EL84's ma Draw Question.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Red Planet, Feb 16, 2006.


  1. Red Planet

    Red Planet Member

    Messages:
    4,356
    Joined:
    Jan 31, 2004
    Location:
    Atlanta
    I've seen talk before here and there about getting Low Draw Rated Tubes for your amp. That it might not be safe to go in the other direction.

    I would like to know the skinny on this?

    It seems to me in a Cathode Biased EL84 Amp if you got a set of tubes rated for say 27 ma when the amp biases itself the plate volts are gonna go up and that seems more dangerous to me. As apposed to say putting a set in rated at 44 ma the plate voltage is gonna go down.

    I just plugged in a set of EI EL84's rated at 44 into my 18 watter they sound great. I then plugged in a set of rated at 27 and they sound fine though out of some I purchased one went bad almost on the spot. I chunked it and grabbed another set rated at 27.

    I'm very curious about this if anyone who really knows can despell this for me.

    Thanx
     
  2. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    The thing about this is that you are only stating the rating-

    To really determine bias you need to do several measurements and make several calculations.

    1: As it's Cathode Biased, measure the Cathode Resistor and write it down as Rk.

    2: Locate screen resistor, pin 9 in an EL84. Measure it and write it down as Rs. (In most cases this is 100 Ohm resistor.)

    3: Turn amp on and let it warm up. When hot measure the voltage across Rk and write it down as Vk.

    4: Measure voltage across Rs and write it down as Vs.

    5: Measure voltage at pin 7 AND BE CAREFUL AS IT'S GONNA BE 300V OR ABOVE AND IT COULD BE LETHAL! Write it down as Vp.

    6:Turn amp off and get out your calculator, pen and paper.

    7: Figure Vk / Rx = Ik and write it down. (This is cathode current...)

    8: Figure Vs / Rs = Is and write it down. (This is screen current...)

    9: Figure Ik - Is = Ip and write it down. (This is plate current...)

    10: Figure Vp X Ip = IPD Idle Plate Dissipation and write it down.

    11: IPD for a cathode biased EL84 should not be above Max Plate dissipation which is 12W. If it is you need to change the cathode resistor to a higher value.

    Best I can do to give you a blow-by-blow of how it's down...

    -Jon
     
  3. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    Jon's analysis is good. In simpler terms: what you are doing when you hand select tubes by current in a cathode biased amp is essentially the same thing you are doing when you adjust the bias current in a fixed bias amp. You are "adjusting" the idle dissipation. Changing the cathode resistor is another way to do the same thing, but is usually more of a hassle. You'll find many EL84 amps exceed recommended maximums for the tubes.
     
  4. Red Planet

    Red Planet Member

    Messages:
    4,356
    Joined:
    Jan 31, 2004
    Location:
    Atlanta
    I see said the blind man. So I can just use my Bias Rite and figure it like I normally would. Its so simple I didnt see it. :jo
     
  5. Blue Strat

    Blue Strat Member

    Messages:
    29,974
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    What Jon said. Plus, you can't draw any conclusions (27mA, 44mA) from 2 pairs of tubes. One of your 27s probably would have failed regardless.

    Also, plate voltage tends to drop with tubes that have higher current draw, it doesn't increase.
     
  6. BozoTone

    BozoTone Member

    Messages:
    542
    Joined:
    Jan 29, 2005
    Location:
    Nashville, TN.
    Man, how did I miss this thread 1st time around...anyway, so why reccomend a low mA draw tube for an amp with high plate volts if a high mA draw tube will lower plate volts...seems like keeping plate volts low on an EL84 would be the ticket...does this effect the grid voltage the same way?
    BZT
     
  7. Blue Strat

    Blue Strat Member

    Messages:
    29,974
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    It's not plate volts that you're supposed to worry about. It's idle plate POWER (which is plate volts X Plate current). If the inherent idle current of the tubes (low or high current) is low, you're more likely to run within the maximum POWER spec of the tube than at 150% or higher which is often the case with EL84 "self biasing" amps.

    Note that "self biasing" is somewhat of a misnomer since amp "designers" choose cathode resistors which cause their amps to "self bias" way above the spec of the tubes. Why they do this I'll never understand. By increasing the value of the cathode resistor, you'd be able to use medium or high current tubes without them melting down.
     
  8. BozoTone

    BozoTone Member

    Messages:
    542
    Joined:
    Jan 29, 2005
    Location:
    Nashville, TN.
    Thanks Mike.....
    BZT
     
  9. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    What Mike said...

    Mike: I canna understand it either, lad!
    On the other hand I find Orange and Bad Cat examples to be biased at near Class A, but not excessive.

    Dr. Z Karmen Ghia always comes in a bit heavy, (usually about 12.88W if you take screen current into account.) Interestingly enough, Dr. owners really don't care how often they have to replace tubes! I guess if you have that kind of $ a pair of JJ EL84's is far less than the bar-tab.

    The JJ EL84's are all Very Close cathode and screen current here- (I get them from CE.) JJ E34L's and 6L6 are all over the map for these rating...
    Appears they can build an EL84 like the old-days when everything was about equal...

    see ya soon,
    Jon
     
  10. 59Vampire

    59Vampire Silver Supporting Member

    Messages:
    4,035
    Joined:
    Dec 17, 2005
  11. LesPaulPlayer

    LesPaulPlayer Member

    Messages:
    223
    Joined:
    Mar 9, 2004
    Location:
    Central CA
    I have a Budda Verbmaster and it runs as hot as the hottest of them out there. Ei EL84's just fry in there. They recommend a re-tube every 6 months. Would gaining tube life through the increased cathode resistor change the tone?
     
  12. dogg

    dogg Member

    Messages:
    35
    Joined:
    Sep 26, 2004
    Slightly - but maybe you like it?
    Try it with 100 Ohm 5W pot wired
    beetween Rk and ground and set bias 'by ear'.

    V.
     
  13. reaiken

    reaiken Member

    Messages:
    1,775
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    Perhaps most importantly, don't forget *screen* dissipation. Multiply the screen-to-cathode voltage by the screen current to get screen grid dissipation. Measure and calculate it both at idle and at full power.

    An EL84 screen grid is rated for 2W max and 300V max. Exceed these, and you are going to get screens that glow bright orange when you hit a chord, or even at idle if the dissipation is high enough. You can look through the holes in the plate to see the screen glow modulated by the signal. In my experience, this is usually what kills the EL84s the fastest, not the plate dissipation.

    The only solution is to either use a larger value screen grid resistor (you can go up to 2.5K or so for very high screen voltages) or use a lower screen voltage supply.

    Randall Aiken
     
  14. hasserl

    hasserl Member

    Messages:
    4,734
    Joined:
    Oct 24, 2004
    Location:
    So Cal
    I hate to disagree with one of the most highly respected amp designers and builders in the industry, but I work with many EL84 powered amps that run plate voltages in the 390's to low 400's, and screen voltages just a few below that. The set of tubes in my personal amp are old pulls from an old church organ with several decades of use on them befroe I got them, and I've been running them like this for over two years, using the amp at regular band practice and gigs, and the tubes are still fine. I've personally worked on and sent out to customer's all over the country dozens of amps running the smae voltages, all with excellent results. These amps are built by a larg manufacturer (Carvin) that has thousands of these amps all over the world.

    IME EL84's will take substantially more voltage at both the plates and the screens than the traditional rating, both old tubes and new tubes such as the Sovteks and JJ's; and sound fantastic doing it for a long, long time.

    If I was you I wouldn't worry about it. Play the amp and enjoy it.
     

Share This Page