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Additional 12AX7s

Discussion in 'The Workbench' started by ShetlandIslandRocker936, Jun 20, 2021.

  1. ShetlandIslandRocker936

    ShetlandIslandRocker936 New Member

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    Hi guys.

    Was wondering if anyone has tried adding more than 1 additional 12AX7 into a Superlead circuit, would this stress the transformer too much?

    Currently in the process of adding a buffered effects loop & was considering a tube reverb circuit too.

    Cheers.
     
  2. 351

    351 Member

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    One preamp wont stress transformer.
    Tube reverb however means what, 2 more tubes, that's 3 in total.( 1 additional amp)
    what you could do is load it down and take a voltage reading of filaments, if its under 6.15vac I'd say no.
    I can tell you right now putting a fender style reverb into a superlead will make it sound like garbage.
     
  3. mickeydg5

    mickeydg5 Well-Known Member

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    Transformers can handle more than the shown rating but do not go to far for safety sake.
    The filament/heater circuit is what needs attention.
    Most Marshall 100 watters have PT rated at 7A for heater. So adding a tube or so will not kill it but if you want more than that go higher amperage to suffice.
     
    Last edited: Jun 23, 2021
  4. NewReligion

    NewReligion Well-Known Member Double Platinum Supporting Member VIP Member

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    I too would pass on the tube reverb.

    BUT, just add a 12/14VAC transformer inside the Chassis rectified to 12 or 6 VDC and heat the 2 addition stages with this. In fact you can safely pick up a 3rd stage.

    Outside of the box. Just my $.02

    David Hopkins
     
  5. ShetlandIslandRocker936

    ShetlandIslandRocker936 New Member

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    Thanks for the advice, much appreciated!

    Is there any reverb circuits you'd recommend for a superlead? I did think about trying copy the 2959's but maybe a solid state circuit would be better?
     
  6. 351

    351 Member

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    Lets keep thinking a bit more outside the box David, with todays precise transformer winding there is no reason DC cant be used on the whole amp for filament voltage, but they are still stuck in the year 1946. In a marshall 1987 you would need 4.8 amps of dc which if the power transformer had a precisely wound filament winding would need no regulator just diodes and say a 10,000uf cap and you would have precisely 6.3v along the rail.
    Or if they insisted on AC for the output tubes, have a 1A transformer for preamp tubes again wound precisely for diodes a cap and no regulator which if you ask me is the weak link.
    10,000 uf 20v caps cost 1.50$ each. - Retail
     
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  7. NewReligion

    NewReligion Well-Known Member Double Platinum Supporting Member VIP Member

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    Yes. It is a simple solution IMO. But can be done with the existing circuit at little cost.

    There are many ways to skin the cat.

    David
     
  8. XTRXTR

    XTRXTR Active Member

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    Remember to make rectified 1A DC you need 2A AC of your winding
     
  9. Matthews Guitars

    Matthews Guitars Well-Known Member

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    If I were to want reverb in a Marshall, I would definitely go all out and install a Fender reverb circuit in it which takes TWO preamp tubes, one a 12AT7. But that's quite a mod on a Marshall, so I'd just not do it. I have YET to hear a tube reverb circuit that uses a single tube for both drive and recovery that sounded good enough to satisfy me.

    I think we know this: Marshalls are great sounding amps but Marshall has never matched Fender for quality of reverb or vibrato circuits.
     
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  10. Matthews Guitars

    Matthews Guitars Well-Known Member

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    And as for making use of an added preamp tube, don't think you'd have to use both triodes in a 12AX7 if you choose to go with the added gain stage. Just one half of the tube has the capacity to give you PLENTY of additional gain. Using both halves might not be beneficial.

    Although you could certainly have all kinds of fun experimenting. Let me think of a few possible topologies....

    Gain stage plus cathode follower.

    Strapped parallel triode gain stage like a reverb driver.

    Gain stages inserted in two different parts of the circuit. (Pre and post tone stack)

    One triode wired as a saturator, the other wired as a cold clipper, each with its own level control and fed into a mixer feeding the next stage in the amp.

    If you want to experiment, have fun with it. Be creative. Don't settle for one circuit implementation until you've tried as many as you can think of.
     
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  11. NewReligion

    NewReligion Well-Known Member Double Platinum Supporting Member VIP Member

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    Great advice.

    I am finishing up my 4 & 5 High Gain stage Marshall. I am building loop boards from Headfirst Amplification now which are consistent with Friedman loops which are a logical reversal of George Metropoulos’s Zeto Loss Loop.

    In addition a coring circuit is not necessary but may be included as will a Todd Langner type compression mod on a switch.

    I will beta the amp a bit.
    I put up the 5 stage before. Seemed to please the ears but I took it down.

    I would add a simple clean but guys like me buy an amp for a single tone. If I want clean I switch over to my Terry Shaffer ( Guitar Rocker ) built Fender 6G3. I put a passive loop in it. Absolutely beautiful. Think Chris Isaak’s Wicked Game.

    Sorry for the rant.

    David
     
    Last edited: Jun 30, 2021
  12. SkyMonkey

    SkyMonkey Well-Known Member

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    Last edited: Jun 30, 2021

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