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Mesa Boogie question

Discussion in 'Amp Tech Center' started by JL_LI, Jan 26, 2020.

  1. JL_LI

    JL_LI Friend of Leo's

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    I know there was a how long to Mesa Boogie tubes last this past week. I have a specific question. My Mesa Boogie Express 5:25 periodically cuts out with volume fading out over about 10 seconds. Switching the amp into standby for 30 seconds and switching it back restores output. I tipped the amp when it faded this morning and it looks like one of the EL-84 power tubes was red plated. Tubes are cheap enough from Sweetwater. I can probably get matched EL-84 tubes locally but I'm not sure they'd be Mesa Boogie tubes. My two are, will I damage the amp if I continue to use it by switching it to standby and back? Is red plating caused by a failing tube or something else? Mesa Boogie amps are fixed bias amps. How careful do I need to be about replacement tubes? I'll really appreciate hearing from someone who knows for sure. Internet searches bring up too much contradictory information from sources of unknown provenance.
     
  2. Guitarteach

    Guitarteach Doctor of Teleocity

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    Sounds like bad tubes, Redplating or space charge changes are a bad sign. So I’d have a spare set or three ordered asap. It will sound fuzzy and blow fuses if you keep playing.

    if you find a reliable supplier of tubes, they should be able to send a mesa matched pair to eliminate the tubes first.

    i play a Mark series boogie and use non mesa matched power tubes with no issue.

    In the past, i’ve had this exact issue happen and soldiered through a gig with cycling the power every song or two but its not a long term fix. I always carry spare power tubes..
     
  3. marshman

    marshman Poster Extraordinaire Silver Supporter

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    Well, if your amp is still under warranty, that’s something to consider...if I remember correctly, using non-Mesa tubes will void your warranty.

    That said, el84s are typically run very hot, and seem to have a much shorter lifespan in my (limited) experience. Keeping spares should be required.

    If you can, switch the tubes around and see if the same tube redplates or if the problem stays with the socket. If the problem goes with the tube, then you’ve found your culprit. If it stays at that socket, something else must be amiss.
     
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  4. Wally

    Wally Telefied Ad Free Member

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    That amp, like most Boogies, is a complicated beast. It runs in three modes. The 5 watt mode is single-ended and cathode biased. I would suggest swapping the tubes you have to see if the redplating follows the tube or stays I the socket.
    if the redplating stays in the socket and if the amp is under warranty, I would then contact Mesa. If it is not under warranty, then it is tech time...whether you send it to Boogie, have a tech or do that on your own.
    If the redplating follows the tube, then a new set of EL84’s is in order. Plus one with the above thought that one should use Mesa tubes if it is under warranty. I have no idea how Mesa biases this EL84 amp in the fixed biased modes. If they bias it hot, that would be out of line with what I see in other Boogies.
     
  5. Jon Snell

    Jon Snell Tele-Holic

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    Messa derive the bias from the HT winding on the mains transformer, through a diode, 1N4007, 120k resistor to a 150u smoothing capacitor and then through a 15k to the network. The grid voltage when in fixed bias is around 11volts, (10.8 for Fender) and cathode bias is almost zero on the one valve.
    It uses a clever patent pending circuit in the output stage and a zener diode in series with the screen grids in one of the modes.I bet the EL84 that red plates is the valve that is in cathode bias mode.
    Check the effects loop sockets for bad connections, that causes fading/distorted sound.
    Anyway Boogies are always too loud and I never plug the mics in when micing up a back line, they don't need it!
     
  6. JL_LI

    JL_LI Friend of Leo's

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    Thanks everyone. I reread a few other Mesa Boogie threads and something struck me. I read in a thread just last week about getting 3 months or a year from power tubes. My Express 5:25 is almost 8 years old and the power tubes have never been changed. I ordered a pair of Mesa Boogie branded tubes from Sweetwater. I played for about an hour after my initial post and didn't lose volume. I looked at the power tubes before I shut the amp off and didn't see red plating. I wonder what causes that to happen sporadically. I think I'll keep using the amp until I see which tube red plates. Then I'll replace both of them but hold on to the tube that didn't red plate as an emergency backup. I've probably gotten such long life from the tubes because I don't turn the gain past 11 o'clock on the two low gain channels and rarely have the volume above 11. Advice above suggested checking the effects loop but I don't use it. The only pedal I use is a compressor from time to time between the guitar and main input.
     
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  7. Dacious

    Dacious Poster Extraordinaire

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    All tubes change over time, the bias can be right at first and then wander. 8 years from a set of EL84s pretty astounding. Mesa matches the tubes but also carefully grades them, so tubes that are too hot or too cold get returned and sold to someone else. You pay for that but at least Mesa packaged tubes are consistent new.

    They can't predict how each tube may vary with age.

    Mesa's Dyna-power patent involves using big transformers land capacitors to store power so the power sections don't sag. That's why they seem to punch out lots of power, coupled with efficient speakers.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    That is a long time for modern EL84 tubes. You will probably notice an improvement in sound compared to what you have been hearing. The hours of usage would have a lot to do with how many years you get out of the tubes. Also, if you do run the amp in the 5 watt mode, you might want to swap the power tubes positions at regular intervals to balance out the wear on the tube that is used in that one socket that is operating in the single-ended mode. How do you tell which tube that would be? Run it in the single ended mode for a few minutes. Carefully using your finger, quickly touch the power tubes for PART of a second. One will be hotter than the other, the how one is the tube that is in the socket that is operating when in that 5 watt mode.
     
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  9. JL_LI

    JL_LI Friend of Leo's

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    Thanks again for the helpful suggestions and information. I ordered a matched pair of EL84 tubes from Sweetwater and installed them after I identified the tube that was red plating. They've been in for a little more than two weeks now without incident. What I noticed immediately is that the tube hiss is gone. There was no discernible change in tone on either of the channels I use. The amp also stopped popping when I switched it to standby or shut it off. So that's three things that should serve as a warning for me that the tubes need to be replaced. I play 30 to 60 minutes most days, longer on weekends, almost always at indoor volume. I think I need to put a tube change on a regular maintenance schedule with two years the max before changing them out. I replaced the 12AX7 tubes two years ago. They'll go on the maintenance schedule as well. Thanks again everyone.
     
  10. Wally

    Wally Telefied Ad Free Member

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    JL_LI, preamp tubes do not wear as quickly as do power tubes.
     
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