Mesa Boogie Mark V:90w

Discussion in 'Amp Tech Center' started by Veltek, Nov 16, 2019.

  1. Veltek

    Veltek Tele-Holic

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    Hey ya'll

    Just got a quick question. I have a 90w Mesa boogie combo, that I converted into a head. I have not played it in a while, and recently have plugged it back in. For some reason when I turn it on, after about 10 minutes of playing, it will power off, then back on very quickly. After that it seems fine. Any Idea what may be causing this?
     
  2. Wally

    Wally Telefied Ad Free Member

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    The question was quick, but the answer cannot be. First question back... Powers off and back on? Does this mean that the power light goes off? If not, does ‘power off’ mean that sound production ceases while the indicator,lamp is still bright?
    If the answer to the first question is yes, then one has to suspect the AC power supply, connections, or the PT——I doubt the PT is the problem. The lamp is tied to the 6.3vac heater filament. If a connection goes bad there, then the lamp is dark and sound production will cease due to the lack of heater filament voltage to the majority of the tubes.
    if the lamp stays bright but sound ceases, then check the heater filament glow in all tubes. Take a very hard look at th e first 2 or 3 preamp tubes. The first 2 or 3 are DC heated and have a separate circuit from the other tubes. If that DC voltage is lost, the amp will not produce sound.
    I would suggest having the back of the amp off so that you can take a look at the tubes when the silence occurs.
    If the heater filaments circuits are not the problem, then other questions arise. Obviously, this is a complicated amp...got a tech?
     
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  3. Veltek

    Veltek Tele-Holic

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    One tech in town who is certified to work on Boogies, he hates working on them, and shows it in his pricing :/

    When I say off and on real fast, I mean that the sound cuts out (chording or any playing is replaced with crackle for like 0.5 seconds), the power light, and all lights on the amp fade, then recovers quickly. Imagine some one carrying groceries and stumbling, but catching themselves before they fall on their face. It does not Completely shut down.
     
  4. Dacious

    Dacious Poster Extraordinaire

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    You have I think a bad tube or tubes. I think those amps have a thermistor in the power supply. It is like a dynamic fuse.

    If something pulls too much current it cuts off for a short period then re-allows current. That saves the power and output section from melting something.

    If you can position it where you can see the tubes, and play it. Do any of the tubes glow red in the grey plates inside like the one below?

    4996-1305079819-bbd3492b0dce7e55b25a8031385ab294.jpg
     
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  5. Wally

    Wally Telefied Ad Free Member

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    Dacious, there is a varsity or in the AC power supply.
    https://el34world.com/charts/Schematics/files/Mesa_boogie/Mesaboogiemark_v.pdf



    Veltek, these are complicated amps. Complicated means time will be spent...in many situations. Time = money.
    I suggested watching tubes as did Dacious. How old are your power tubes? I have repaired a MK V’s DC heater filament circuit. It is not as simple as working on a handwired 1960’s amp.
    Does this problem occur in all playing modes? In the 45 watt Class A mode, only the middle two 6L6’s are in operation. If the amp does not fail in this mode, one might surmise that there is a problem in the two outside tubes or their circuits. If there is a problem in this mode, one might surmise that there is a problem with those two middle tubes or their circuits. pin the 10 watt Class A mode, the two 6L6’s close to the 5U4 are in parallel class A operation....same thing, is there is a problem in that operation or not?
    the middle two tubes run in Class A full time even when in the 90 watt mode. This class A mode might be a bit harder on tubes than is the fixed biased mode in which the outside two tubes operate when in 90 watt mode.
    I will observe this. When one buys a high dollar, complex amp; one might expect that repair charges will of necessity be higher than with simpler circuits. If it were my amp and I wanted to try to avoid the tech charge, I would order a new set of power tubes from Mesa and see if that cures the problem. You are going to need the tubes at some point, anyway, correct?
     
  6. tubelectron

    tubelectron Tele-Holic

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    Unfortunately, I understand him...

    Are the panel light - notably the pilot light - monmentarily completely shutting off or diming/fading ?

    In the 1st case, it would mean a bad contact in the AC mains circuit of the amp (or external)

    In the 2nd case, it would mean a bad contact on the supply somewhere, or an issue as supposed by @Dacious ... Or anything else since these amps are as complex as a nuclear plant. It's hard to tell... :confused:

    -tbln
     
    Last edited: Nov 21, 2019
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  7. Veltek

    Veltek Tele-Holic

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    Thank you all for your replies. I am more so trying to figure out if I need to get out from under this amp before it becomes catastrophic.

    So, I would say they dim/fade. but very quickly. Maybe within 10% of being totally turned off? It is hard to tell because it happens so quickly, and the timing is random.

    I am actually wondering if I am not letting the tubes 'warm up' enough. I recently moved the amp onto a outside facing wall, and it has also dropped 40 or so degrees here in the south. I know some people don't believe in using standby to warmp up the amps, but Mesa Boogies manual recommends a 30 second warm up period.

    I've been trying to let it warm up for 2 in standby, and have not been having the issue. I dont know, does this sound right to ya'll? Or am I masking the problem this way?

    (I will be watching the tubes the next few times I turn it on, will report back with results)

    Also, the Settings I use are 90W in channel 1, 45W in channel 2, and 90W in channel 3. I feel like it happens more frequently in channel 2? but Its not something I have specifically taken note of. So I will also start paying attention to that.
     
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  8. Veltek

    Veltek Tele-Holic

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    Also, he actually posts his workings on youtube. Any time he gets a Boogie in, he goes OFF about how bad mesa is at manufacturing, and how he hates that they do certain things, or certain values they use. Its really entertaining. Lol
     
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  9. Wally

    Wally Telefied Ad Free Member

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    Complex amps..... I repaired the DC heater filament circuit in one, but I did not have much time with which to figure out all of the configurations there...and what they can do. The ability to assign different operational parameters for each channel is definitely a game changer. The MKIV s my favorite Boogie up to this point, but I could sit with one of these MKV’s for a while figuring out what I would think off it. The MKIV has 8 power levels, but they are ‘pick one and use it’ rather than assignable by channels. The MKIV I had could be manipulated to do everything from early 50’s tweed to screaming high gain. After I replaced the Celestion OEM speaker, the amp became much more musical.....with an Altec Lansing 417H in it.
     
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  10. Wally

    Wally Telefied Ad Free Member

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    that would deter me from letting him work on the amp....unless he is so good as a tech that he can disrespect and repair at the same time. Those two things are contradictory imho. I try not to put things on my bench which I feel should not be working. I have a couple of Fenders like that here right now....and they will likely be the last of that type I will work on. Complexity is one thing, but cheap production values is another. Mesa builds well, as far as I am concerned.
     
  11. Veltek

    Veltek Tele-Holic

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    He is a good tech. But He has that old school mind set that nothing is better or as good as a hand wired fender/vox/marshall from the 60s or 70s.
     
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  12. JDB2

    JDB2 Tele-Meister

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    I know there is no real comparison to the wizardry of a Mark V but I feel like my Fender Prosonic is not too far off that description, yet it's basically a one channel amp with a switchable boost circuit and three power options.
     
  13. tubelectron

    tubelectron Tele-Holic

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    I agree completely ! :)

    He's right - at least for the repair and service standpoint. I wonder if those fancy multi-channel multi-PCB amps will be serviceable in the future... I'm afraid that the answer would be negative : look at the picture below... o_O

    [​IMG]

    Personally, I wouldn't use the term "wizardry" but rather "complexity" or/and "complication" instead...

    But it's me, OK ? :D

    -tbln
     
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  14. Wally

    Wally Telefied Ad Free Member

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    JDB2, I am a ProSonic owner and fanatic, and yes, the Pro Sonic has the same range of versatility, while being a much simpler amp. Fwiw, my Super Champs are much the same. I just finished modifying a Fender 75 yesterday. It turned out to be a much more usable amp with much richer tonality. Very versatile amp, too....BF to tweed to high gain. It also is much simpler than those Boogies....and it is handwired...as is the Super Champ. The Pro Sonics have a PCB, but the chassis mounting for all jacks, pots, and tube sockets is a superior approach, ime. Bargain amps in the market, imho.
     
  15. Wally

    Wally Telefied Ad Free Member

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    I am old school as well, but one has to admit that those old amps are limited in their versatility. I like tube overdrive, and attaining that demands some added complexity compared to those old amps. I have put amps to the test.....high dollar pedals versus onboard tube overdrive. There is a difference. Now, a good tube preamp unit that is wall AC powered is a useful tool with an old simple amp.
     
  16. Veltek

    Veltek Tele-Holic

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    I really enjoy the tones I get from my Mark V. Channel 1 is warm and fat and crystal clear cleans, usually with a healthy dose of reverb. Channel too is my blusey crunchy channel. and Channel 3 is my high gain lead territory. It is nice having all of that in one amp, then being able to add in its EQ, a solo boost, and switch channels at will. I do have a nice little pedal board with a couple OD's, Reverb, etc, but the Boogie does not need it. (I use my pedal board for my Quilter toneblock)
     
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