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Dual triode - can both sides be run in parallel ?

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  • HaroldBrooks
    replied
    Originally posted by Helmholtz View Post
    Good idea!

    A cascode is another way of wiring 2 triodes is series. Achievable gain with the 6SN7 is around 100 (a little more than with the ECC82 in Merlin's example), using a plate resistor of 67k @ Ip=1.9mA.
    Thanks for figuring that out Helmholtz, I rely on you to be the brains behind whatever low level "Experimental" stuff I am doing at this point !

    By the way, part of my mixed heritage is from Germany, and my Father spoke fluent German and was a big clock repair man and fabricator, amongst other hi-level things. Hope to visit Germany in the not to distant future !

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  • HaroldBrooks
    replied
    Originally posted by Bloomfield View Post
    Another option to get more gain with a 6SN7 would be to set it up as a cascode. Never tried it myself but there is some information here:

    http://www.valvewizard.co.uk/cascode.html
    Thanks Bloomfield. I think I am going to try this and see how it works out, but on one of my "rolling test lab" hi-fi chassis amps first. I have a few that run 6J5 tubes in V1, so I believe I can sub a 6SN7 in the same Octal Socket, and rewire the whole thing to work as a cascade. This wouldn't breach the integrity (if there is any left !) of the initial Zenith Hi-Fi design of the amp in question, as it is my understanding that a 6SN7 is basically two 6J5 tube in one glass envelope, so it should work out nicely, as long as I adhere to the adjustments necessary for the Cascode.

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  • Helmholtz
    replied
    Originally posted by Bloomfield View Post
    Another option to get more gain with a 6SN7 would be to set it up as a cascode. Never tried it myself but there is some information here:

    http://www.valvewizard.co.uk/cascode.html
    Good idea!

    A cascode is another way of wiring 2 triodes is series. Achievable gain with the 6SN7 is around 100 (a little more than with the ECC82 in Merlin's example), using a plate resistor of 67k @ Ip=1.9mA.

    Leave a comment:


  • Mick Bailey
    replied
    Doug Hammond's Firefly uses a cascode front end and it works superbly.

    Leave a comment:


  • HaroldBrooks
    replied
    Originally posted by Bloomfield View Post
    Another option to get more gain with a 6SN7 would be to set it up as a cascode. Never tried it myself but there is some information here:

    http://www.valvewizard.co.uk/cascode.html
    Nice ! Always learn something new here !

    Leave a comment:


  • Bloomfield
    replied
    Another option to get more gain with a 6SN7 would be to set it up as a cascode. Never tried it myself but there is some information here:

    http://www.valvewizard.co.uk/cascode.html

    Leave a comment:


  • Helmholtz
    replied
    I'd thing the effects are the same, regardless of which dual triode he is using.
    The principles yes, but not the results. Clipping characteristics and clean headroom strongly depend on input stage gain. And the different triodes vastly differ in gain (G).

    6SN7 : single triode G=14, 2 triodes in parallel G=16, 2 triodes in series G=196 max.
    12AX7: single triode G=57, 2 triodes in parallel G=76, 2 triodes in series G=3,249 max.

    Generally 2 triodes in series introduce additional odd harmonics as the signal is inverted twice.

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  • Justin Thomas
    replied
    I'd thing the effects are the same, regardless of which dual triode he is using. As in, more than a single by itself but less than in series. I'm not expecting it to compete with a 12AX7.

    Justin

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  • Helmholtz
    replied
    a parallel 12AX7 was juuuuuuuust right. The two stages cascaded resulted in a great RAWK amp, but with no clean headroom.
    But he is using a very low 6SN7, resulting in less than 1/4 the gain of a 12AX7. Gain values with Ra = 100k and bypassed cathode resistor 6SN7: G=14, 12AX7: G=60.
    With the two 6SN7 triodes in parallel gain will increase from 14 to 16 or by 1.16dB.
    Last edited by Helmholtz; 11-29-2019, 03:50 PM.

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  • Justin Thomas
    replied
    I thought the parallel triode input stage was more useful for "reducing" noise than boosting gain, or at least that was the primary intended effect. In my experiences there were times when a single triode wasn't enough, an ef86 was too much, & a parallel 12AX7 was juuuuuuuust right. The two stages cascaded resulted in a great RAWK amp, but with no clean headroom.

    Note: I build simple amps. I usually don't have room to add more controls, so switching from a one-knob tone stack or adding extra volume controls aren't an option.

    Wouldn't hurt anytning to try; just twiddle cathode & plate resistors to match.

    Justin

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  • Helmholtz
    replied
    Originally posted by the fatch View Post
    In the usual Fender input circuit, the single 12AX7 triode has a gain of around 60. According to the Valve Wizard, the two 12AX7 triodes wired in parallel in the Matchless Lightning input stage achieve a signal swing of 295V p-p and gain of 76, an improvement of just+2dB over the single triode. https://el34world.com/charts/Schemat..._lightning.pdf
    True and the extra dB gain with the 6SN7 triodes in parallel would be even less as this is already a very low impedance (Ra) tube.

    OTOH using a single 12AX7 would increase gain from 14 to 60 corresponding to +13dB.
    Last edited by Helmholtz; 11-29-2019, 03:01 PM.

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  • HaroldBrooks
    replied
    Originally posted by Mick Bailey View Post
    Cascading the triodes and using a dual volume pot works well. My main amp has this arrangement and will go from really clean to more than enough gain off a single knob.
    Not sure if it's the same thing, but someone in the distant past took the three inputs and assigned one to one side of the 6SN7, and the other two to the other side of the tube. They also have two volume knobs on the amp, so I can jump between two inputs and get both channels working, it just requires that extra plug. It sounds beefier, but not necessarily better for this amp and what I need, so there you go.

    I will try the 12AX7 adapter and I will order one and let you guys know how it worked out. I can tell you I did that very thing with a Zenith Hi-Fi amp I converted, that is replace the 6J5 week tube in that amp with a 12AX7 using an adapter, and I actually liked the 6J5 better ! running a boost in front sounded better with the older and weaker tube, who would have guessed. Just trying and exploring a lot of things, so then I know how it worked out rather than speculating based on the pure maths of the change. That's useful, but your ear is the final judge of the outcome.

    Leave a comment:


  • HaroldBrooks
    replied
    Wow, crazy how little you gain, but that's why you don't see it that often, and that's why they came up with more powerful tubes like the 6J7, 6SQ7, and the newer noval 12AX7.

    I think I am going to back off on doing the parallel trick because the net change is minimal. I may in the near future get an adapter to mate a 12ax7 to the 6sn7 socket, and I am sure one is out there. Then I can test the effect of a higher preamp gain on the circuit without committing to rewiring everything, and changing the plate and cathode resistors to match the new tube better.

    Leave a comment:


  • Mick Bailey
    replied
    Cascading the triodes and using a dual volume pot works well. My main amp has this arrangement and will go from really clean to more than enough gain off a single knob.

    Leave a comment:


  • the fatch
    replied
    In the usual Fender input circuit, the single 12AX7 triode has a gain of around 60. According to the Valve Wizard, the two 12AX7 triodes wired in parallel in the Matchless Lightning input stage achieve a signal swing of 295V p-p and gain of 76, an improvement of just+2dB over the single triode. https://el34world.com/charts/Schemat..._lightning.pdf

    Leave a comment:

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