407 Volts on EL84's

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by VanR, Jan 12, 2006.


  1. VanR

    VanR Senior Member

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    Pulled the chassis out of my H&K Tube 20 the other night and measured 407 volts on the plates of the EL84's (pin 7). Wow. What is that about? I was expecting maybe 350 or so. But 407? I guess the JJ EL84's I have in there won't last long at that voltage. Maybe I should order some of those Sovtek EL84M's or find some 7189's. What do you guys think. Should I worry or just be prepared to replace them every few months or so? Guess I need to order a bias tool and make sure the bad boys are biased correctly. They aren't redplating and sound great to tell you the truth.
     
  2. teefus

    teefus Supporting Member

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    did you check the current too?
     
  3. fullerplast

    fullerplast Senior Member

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    You need to subtract out the cathode voltage if it is cathode biased. The tube will need to be in place to measure that and you need to know if there is one or more cathode resistors between power tubes.
     
  4. VanR

    VanR Senior Member

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    The amp is adjustable bias not cathode biased. I have been meaning to buy a Weber Bias tool and replace the tubes and rebias. But I didn't expect to see the voltage that high at all. The tubes were in place and working when I did the measurement and I checked it about 3 times on each socket to make sure it was correct.
     
  5. hasserl

    hasserl Member

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    I have seen plenty of EL84's running in the low 400's, they seem to do just fine. I realize that is much higher than the specs call for, but these are tough little tubes. I wouldn't worry about it at this point, they haven't failed on you yet have they?
     
  6. VanR

    VanR Senior Member

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    No the JJ's seem to be quite happy in there. Sound great. I was just surprised at the plate voltage they were getting. They are just about 2 months old. I popped them in there in an emergency and never have had a chance to rebias them yet.
     
  7. Matt H

    Matt H Guest

    with higher plate voltages, the real questions you should be asking are plate dissipation and SCREEN voltage...

    lots of tubes can run WELL over Va spec if the screens are kept low. IIRC, there was a seymour duncan amp that ran something like 550V on the plates, but only 250 or so on the screens on el84's...

    also- adjustable bias vs cathode bias is hardly a valid comparison... cathode biased amps can be adjustable very easily... as can fixed biased amps... but that's another story that i'm sure has been discussed ad nauseum...
     
  8. Roccaforte Amps

    Roccaforte Amps Member

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    There is also a 500V Rivera EL84 amp, and the Crate Vintage Club 50 is 500V. However, these are not cathode biased amplifiers.
    There are many old British made EL84 amplifiers that run EL84's hard,
    vintage Marshall Lead 20's are over 400V, cathode biased.

    Just goes to show you that published maximum ratings are in no way exactlly what a tube can withstand.
     
  9. fullerplast

    fullerplast Senior Member

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    It's pretty interesting to see the range of recommendations....you have a group that strongly recommends operating tubes at no more than 60-70% of published operating maximum specs and you have a group that basically ignores the specs and runs tubes at whatever they will actually handle, specs be damned.

    Here's a case in point where the amp was designed to run at over 135% of specified maximum ratings! :eek: You can't just go out and plug in any tube with an amp like this. You need to search out the brands and maybe even batches that will handle the abuse. You also end up replacing tubes at a very rapid rate.

    I understand the tone tradeoff, and I do have amps that run over spec too, but I actually can't think of too many other new consumer products that encourage design beyond maximum component ratings like this....:confused:
     
  10. Roccaforte Amps

    Roccaforte Amps Member

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    Nobody is encouraging anyone to do anything, people are free to make their own choices, and design amplifiers anyway they seem fit.
    I do agree with you, you can just run any tube in any circuit.
    There are many NOS tubes that aren't worth a crap in most guitar amplifiers, and if you don't know which ones they are, you'll
    be setting yourself up for a big dissapointment.
    For example, there are quite a few NOS 6V6 tubes that won't
    handle over 350V for very long, yet there are others that will.
    Ask your tech what to buy, it can save you in the long run.
     
  11. fullerplast

    fullerplast Senior Member

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    I think that indirectly, designers are being encouraged, especially in the case of EL84 amps, simply because they sound better running hot. They're encouraged by the marketplace and competing amp designs. I just find it interesting that I can't really think of another consumer product that is designed so far out of spec on purpose.:cool:
     
  12. Roccaforte Amps

    Roccaforte Amps Member

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    Oh, I understand what you're saying now.
    But then again, what is really out of spec here?
    There really isn't any set of specs for tube guitar amplifier building.
    Tube manuals in the past had the design maximum
    ratings lowered on purpose to insure few tube failures,
    I've been told this more than once by old tube
    engineers, and it certainly appears to be true.
     
  13. fullerplast

    fullerplast Senior Member

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    Yeah, I'm just saying that the designs are running the components beyond their recommended ratings....kind of like if somebody used .25 watt resistors running at .4 watts because they sounded better than using half watt resistors. That kind of thing is generally frowned upon for consumer products especially, just because of warranty and safety issues. That's all I'm really saying....

    The problem with the tube specs is that not everybody has the same safety margin built in. So we get all the old Sovtek 6V6 fireworks in Deluxe Reverbs, where RCA's work fine. The Sovteks probably met the spec, but just barely. It makes it more complicated for the end user because the parts are no longer generic. I think we are both in agreement, I'm just thinking out loud....:cool:
     
  14. pepi

    pepi Member

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    My 18 watter runs at 406.
     
  15. jblues

    jblues Guest

    hahaha guitar amps that engineers designed were never meant to be played distorted, they expected you to buy a bigger amp, and play clean!
    so.... we're playing these amps out of spec, when you turn the volume up to 8 say , it starts to saturate so you were expected to turn down, because they figured..."no one plays on ten!only 5 and a half or six."
    just watch the plates forthe cherry red color or it's over....
     

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