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Thread: Using EL86 with EL84 OT

  1. #1
    Junior Member
    Join Date
    Apr 2017

    Using EL86 with EL84 OT

    Hi all... So, if I modify all the voltages in the circut to fit the EL86 but I don't have enough money for the new OT, can I use the EL86 with the old EL84 OT from the late 50's, and if I can, what would i have to do with the bias section and how should I adjust the bias so that the tube(EL86) handles a little higher EL84 plate voltage in the circut (don't worry for the screen grid, just bigger resistor) and that transformer that was not meant for that tube? Any opinions and posts would be very useful.

  2. #2
    Supporting Member Chuck H's Avatar
    Join Date
    May 2007
    pacific north west
    A brief glance at the datasheet suggests that the el86 is better with lower voltage, higher current and a lower Raa (primary impedance). Power is about the same so that's not an issue. You would need to know (or discern) the primary of the OT in question. And how much "higher voltage" are we talking about??? Further, a bigger resistor for the screen feed in the HV rail is "ok" but not ideal. You can quickly pigeon hole yourself into a very specific (and saggy) tonal character. This assumes you are building a guitar amp.
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

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    "Shut up, you big dumb poopy-head!" Justin Thomas

  3. #3
    Senior Member
    Join Date
    Jul 2011
    Adelaide, South Oz
    What Chuck said.
    See the Class AB Push Pull specs on this datasheet:

    The higher voltage (than +250V recommended for EL86) would require a higher Raa OT so the (almost certainly) 8K Raa for EL84 may be fine.

    Set bias for the idle current to be less than what the data sheet says by the ratio of the B+ you have with respect to 250V.

    Chuck H likes this.

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