Ei EL84s in a Matchless???

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by illinimax, Sep 30, 2004.


  1. illinimax

    illinimax Gold Supporting Member

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    Anyone have any experience with this setup? I'm curious as I've just tried a couple of quartets and proceeded to take out a resistor. I've read that the Ei tube is wired like most NOS EL84 tubes and may be problematic for some amps.
     
  2. hunter

    hunter Supporting Member

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    Can't speak for the 30 watters but I have been using Ei's in a couple of 15 watters (Spitfire and Lightning) and a BadCat Cub for several years. One way to be sure is to pull the chassis and check the socket pins.

    hunter
     
  3. fullerplast

    fullerplast Senior Member

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    I don't really follow your last statement...the EI *is* an EL84.

    The only thing that may be problematic for *any* EL84 is an amp that is designed or set up to exceed the maximum recommended ratings of the tube (and there are quite a few amps that are designed like this). For these amps, you either have to hand screen tubes for low current, or change the circuit to reduce the voltage and/or current. Usually this can be as simple as a changing a cathode resistor. Some tube vendors (LV for one) will screen the tubes for you.
     
  4. hipfan

    hipfan Member

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    I don't have any technical basis to question your analysis, but I had a Laney VC30 that did not work with some NOS Philips EL84's that I had purchased. It worked fine with JJ's. I later found out from what I believe was a reliable source that the JJ's and the Philips EL84's had different pin arrangements, and the Laney was designed to accept the JJ-style pin arrangement only.

    Maybe I was misinformed? Any tube gurus out there know the answer?
     
  5. fullerplast

    fullerplast Senior Member

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    You were misinformed. The pinouts are the same. Their ability to handle high voltages and currents are different, even though the spec sheets are the same. Recent JJ's have been more tolerant of high voltages in general.
     
  6. illinimax

    illinimax Gold Supporting Member

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    The tubes in question were in fact purchased from Lord Valve. The pinout as I understand from a recent thread on Ted Weber's board is that the Ei EL84s are similar to many NOS tubes in that the pinout is different than most modern EL84s. They work fine in some amps and in others they cause big problems such as the one I experienced.
     
  7. fullerplast

    fullerplast Senior Member

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    The EL84/6BQ5 has the following pinout:

    2- grid
    3- internally jumped to both cathode & suppressor
    4 & 5- heaters
    7- plate
    9- screen

    These are the only pins that are guaranteed for any EL84 tube.

    1, 6, & 8 are described as internally connected. There are no guarantees for these pins and that is where some amp designs run into trouble.

    If some EL84's don't work (outside of exceeding voltage and current limits), it's an amp design issue, not a tube problem. The designer made some bad assumptions.

    On many EL84's, both NOS and new, pins 1 and 2 are connected internally. If an amp uses that pin as a convenient "tie point" (as Laney does) for something like the screen resistor, that connection will not be floating with many tubes.

    This means that many EL84/6BQ5's will not work in a Laney unless it's modified. This is because of false assumptions made about the unused internally connected pins. The fix is to move that tie point to another place that is definately not connected to anything.
     
  8. hipfan

    hipfan Member

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    Yes! That makes sense. Thanks for the explanation, fullerplast.
     
  9. Roccaforte Amps

    Roccaforte Amps Member

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    Take out what resistor?????

    The EI EL84's run hotter than all the others,
    you will see the plates glow in cathode bias amplifiers.
    The solution is to rebias these tubes to the correct idle
    current for the particular circuit.

    I use the JJ EL84 for everything, best EL84 out there.
    Doug





    PS, Fullerplast makes a good point.
    Some old AC30's will have pins
    1+2 connected together by the grid stop
    resistor. With some modern tubes like
    the Russian EL84, they used pin 1
    for a plate support rod.
    If you stick this into one of these AC30's
    that has pins 1+2 connected this way
    you'll have a direct short. I've seen it happen.
     
  10. TheAmpNerd

    TheAmpNerd Member

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    So Doug,

    If you have one of these AC30s, I guess you just separate pins 1 & 2 and make sure the grid stoppers go to pin 2, eh?

    Then this should let you plug just about any EL84 in the amp.

    On the AC30 thing, Kevin O'Connor discusses screen and grid stopper fixes in the amp by raising the values to 1K for the screens and 2.2K for the grids (TUT3).

    Then he says to compensate in the preamp section by using a small bypass cap.

    Have you tried this? Does it work?



     
  11. Roccaforte Amps

    Roccaforte Amps Member

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    Yes and no. Depending on the amp and voltages,
    1K screen resistors can take away some tone.
    What I do is very the value to reach a point
    that lightens up on the tube, and yeild
    good tone.
    Increasing the grid stop resistors is an excellent
    idea, 5-10K is better.
    Doug
     

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