Marshall 4140 Help

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by RUSHFANnLV, Apr 20, 2008.

  1. RUSHFANnLV

    RUSHFANnLV Member

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    Before I take my amp to Thermal Relief here in Las Vegas (best amp guys in the city...all the other stores take their amp repairs to these guys!) I was wondering if I could at least diagnose the problem here. My amp started crackling really loud on day when I turned it on. To isolate the problem, I unplugged from my pedal board and plugged right in to the amp. It played OK for a few seconds then started crackling again and the sound decreased by about half. I turned it off and didn't touch it for a week. Today, it wonÂ’t make any sound. One of the 12AX7 tubes is not glowing but I'm not sure if that's the only problem. Any ideas or suggestions? I'm really just trying to gauge how hard of a hit this is going to be. These amps sound great but maintaining them gets to be expensive.

    Here is a pic that shows which tube is out.

    [​IMG]
     
  2. SatelliteAmps

    SatelliteAmps Member

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    It could be a lot of things. Hopefully it is the easy one, which would be the preamp tube going. It could be things involved in the circuit board though, and that could get expensive. The tech should be able to give you a bit of an estimate before working on it.
     
  3. stratman_el84

    stratman_el84 Member

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    It's a good idea when you own a tube amp to keep some known-good preamp tube (and rectifier tube if your amp uses one) spares around for diagnosing problems. You can swap out a preamp tube or rectifier tube with another of the same type with no adjustments to the amp needed, and it may solve your problem and save you some aggravation, time, and $$$.

    Don't try this with power tubes unless you're capable of setting power tube bias, as you can do serious damage even if the power tubes are the same type & brand. (One exception are Groove Tubes power tubes. You can swap out a set of power tubes of the same type and GT-rating and be OK in that the bias will be in a safe range, if not "ideal".)

    If you swap out preamp tubes and it doesn't fix it, take it to a tech if you're not experienced with working on tube amplifiers. Good Luck!

    Cheers!

    Strat
     
  4. burner

    burner Member

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    Yep, try a new tube first.
    Q: Are you using mixed brand (or types) power tubes?
     
  5. rob2001

    rob2001 Member

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    Most likely a tube issue. Swap out the pre tubes yourself.

    Side question... what amp is this? 4140 or 4104?
     
  6. RUSHFANnLV

    RUSHFANnLV Member

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    Thanks to all for the great suggestions. I realy appreciate it. I will swap out the pre tube as suggested. I should just learn to do that myself. I'm just so scared I'll mess something up.

    My amp is a marshall 4140 club and country combo.
     
  7. Swarty

    Swarty Member

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    Try the tubes first, but there is a good chance that you have a bad solder connection somewhere. Try smacking the amp with your fist and see if it crackles. I had one of these and there is a large filter cap on the circuit board that came loose and caused a similar problem.
     
  8. stratman_el84

    stratman_el84 Member

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    As long as you're just swapping preamp or rectifier tubes with the same type (IE: 12AX7 for another 12AX7, GZ34 for another GZ34, etc) and leaving the power tubes alone, you're perfectly safe even if you don't know anything about amps.

    The tubes in vintage amps were meant to be swapped out by the average Joe when they were originally designed and built. In the old days, people took entire television sets full of tubes (10 to 24 or more) out and took them to the local drugstore and tested them on the drugstores' tube-tester, bought replacements for weak/bad ones, and stuck 'em in when they got home.

    (Ahh, memories! I remember as a kid helping my father with this every-couple-months' task. It also meant I got a treat at the drugstores' soda-fountain.:drool )

    Back in those days, power tubes were mostly made right here in the USA by the big names, and they were very high-quality, low-cost, and very consistent. You could re-tube an amp for around $10-$15 depending, and it was common to just replace all the tubes every couple months, just because.

    Back then, you didn't take it to a tech, you just bought a bunch of power tubes and found the ones that sounded/worked good in that amp. They were like lightbulbs, disposable. You could buy a case of power tubes for the price of a trip to the repair shop.

    That's why many older Fenders and others had just a "balance" adjustment for bias, simply to account for small matching errors to keep one of a pair from running hotter. The bias was in most cases in a safe range, if not ideal if you replaced the power tubes it came with, with the same brand and type.

    These days, power tubes are NOT very consistent, and you should get matched power tube sets and have power tube bias checked/set. You can do damage to an amp without having the bias checked/set on a new set of tubes.

    Preamp and rectifier tubes because of their nature and that of the circuits they're in don't need any adjustments regardless of brand.

    Cheers!

    Strat
     
  9. rob2001

    rob2001 Member

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    Ahhh, the club and Country... I thought so. Having this amp and a name such as rushfan makes sense!! Lifeson used these yes?
     
  10. RUSHFANnLV

    RUSHFANnLV Member

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    Yes he did. I have a pair of them and run them in stereo via a classic CE-1. I really nail that MP/Signals/P/G tone with these amps. Without the chorus, they are also very nice sounding amps for some of the earlier Rush tones.

    Well, it turns out that the tube that is in question is the only one of the pre tubes that is not a Groove. I will be taking it in today anyway for a full check up.

    Thanks to all of you for your advice. I learned a lot just by posting this thread. I didn't know I could change out the pre tubes without biasing the amp. I will keep a spare set from now on.
     
  11. RUSHFANnLV

    RUSHFANnLV Member

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    UPDATE!

    Turned out to be a pair of power tubes that went out...blew a resistor as well. Replaced all four with GT 6550's. The pre tube that seemed to be burned out was so that was replaced as well. Going to pick her up tomorrow.

    Again, I want to thank you all for being so nice and willing to help me. I really appreciate it. I've always respected the members of this message board for their attitude and patience with people like me who like to use older equipment but don't know too much about it. Thaks to you, I'm learning.:)

    Mike
     
  12. rob2001

    rob2001 Member

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    Thats great news! I agree about how helpful folks can be here. I use older JCM 800's and i've worked through some issues here as well with help. It's a great resource. Have fun when you get your amp back. :dude
     
  13. Mr. Bertha

    Mr. Bertha Member

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    How did it end up sounding when you switched to 6550s? I've got a 4140 with el34s and Celestion V30s instead of the stock McKenzies. I'm considering getting 6550s or KT88s to really maximize the clean tone.
     

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