6V6 to EL84 converter. Problem/question

Discussion in 'Amps and Cabs' started by ekkybedmond, May 24, 2015.

  1. ekkybedmond

    ekkybedmond Member

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    Hi there

    I bought a pair of these converters from Thomann, Europe. I have this Matchless Lightning replica, but with 6V6 output, instead of EL84.

    Converting this amp to EL84 with the adapters just does not work. With the GZ34 rectifier, I get 'some' sound/signal, with 5R4 rectifier, I get NO signal/sound.


    PV with 5R4 rectifier is around 400, with the GZ34 it will be higher (did not measure).Any clues on why this does not work. Something to do with the fact the Lightning is not originaly a 6V6 amp, though it works perfectly with the 6V6.


    Try a 6L6 to EL84-converter...?Thxs for any thoughts..
     
  2. swiveltung

    swiveltung Member

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    I made my own EL84 adaptors once, it didn't work either. Not sure why. I thought maybe I didn't get the pin outs right, but couldn't find that. Now you are confirming my problem...
     
  3. HughesP

    HughesP Member

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    There's a whole bunch of variables that could do this, but these would be my guesses. I've never opened up an "adapter" to see how the actually work, but there's lots of assumptions that would have to be made with creating one which might not be true with your amp.

    Possibility 1:

    If the adapter is simply connecting similar pins (no resisters/circuitry in the adapter), then your amp is trying to bias el84s with the resistor value intended for 6v6 tubes. If that is the case, it may be that your cathode bias resistor on the 6v6 tubes (which could be around 470ohms?) is creating REALLY REALLY cold bias - which in a cathode bias arrangement could make the output super low.

    Possibility 2:

    If the adapter was intended for swapping from a 6v6 fixed bias amp to convert it to el84 cathode bias, then by using that kind of adpater in a 6v6 cathode bias amp, you are creating the same situation as possibility 1, except you are now making it even worse - probably adding an additional 125-150ohms on top of the 470ohm.

    My guess, based off the fact that you get SOME sound with high plate voltages (which is a result of changing the bias point), that you may be experiencing one of those things. Of course there are other possibilities too:

    Possibility 3:

    Seeing as you clearly have a rather unique homebrew amp, it is possible that an unused pin from the 6V6 is connected to something in the amp - perhaps a tie in point for a screen or grid resistor. If the adapter is expecting that pin to actually be unused, or expecting to see it grounded, it may create a problem of its own. For all you know, your adpater may be grounding the grid or screens, which would certainly create problems!

    Those would only be my guesses though!
     
  4. cochese

    cochese Supporting Member

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    I'm not sure which model of converters you have. THD originally made Yellow Jackets years back but they have several models. For fixed bias amps and for cathode bias amps. They also have ones that don't drop the power at all which is usually recommended for a low power amp like the Lightning. The ones for Cathode Bias amps have a ground wire for cathode bias amps. http://www.yellowjacketstc.com/yellow_jacket/yjc If you go got the TAD version I'm not sure if those have the ability to work with a cathode bias amp.
     
  5. ekkybedmond

    ekkybedmond Member

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    Thanks Chris and HughesP, I'm getting a clearer picture now.

    The homebrew amp (well, just the Lightning schematic adjusted for 6V6) is fixed bias.

    VHT offer one version of the adapter, whether that is a full-power or a diminished power version, I can't tell. Hope VHT responds to a quiry I sent them.
    THD offers indeed a version which just adjusts for PINS, no voltage or current loss. Curious if they offer a slim-version, the regulars will make the EL84 stick very much out, in the direction of the speaker (mine's a combo).

    I have a PV tester in EL84 format, but no mA tester. Tested with a regular 8-PIN mA tester before the converter, the reading was 0, but that be due to the different pin-out/convension. The sound is a weak distorted rattle.
     
    Last edited: May 25, 2015
  6. cochese

    cochese Supporting Member

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    The Lighting is fixed bias? The Matchless were cathode bias. I've played through original Lightnings. I owned a Top Hat Club Deluxe that my tech said is basically a tweaked Lightning which is a tweaked AC15 style amp. The Club Deluxe had plenty of chime with the 6V6's. The converters are a quick solution but if that amp is really fixed bias you might want to get that changed to cathode bias with no negative feedback. It's more than just the cathode bias. My tech modded a Fender Deluxe Reverb for me with a switch to go from fixed to cathode bias no negative feedback and that makes the amp very aggressive and snarly as opted to the tame fixed bias sound.
     

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