Mesa Boogie Tube Problems

I had posted about retubing my Mesa Boogie Lone Star Special amp here and here.

4 JJ EL84 power tubes, 5 JAN-Philips 12AX7 preamp tubes and 1 Sovtek 5Y3GT rectifier tube. At first, everything seemed fine. As the tubes burned in, the sonic character of the amp changed noticeably. Where I had kept channel 2 as a higher gain channel — clean with some dirt if driven hard from the guitar — I engaged the Drive to push more of an overdriven sound. The distortion sound was rich in harmonics and sounded awesome. Far sweeter than the old tubes. But, during rehearsals a few weeks back, and ever since, the volume of the amp randomly drops. Dramatically. It’s almost as if the amp loses headroom.

Obviously, tubes might be an issue. Or perhaps a transformer.

Searching the web, I found some folks had experienced the same issue. And specifically highlighted the Cathode Follower Tube as suspect.

From Mesa Boogie:

SOUND “DROP-OUTS? / INTERMITTENT SIGNAL

One Possible Cause

Most (but not all) Mesa/Boogie amplifiers have one or two “cathode follower? tube stages in their preamps. Tube selection is CRITICAL in these stages.

Specifically, in a 12AX7 tube used as a “cathode follower?, the voltage difference between that present at the cathode, as compared with the heater voltage, can be withstood or tolerated by certain types of tubes, whereas other tubes will fail. The failure of a “cathode follower“ tube will cause sound dropouts or signal loss.

For the past few years, Mesa has been using two types of 12AX7 tubes: ones originating in Russia (Sovtek EH), and ones originating in China. The Russian (Sovtek) tube is NOT reliable as a cathode follower. Of the tubes we are using today (March 2008), ONLY THE CHINESE 12AX7 IS RELIABLE AS A CATHODE FOLLOWER.

In conclusion, if you are troubleshooting for signal dropout in a Mesa/Boogie amp, suspect a cathode follower tube, and try replacing it with a Mesa 12AX7 that is marked as “CHINESE? (silk-screened on the tube itself).

Below is a partial list of Mesa amps and cathode follower tube locations:

GUITAR AMPS BASS AMPS

Lone Star & LS Special: V3
M-Pulse: V2
Stiletto: V3 & V4
Venture: V2
Road King I: V3 & V4
Big Block 750: V4
Road King II: V3 & V5
Titan: V4
Roadster: V3 & V5
M2000: V2
Dual & Triple (2ch or 3ch): V3 & V4
Bass 400+: V2
Tremoverb: V3 & V4

ANOTHER POSSIBLE SCENARIO may occur in the “SPONGY? (or on some models, “TWEED?) power setting: the reduced filament voltage may cause very low output from a RUSSIAN preamp tube. Again, the recommended fix is to replace the “sagging? tube with the CHINESE type of Mesa 12AX7 – which are more immune to this type of failure.

I will try to source a Chinese tube today. The JAN-Philips tubes are US but I do have quite a few Russian tubes. And it looks like I will have to make a trip to Toronto to find some Mesa tubes if I can’t get this resolved. Lots of playing ahead over the next two months and the Lonestar Special has my tone. But not with random sagging.

13 replies
  1. allan hyggen
    allan hyggen says:

    I’m having the same intermittent problem you describe with my LSS, and I do have a Chinese MB 12ax7 in V3. I was curious to learn if you solved your problem and it was tube related?

    cheers

    Reply
  2. Walt Carroll
    Walt Carroll says:

    I’m also having the problem. It has sagged a few times in practice but actually went out twice at outdoor gigs. I don’t even use the amp anymore because of that (I have the Lone Star). The amp sounds great when it plays…. I’m going to try to obtain a tube… then figure out where it goes..
    thanks

    Reply
  3. Diego Bolanos
    Diego Bolanos says:

    Hi i have the same problem with my lonestar. I replaced all the tubes in my amp I still have the volume drop. I sometimes just doesn’t sound for a couple of seconds and the sound comes back up. What could this be?

    Reply
  4. Labrador
    Labrador says:

    I believe, problem is in Loop section. Try put hard bypass switch down. Everything works fine. Second solution: put one short cable in loop (one side in Send – other in return).
    And, try your amp.

    Reply
    • Francisco J. Roa
      Francisco J. Roa says:

      Hi Labrador.
      I have the same problems. Do you know the exact reason for this malfunction?. I have tried bypassing tne loop section with a short cable and it seems everything is ok but if I disconect the cable the problem comes again. Do you know how to fix it?
      Thanks a lot

      Reply
    • Martin Cooper
      Martin Cooper says:

      I have the LSS head and the 4 x10 combo. They both have the intermittent volume problem and I too, fixed them with the loop bypass, short cable solution, above. I am seriously disappointed by this, for 2 reasons:
      1. The SOLO feature of the LSS is lost, which is an excellent feature and benefit if it works.
      2. The resale of the amp drops because I personally could not sell it to someone without telling them about this issue.

      I hope there is a permanent fix for it and that MB is straight-up and communicative about the issue.

      Reply
  5. Tim Robertson
    Tim Robertson says:

    It could be in the output stage: Check to see if you running speakers of the correct impedance and that it is plugged into the appropriate jack.
    I have seen this problem with the Lonestar Special head,
    The EL84’s were overheating, glowing orange like a space heater and the thus a volume drop out. This is also damaging to the output transformer.

    Reply
  6. Craig
    Craig says:

    I had the same issue a few years back, took the amp to a mesa authorised repair guy and found out that it was in the loop section. He told me there was a risistor in the loop that went bad. He replaced that and I’ve never had the problem again. Hope that helps save some $ on tubes for a few of you.

    Reply
    • Francisco J. Roa
      Francisco J. Roa says:

      Hi, I have same problem with my Lonestar Special. I have tried with a cable from send to return and until now the problem seems have been fixed. Please any people know what the real problem is?. What resistor (or another component) is bad?. I live in Spain and the Mesa tech service has not fixed the problem. They only change some valves and in a few months I have the problem again. I’ve try with other tech and he has done the Mesa modification but amp continues failing.
      Why Mesa has not fixed the problem?. There are a lot of people with same problem and this is not acceptable for a boutique amp at a very high price.
      Please I need help to know what needs to be repaired for my tech service and to fix definitely my amp. Thanks a lot:
      Paco Roa

      Reply
  7. Gottfried
    Gottfried says:

    I’m facing the same issues with my LSS. In hard bypass mode the issue goes away. There must be a problem with the FX loop section. I will try replacing also all tubes. If one of the resistors is bad how can I figure out which one it is?

    Reply
  8. Gord Wait
    Gord Wait says:

    I’ve had various similar symptoms, and am still having issues.
    I had random jumping up and down of levels, with intermittent squealing.
    I opened up the amp, unbolted the power tube board and found a fractured 100 ohm resistor.
    As it would heat up it would of course thermally stress and act up.

    Swapped in a new resistor, seemed OK for a week or so.
    Then I got the dreaded “runaway red glow” issue on two of the power tubes.

    I read how Mesa avoids biasing by selecting tubes that match the fixed bias point, so I sprung for two matched pairs of true Mesa Boogie EL84 tubes.
    One each from each pair would run away red hot.
    Took those back and exchanged, got all four behaving..
    For a couple weeks..

    Last rehearsal the amp stopped making sound mid song, and I noticed a very strong burning paint smell.
    I shut it off.

    Took it home, opened it up, didn’t find any burned parts, so I tested it again, red glowing tube plates after about 5 minutes.
    I’m thinking something is off in this amp, and causing the tubes to run away thermally.

    I find it hard to believe I had so many bad new EL84 from Mesa..

    I had to pick up another amp today for rehearsal (Peavey Jack Daniels, does ok, no Lonestar, but it doesn’t fry itself either!!)

    Love my Lonestar Special when it works, but twice now it’s died at a gig. Don’t know if I trust it for gigging anymore.

    Reply

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