Mesa 400+ Tube Question

Discussion in 'Amps and Cabs [BG]' started by sincity, Oct 23, 2003.

  1. sincity


    Oct 16, 2002
    I just purchased a used Mesa 400+ from the mid 90's in excellent condition.

    The guy I bought it from shipped the tubes separatley from the amp (per my request), because I did not want the tubes showing up broken.

    All of the power tubes are Mesa 5881 6l6 425 and look new. The owner told me the unit was recently retubed.

    Eleven of the power tubes are color marked with a brown dab of paint near the bottom of the tube socket. One power tube is color marked with a green dab of paint at the base.


    What gives with the green marked tube?

    Also, how do I know which tubes are matched? The previous owner did not mark them when he packed them in foam.

    If I plug the power tubes in, will they function normally if I don't have the matched tubes in the right order?

    Thanks for your help.
  2. sincity


    Oct 16, 2002
  3. Those colors are Mesa's way of marking what "grade" the tube is supposed to be. Evidently, you got eleven matched tubes and one of a different grade. I strongly recommend you forget about the the Mesa tubes anyway. They're low quality and their 'matching,' even if is accurate now, will drift as they age. The amp needs a bias adjustment and a good set of 6L6GC's. I reccomend JJ Electronics tubes from:

    You don't.

    Doesn't matter if the tubes are actually matched. That's why they're done like that; you don't have to balance each side of the push/pull output if all the tubes are the same.
  4. sincity


    Oct 16, 2002
    Psycho Bass Guy,

    Thank you so much for your response. It really put my mind at ease. When I have the cash I am going to change to the JJ Electronics Tubes. I seem to see an overwhelming support for them on the TalkBass.

    All the Best.
  5. BTW, that "if" in italics means "when." I just rearead that and you can take it to mean that matching doesn't matter. It should read:

    Doesn't matter when the tubes are actually matched. etc
  6. The 5881/6L6 425 STR variation is a Russian 5881 military tube. I went through the same thing. It is the one with the band missing around the base. I had the same situation.
    This tube is super rugged, but is a lifeless sounding tube compared to the JJ/Tesla 6L6GC.
    I got mine from Bob Pletka at also. They were $223. delivered.
    I also followed Psycho's recommendation to have a variable bias installed so you can fine tune them.
  7. sincity


    Oct 16, 2002


    I fired up the head today, and everything works fine. I think the tone is a little on the bassy or muddy side, especially with my P-bass. Better sounding with my Stingray.

    Do you think the JJ/Tesla 6L6GC will tighten up the tone by adding more midrange grunt and definition to the head?


    What is a variable bias control, and how much does that usually cost?
  8. Yes! :D I speak from experience.
  9. sincity


    Oct 16, 2002
    So, did you have the same type of tone with your Mesa tubes before you replaced them with the JJs?
  10. I for one seriously appreciated Mesa's "fixed bias" approach especially when out on the road. Occasionally, a tube would self-destruct (quite dramatically at times) And I knew that once the fuse was replaced all I needed to do was stick another "Mesa" labelled 6L6 in there and I was off and running. I mean, what a bitch to have to rebias the entire rig each time a tube needed replacing? On the road, that just wasn't feasable for me. The other col thing of course was that if in the middle of a set a tube went, all I need do was yank it out continuing on 11 cylinders (so to speak.) No doubt the overall tone would be improved with a set of JJ's but for touring musicians, the fixed bias concept plus the fact that the amp already sounded rich and fat with whatever cheap chinese stuff Mesa labelled was reason enough to leave it be.
  11. Yeah, the bass was muddy and the tubes would clip a whole lot faster. the JJ ecc83s are amazing. And the 6l6's are stable and give you nice definition. Aside from some NOS tubes, JJ's are the best production tubes IMO right now.
  12. JJ's have plenty of bass, but they are more balanced. They don't have that wierd mid honk that the Mesa tubes had.
    Check the other threads for tone settings. There is a million of em. You prolly wanto back off the bass control to 5 or so.
  13. Tube biasing and matching--a topic about which there is complete agreement and no possible room for dispute...right?? Ha, ha ,ha.

    I did a pretty exhaustive search on the 'Net a year or so ago as I was restoring my Ampeg. First, there is at least one point most everybody agrees on. Fixed bias (as opposed to cathode bias) merely means that instead of an adjustment potentiometer [variable resistor], the manufacturer has soldered in a resistor that yields the idle current they have pre-determined. Adjustment is possible, but only by desoldering the existing resistor and putting in one of a different value. That's the way my Ampeg is.

    Now the disagreements start. Many people advocate replacing the fixed resistor with a trim potentiometer, allowing bias to be easily adjusted. Yet there is an argument to be made that anything adjustable, such as a bias pot, can become un-adjusted, sometimes with dire consequences. For an amp that gets knocked about on the road, that kinda makes sense.

    Next point of contention: how to set the bias (whether there is a bias pot or whether the tech is replacing resistors). One theory says to put the amp on a scope and look at the waveform. In theory, great, in practice, not necessarily easy. Other methods involve measuring voltage or current in one of several possible ways. How, how can you go wrong here, huh? Well, there isn't a specific number to set the tube bias to. It depend on the plate voltage, screen voltage, and personal taste. The tube manuals give a maximum allowable idle value and people shoot for something like 60 to 70 percent of that value. Again, the exact number is a matter of debate.

    More disagreements arise on WHEN to set the bias. One somewhat heretical site (Tone Lizard) suggests that the bias isn't terribly critical, as long as it's in a certain range, leave it as it is when you replace tubes, as long as you go with a similar tube (same brand and tube designation). Other people go to the other extreme, actually putting a bias pot on each individual tube and tweaking each tube's bias.

    Further disputes arise when it comes to tube matching. The Tone Lizard proposes that tube matching is pretty much a marketing gimmick. Other people argue that tubes MUST be matched with this method or that method (Transconductance! No, current draw!! No, our super-secret tube matching method!!). Some people suggest that deliberately mismatched tubes have been found to yield good tone.

    My point (if there is one): Get as much info as you can. I've taken a middle-of-the road approach. I put in a matched set of tubes, but left the bias set as it came from the factory--for now....

    This much I can attest to: At one point I plugged in 4 completely different tubes (though all 7027's) All old tubes, too. A GE, a Magnavox, a Conn, and a no-name that didn't resemble any of the other three. The amp sounded fine. I put it on the scope, and the waveform looked fine. It delivered the expected 100 watts as the waveform started to show visible clipping. I put in the new matched set of Sovteks. Same waveform appearance, same 100 watts. Did it sound better? I dunno, this wasn't a blind experiment.

    Fixed bias or adjustable bias, if a tube starts going out on you in a gig, pop in one of the same type and keep playing, don't sweat it IMHO.
  14. That's a whole lotta words. Wonder if I should read all that or not.
  15. Tall Cat

    Tall Cat

    Oct 14, 2003
    I agree that you may want to retube the amp you'll get better performance from matcher tubes and there are better tubes than the mesa's out there. Try I just retubed my Bass 400+ with the sed 6l6's and it's never sounded as good.
  16. This is probably the best summary on biasing I've ever read, no kidding! Here's my 2 cents:

    Fixed bias, oddly enough, is the form of biasing that CAN be adjusted. Cathode biasing let's the tube "do it itself," but it's much more innefficient and produces a whole lot more heat. Many amp manufacturers misleadingly call cathode bias "Class A" but it's not.

    My feeling is that any jolt that would somehow be sufficient to change the adjustment on a pot would no doubt damage much more than just that adjustment, so I'm not afraid to put pots in an amp.

    I'm with Tone Lizard all the way on this one. Current draw biasing is like using a dartboard approach; you might hit what you're aiming for; you might not. It's a gross mathematic oversimplifiaction that fails to take a whole bunch of factors into account. Using an oscilliscope will tell you EXACTLY what's going on with your output stage. There's no guess work.

    It's ENTIRELY dependent on how clean you want your output stage to be. Unbalanced output produces more assymetrical clipping which introduces more even-order harmonics, which the human ear finds pleasing. I like to bias as close as possible to get the amp as loud as possible and still be clean.

    Same story as biasing.

    I've done it myself, but I always at least double-check on the scope to make sure my ears aren't lying to me.

    Yep. If there's going to be a problem, you'll know it. You'll either see the thing cherryplate in less than a minute or your HV fuse will blow.
  17. quote:

    Fixed bias or adjustable bias, if a tube starts going out on you in a gig, pop in one of the same type and keep playing, don't sweat it IMHO.

    Or in the case of a Mesa 400+, pop out the suspect bottle...and keep it out until later, running on 11 cylinders.
  18. Big String

    Big String Supporting Member

    Apr 22, 2000
    Northwest Indiana
    I could be wrong here, but the "11 cylinders" thing might not be good for the amp. If I'm not mistaken the 400+ is wired in paralled which to my understanding requires one power tube in each row of six to be paired with the one next to it in the other row. I thought I read something that said if you have one bad tube you need to pull the pair out hence running on 10 cylinders....

    I always numbered (in my mind) looking from the back of the amp, inside row was 1 -6 outside row 7-12. So, the pairs would be 1/7 2/8 3/9 4/10 5/11 6/12.

    Set me a blaze if I'm wrong, just trying to help not discredit anyone.
  19. Big String: that's 100% right.

    You should definitely take the blown tube's mate out in the 400+ and run it on 10 tubes if you're faced with that situation.
    This should not happen very often at all. I suspect that if it happens with any regularity the tubes are really crappy (sounds about right for MB's) or there's something wrong with the amp.
  20. When I had my 400+ rig 10 years ago whatever tubes they had labelled sounded absolutely cavewrnous with rich sweet mids. I swapped for my older chinese 6L6's and the new set had too much bottom frankly, No doubt far different tubes now.
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