What EL84 type can survive

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by DonP, Oct 20, 2019.

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  1. DonP

    DonP Member

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    448 volts on the plates
    398 on the screen grids
    bias at -17.91 volts.

    I don't have an easy way to check the current as the cathodes go to ground, but I plan on soldering in some 1 ohm resistors.

    This is in a Lafayette LA-350. Specs call for 7189A's which are hard to find. I have a new quad of GT EL84 tubes (no M) that I bought in the mid-90s. They have a 6 rating. I don't want to kill these new tubes, so what tube can take this beating?

    Thanks.
     
  2. oneblackened

    oneblackened Member

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    6P14P-EV (Sovtek EL84M/Tung-Sol EL84) and really that's about it. JJ, maybe, but doubtful.
     
  3. WillLane

    WillLane Member

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  4. SnidelyWhiplash

    SnidelyWhiplash Member

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    That's a lot of juice for an EL84 tube type. My guess is that it would end up as an instant firecracker.
     
  5. slider313

    slider313 Silver Supporting Member

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    It may be time to consider real 7189A's and save money in the long run.
     
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  6. DonP

    DonP Member

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    Been working with EL84's already. That's how I go the voltages. But I don't expect them to live for long at these voltages.
     
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  7. DonP

    DonP Member

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    I bought a used quad. 2 were good and 2 were worn. I was going to send them back today. Seller offered me 50% off. Maybe I should take it and try to find two more?

    I have a "slightly used" matched quad of gold grid 6P14P-EV coming in from the Eastern Block in 7 to 10 days.

    But I thought there was an EL84M the could take a beating.
     
  8. slider313

    slider313 Silver Supporting Member

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    It's not so much the plate voltage that reduces the life of the tube, it's the current; or not observing the maximum plate dissipation of 70% or less.

    The Sovtek EL84M may be an option but, regardless of which tubes you choose, you'll need to check the bias and observe the current draw.
     
  9. teefus

    teefus Silver Supporting Member

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    as mentioned, the russian military 6p14p-ev would be a good start.
     
  10. pdf64

    pdf64 Member

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    Just measure the resistance of each OT primary CT to plate leg, and then the Vdcs at idle across those resistances.
    To avoid the potential issue of oscillation messing up the readings (bear in mind these are high mu tubes, your probes and meter act as a transmitter mast for the plate circuit, and oscillation may be above audio range) take out the tube in the phase splitter socket, or at least recheck with that tube out.
     
  11. WillLane

    WillLane Member

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    Also it's fun to keep in mind a healthy AC30 often runs the EL84's past 100% dissipation.
     
  12. slider313

    slider313 Silver Supporting Member

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    The Vox AXC30 is cathode bias and runs about 330v on the plates. The amp in question is fixed bias, running 448v.
     
    Last edited: Oct 21, 2019
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  13. Pete Cage

    Pete Cage Member

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    The good news is that it's unlikely you'll be running a hi-fi amp into clipping, so it's not going to be quite as rough on the tubes as a guitar amp running deep into overdrive with the screen grids glowing orange...
     
  14. DonP

    DonP Member

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    Good point.
     
  15. ddog

    ddog Supporting Member

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    check perry's listing here...good guy,great tubes
     
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