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THD Yellowjacket theory?

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  • THD Yellowjacket theory?

    Hello again,

    I'd like to use an EL84/5881 combo in my amplifier, but my problem is a 450V plate voltage. The THD website claims EL84 conversion in amps with a plate voltage 'under 520VDC', but I'm skint and not willing to pay $100 for a pair .

    TUT4, 6-32 explains the yellow jackets. I understand the pin conversion, cathode resistor and bypass cap, 'coupling cap to block the control-grid bias voltage while coupling the audio signal to the EL-84', and grid leak resistor to ground. What I don't understand is how its able to tank an EL84 to 520VDC when they're rated for a maximum of 300V plate voltage! (although I've heard of cathode biased EL84's in AB @ low 400V)

    Can anybody shed some light on this? Is there an easy way to drop the plate voltage for a single tube? Something I've missed?


  • #2
    First, some voltage is dropped over the cathode resistor. Second, datasheet voltage ratings are *very* conservative. Fender and many others routinely run power tubes way over their specification for many decades and it seems to work OK. Third, some modern production tubes like JJ EL84s are known to stand up to much higher voltages than their NOS equivalents.

    520V sounds a bit much, but if they say it works, it probably does (although the tubes may wear out a bit quicker than with lower voltages).


    • #3
      im not familiar with the yellow jacket, but it sounds like it might be a socket that you plug into a octal socket for another tube (6v6?) and it lets you plug the 9pin el84 into the amp. if this is the case there is quite possibly a dropping resistor in there that drops to voltage to something more suited to an el84. i could be wrong, and that yellow jackets may bve something completely different.


      • #4
        There are various different Yellow jacket adapters. They are used to switch between different kinds of power tubes. The most commonly used is probably the octal (EL34, 6L6...) to EL84 adapter.

        It maps the pins to the EL84 tube and inserts a cathode resistor (so no bias adjustment is necessary). It also inserts a coupling cap from the grid to block out the amps regular bias voltage. Very simple and rather clever.


        • #5
          Tube data sheets are like speed limits on highways, you are not supposed to pass them, but they car doesn't blow up when you do.

          Tube specs are what the tube maker suggests for long tube life and reliable service in consumer goods. They were for things like table radios and TV sets, and they expected tubes to last a long time. Dad was not going to be happy changing his TV tubes once a year all the time. We however beat the crap out of tubes and expect to change them at least yearly. At 300v sitting in a radio, that EL84 should last a nice long time. At 400v in a guitar amp, it lasts... long enough.
          Education is what you're left with after you have forgotten what you have learned.


          • #6
            I run a HRDx with EL84s using the Groove Tube SubstiTube adapters - same thing as Yellowjackets except tougher construction in the GTs. I biased my amp right before installing them and tested it right after and it was dead on with no change on the bias. What it did to the amp is awesome. I like the tone MUCH better and volume of it much better now and can actually crank it up a little in clubs without having sound techs coming after me with pitchforks and torches.

            Between using the adapters, good vintage GE preamp tubes and a much better speaker (80s era 100w Carvin British Series), the normally boxy & farty bass heavy HRDx is a much better balanced amp now. The cleans are still very Fenderish, but the drive went from Fender-fizz to a cross between early Vox/Marshall overdrive. I'm loving it now, and all without touching a soldering iron to the somewhat finicky PCB.

            Check craigslist and ebay for Substitubes or Yellowjackets, they turn up used regularly. I got mine including EL84 JJs for $45 for the pair.

            - JJG
            My Momma always said, Stultus est sicut stultus facit


            • #7
              Thanks for the replies. Silly tube data sheets. So whats the general consensus of running EL84s @ 450V? And regarding the dropping resistor that black_labb mentioned, is it as simple as a resistor in series with the Va and the tube plate?

              JJG, I'm all the way over in New Zealand, but I'll have a squiz for some international auctions on ebay.