EL84 question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by psxguru, Nov 2, 2003.

  1. psxguru

    psxguru Guest

    Do EL84 valves/tubes vary a lot in body diameter?

    The reason I ask is that I got hold of some Mesa Boogie EL84's to try in my Marshall DSL401 - the EI valves that are in there have been in there since I got my amp over 4 years ago.

    It appears these Mesa tubes are a bit 'fatter' than the EI's and they wont fit throught the holes in the chassis casing. doh!
     
  2. John Phillips

    John Phillips Member

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    Another brilliant piece of design on this amp... :mad:

    Yes, they do vary. I just looked at the ones in my Mesa Maverick and they are definitely the 'fat' type.

    Short of filing out the holes in the chassis (not a simple job since you'll need to take the board out - although if you do that you can cure the overheating rectifier while you're at it :rolleyes::mad: ) there's no easy solution other than just to find narrower tubes.

    I've just looked at some JJs I have and they are the wider type too.
     
  3. psxguru

    psxguru Guest

    Thanks for the info John.

    I think I'll give Watford Valves a ring and get them to tell me which brands are skinny ones!

    I take it you've fixed a few of these amps with the dodgy rectifier then? I had to get mine fixed as well :rolleyes:
     
  4. John Phillips

    John Phillips Member

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    No, only one so far - I made the mistake of taking a DSL201 in a trade, and even though I knew about this fault in advance, it still needed doing, along with another more serious problem that I didn't know about until after I got it. I avoid working on these amps if at all possible - often they're still under Marshall's (actually generous) 3-year warranty, so I get the owner to send it to the nearest Marshall dealer and they can have the 'fun'. They are however notorious for this issue. There's some advice going about that the right way to cure it is to fit a cooling fan. This is wrong and doesn't actually address the cause of the problem.

    The right way is to fit a heatsink to the rectifier, which cools it directly. Fan-cooling the outside of the chassis will do almost nothing since the rectifier is not in thermal contact with it - it's mounted on top of the circuit board.

    Did the guy who fixed the rectifier on yours fit a heatsink? If he just resoldered the broken pin connections, it will happen again. Probably not today, probably not tomorrow, but eventually. This is simply a piece of bad design by someone at Marshall who didn't do either the math for the heat dissipation or the crude but very effective test of putting his finger on the rectifier at the prototyping stage (and burning it). Either that or they very cynically calculated that the problem would not arise on average until the amp was out of warranty, and since heatsinks cost a few pence... :mad:

    As you can tell I am not very impressed with Marshall. It's sad to see a company which once turned out some of the best-built mass-produced amps around (up until the end of the JCM800 series) go from that to the JCM900 and 2000 series.

    I'd be a bit unfair if I said they were total crap, but they just aren't built with a quality-first culture any more... more like cost-first.
     
  5. Rog

    Rog Member

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    Hi Guys. Just walking through the various rooms to see if there is anyone I recognise.
     
  6. Rog

    Rog Member

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    Hey Kevin, when do you need to know about tickets for the PRS meal thingy? I might be able to make it.
     
  7. psxguru

    psxguru Guest

    I'm guessing he just resoldered it, I'll have a proper look when I bias the amp when I get some new valves (that fit!).
     
  8. psxguru

    psxguru Guest

    you have mail! :D
     
  9. myles111

    myles111 Member

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    psxguru .............

    Tube bottle diameters do vary, but all are currently withing max spec. Marshall has addressed this issue of undersized holes in later production.

    There is a lot of EL84 info on my personal website from the GAB linlk below.
     

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