+ current flowing to ground via those resistors (the main cause of them blowing?) then it's better to keep that fault current flowing so as to blow the line fuse ASAP, rather than the resistors to blow, which may result in the heater circuit being pulled up to B+ (by the tube short). Because then the heater-cathode insulation of all the tubes is at risk.
Therefore uprating the heater virtual centre tap resistors to be able to withstand B+ fault current for ~10 seconds could be a good idea.
However, I've never encountered an amp full of tubes with obviously damaged heater-cathode insulation, even when a power tube and the virtual centre tap resistors have blown; so maybe I've missed something? Pete.

[MWJB]

Hi Pete,

Good point!

[Maintain]

Thanks to all who have provided input. I just joined this site and am impressed by the level of knowledge within. I'm pulling the board today. I will make some additional component checks and if no more problems are found, I'll replace the burnt resistor, install new tubes then cross my fingers.