LSS 30w valve?

[Mork]

Hi, during band rehearsal yesterday my Lonestar Special 30 watt setting on the clean channel crackled and spluttered and the sound out the amp became weak and sounded like a transistor radio. There was a strong fishy smell from the back of the amp and it looked as if one of the valves (possibly next to the 5Y3?) was glowing very brightly in comparison to the rest (center section glowing?). Both the 15/5W sections worked fine as did all 3 power settings on the drive channel, then after the amp had been on standby for a few minutes the 30W section worked fine again. Today the valves are all glowing orange top and bottom (normal?), temperature seems fine and the smell has gone.

Can anyone point me to the correct valve to replace for the 30W power amp section (in the hope this is the problem!) or, seeing how the amp is nearly four years old maybe it's due for a replacement of all the EL-84 power tubes?

Thanks in advance.

 

[ricardo loma vista]

There is no specific 30 watt tube--the amp runs at 30 watts when all 4 tubes are engaged, 15 watts with 2 tubes, and 5 watts with one tube.

I recommend you replace the whole set of tubes. The cost of 4 new tubes is nothing compared to the potential damage and downtime you may experience if one or more tubes fails catastrophically. That one tube has had an intermittent failure is a warning to replace them. You can get the EL-84 tubes directly from Mesa or any number of tube suppliers.

 

[Mork]

Okay thanks Yes I realise there is no 30W tube, I worded it wrong- should've been which end of the row of four is the final tube (if that isn't the wrong way of putting it again!).

Does anyone have any thought's on swapping the EL-84's out for a matched set of JJ Tesla's?

 

[Octavarius]

On the Lonestar Classic, the outer pair is on in the 50W mode, and all four in 100W. In 10 watt the two left 6L6's are running parallel in Class A cathode bias. The 6L6 on far right is still 'on' (but no signal gets to it) to provide proper balance in output tranny. The inner right 6L6 is truly 'off', cathode is lifted. Not 100% sure, but I think the Lonestar Special is based on this same principle, so technically, if one of the tubes was red plating only in 30W, I'd guess it was one of the inner pair. But then again, bias shifts a bit when switching through power settings, so unless you're certain which tube was red plating, there's really no telling.

Check out this section from the manual on how to diagnose and remedy power tube issues:

DIAGNOSING POWER TUBE FAILURE:

There are two main types of tube faults: shorts and noise. Both large and small tubes may fall prey to either of these problems but diagnosis and remedy is usually simple. If a fuse blows, the problem is most likely a shorted power tube and shorts can either be mild or severe. In a mildly shorted tube the electron flow has overcome the control grid and excess current flows to the plate. You will usually hear the amp become distorted and begin to hum slightly. If this occurs, quickly look at the power tubes as you switch the amp to STANDBY and try to identify one as glowing red hot. It is likely that two of a pair will be glowing since the “shorted” tube will pull down the bias for its adjacent mates, but one tube may be glowing hotter — and that one is the culprit. The other two are often fine — unless they’ve been glowing bright red for several minutes.

Because there is no physical short inside the tube (just electrons rioting out of control) merely switching to STANDBY for a few moments then back to ON will usually cure the problem ... at least temporarily. Watch the tubes carefully now. Should the problem recur, the intermittent tube will visibly start to over heat before the others and thus it can be identified. It should be replaced with one from the same color batch, shown on its label. Call us and we will send one out to you.

The severe short is not nearly so benign. In the worst cases, a major arcing short occurs between the plate and the cathode with visible lightning inside the glass and a major noise through the speaker. If this is seen to happen, IMMEDIATELY turn the amp to STANDBY. By this time the fuse probably will have blown. Such a short is usually caused by a physical breakdown inside the tube including contaminate coming loose or physical contact (or near contact) between the elements. Replace it and the fuse with the proper slo-blo type and power up the amp using the power up procedure as we described earlier in this manual.

If the power tubes (or the preamp tubes) haven't been replaced in all those 4 years, I've definitely say it's time for a full retube, though. Get a matched quartet of EL84s from Mesa and five Mesa 12AX7 preamp tubes and you'll probably notice some improvement in tone and feel as well.

Hope this helps!

 

[plan-x]

I'd look in to some JJ's. Cheaper and sound better. Save your old tubes for an emergency. The good ones that is. Pre-amp tubes can last alot longer than power amp tubes. Call Bob at Eurotubes, he'll hook you up. Or run down to GC and score some Groove tubes. I believe if you get the right ones, color code or something, Mesa will still honor the warranty with GT's, if your still under it.

 

[ricardo loma vista]

I think the LSS is different from most 4-tube amps in that the tubes are arranged in left/right pairs. If I remember correctly, the pair nearest the output xformer are the only two providing power for the 15W setting, and the end tube of that pair provides power for the 5W setting.

You don't have to worry about color coding for EL-84's. The LSS is cathode biased so the bias adjusts to the tube.

 

[Steve73]

x2 on the JJ's. Decent price and they sound great. Definitely better than the Mesa tubes.

 

[Mork]

Thanks everyone for the great comments. I've ordered the valves to replace all the power amp section. Cheers!

 

[Bumpville]

check page 18 of your Owners Manual.

TUBE appears to be shorted.

 

[Mork]

Hi Bumpville, I have since my original post found the relevant info in the manual (don't know why I didn't think looking there from the start!) and have replaced the outside pair with a spare pair I already had, and the middle pair with new tubes (all Mesa), keeping two original ones as backup. At some point I will pick up a set of JJ's and give them a try.

I'm surprised at how noticeable the new valves sound, brighter and more twang on the clean channel, but worryingly a little more muddied on the drive channel, not sure what that's all about.

 

[Bumpville]

Hey Bottle Rocket,

The Drive channel may be part of the Pre-Amp Circuit, which is controlled by one or more of the 12AX7 preamp tubes, I believe.

Diagnosing Valves is hard without a Hickok Tube Tester -- http://www.tubewizard.com/recommended_Hickok_testers.htm
(showing my age).. If you have had the amp for awhile and experienced Valve problems on the Finals (EL's), especially a shorted tube, it may have impacted (Overdriven) the preamp Valves.

Not sure which 12Ax7 is connected to the preamp drive circuit, but that may be the culprit causing your muddy drive problems. Like all vintage design valve amps, one thing depends on another. In the old days you could go to Radio Shack and test your tubes on their tester to fund the exact problem, nowadays, it is pretty much swap, hit and miss.

Bumpville

 

[hippdogs]

I recently had a similar problem with my LSS head, only without the smell. Here's my info. 30 watt uses all four el-84's. The 15 watt mode uses the outside tubes on either side (facing the back). The 5 watt mode uses the far left tube, facing the back. so the bad tube was probably one of the middle tubes. I changed out my two middle tubes and perfect. But for the money compared to the hassle..... replacing them all is a good idea. good luck :?