In Search Of a simple 18 Watt Schematic

Discussion in 'The Squawk Box' started by Cygnus X1, Jan 20, 2011.

  1. Cygnus X1

    Cygnus X1 Senior Member

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    Trying to save a little design work.

    Looking for a schem for a 2xEL84 18 watt TMB (no tremolo), with just a single channel.

    Master volume would be good as well.

    I have a transformer set from an RCA console I am thinking about converting.
    Current tube compliment is 5Y3, 2x12AX7, and 2xEL84.

    I've looked around the usual sites and have only found two channel versions.
    Thought someone might know about something already designed that I could save some time with.

    Thanks!
     
  2. nwobhm

    nwobhm Senior Member

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    Aren't most of the 2 channel ones, just splitting V1? Instead of 2 channels, why not make the first gain stage a dual triode, like the high input on the normal channel on a 18watt or LiteIIB. I did one like that years ago, with a master volume. Ceriatone used to have a layout for one with an ef86 channel and a 2nd channel wired up like a dual triode. You can use that as a reference, just leave off the ef86 channel. That was years ago so, I don't know if it would still be there though? ***edit I didn't notice, you had 2x12ax7's. The one I was talking about used 3.
     
  3. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Just leave out the second channel. If the two channel schematic uses mix resistors and goes into one side of the PI, just omit the second channel. If the two channels go into opposite sides of the PI, just omit the second channel and have the now unused side of the PI go to ground through a .1uf cap.
     
  4. Cygnus X1

    Cygnus X1 Senior Member

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    First channel is vol/tone, second is TMB.
    I glanced at the Ceriatone and my tired brain said "whu"?
    Vol 1 runs to V2, Vol 2 runs to V1, and an appearance of a cross-mash in between.
    I'm really trying to cut down on one of the 12AX7's here.

    :laugh2:

    Never try to study when tired...time for coffee!

    :shock::shock::shock::shock::shock:
     
  5. Lyle Caldwell

    Lyle Caldwell Senior Member

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  6. Cygnus X1

    Cygnus X1 Senior Member

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    Yes.
    P1 extreme and Hi Octane don't match my idea of the PP EL 84.

    But tying in only the P1 preamp might be the ticket.
    Hmmm.
     
  7. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Haven't looked at the Ceriatone but it sounds like a cascaded input.

    Roll your own.

    Just take the 50W Plexi schematic and adapt it to a cathode biased 2 EL84 output stage (you can pretty much use the AC15 output section adding in the 50W Plexi's negative feedback and presence control instead of the Vox "Cut" control).

    If you don't want the dark/bright inputs of the Plexi just run V1 in parallel and omit the unused input/volume section.
     
  8. JakeM

    JakeM Senior Member

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    Man Lyle you are FULL of info, I havnt seen you post a reply that hasn't helped someone out...fair dues and nice work

    good luck cyg, I was looking for a kit that was an 18watt marshall with 1input and tmb and a master volume. I gave up after a while.
    jake
     
  9. Cygnus X1

    Cygnus X1 Senior Member

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    That was my first impression.
    Then the fog overtook me.

    I'm somewhere halfway between a "paint by numbers" guy and a "designer".
    I'm just way too familiar with the Fender circuits and my own Cygnus-7 (SEL).

    Thank you Lyle, keep it rolling...
     
  10. moldymealworm

    moldymealworm Senior Member

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  11. Cygnus X1

    Cygnus X1 Senior Member

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    Great minds think alike...I have already built it.
    I played it for about a year and then tweaked it literally to its demise.
    I was trying to get rid of a hum I couldn't track down, and she is sitting on the far corner of my "bench of doom".

    :shock:

    Wait, I found something.

    What do you all (y'all) think of this?
    Not TMB, and not really Spitfire.
    MV should be easy enough to throw in.
     

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  12. Lyle Caldwell

    Lyle Caldwell Senior Member

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    That's not ideal at least as shown. No grid resistor on the input, no presence control, I think that's wide of the mark.

    Thanks viva-la-punk!
     
  13. NSCS

    NSCS Premium Member

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  14. Cygnus X1

    Cygnus X1 Senior Member

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    True, and I think I want more gain.
    P1 preamp married to an EL84 P-P seems to be the mark.

    I had a heck of a time getting a tone I liked out of the Spitfire...it sounded too much like a thin saturated AC15, not a good sound to me.
    It spooked me until I got the EL84's down to realistic operating specs.
    I think it went from some 360 volts on B+ to around 325.
    Closer to their design.
    I read the filament design of the EZ81 may have been the source of the hum issue.
    It needs the filament separated from the rest of the tubes?

    For all that I'd rather just go with a diode setup to avoid the headache.
    But then I'd have some trouble getting the B+ where it needs to be, keeping in mind I'm using old iron.
    Might stick with the 5Y3.
     
  15. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Yeah, no need for the hassle, heat, and current requirements of a tube rectifier. Diodes for this amp.

    And I really recommend a 10H choke.
     
  16. moldymealworm

    moldymealworm Senior Member

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    I think you're selling yourself short here...
     
  17. Cygnus X1

    Cygnus X1 Senior Member

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    I do like that idea, but there is no way a power transformer designed for only four tubes could support all of that current.
    And my planned chassis is only 13.5x5x2 inches.
     
  18. Cygnus X1

    Cygnus X1 Senior Member

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    :laugh2:
     
  19. Cygnus X1

    Cygnus X1 Senior Member

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    Good point on the choke.
    It would thicken the tone in the recipe.
    I have a 9H/120ma kicking around, which should be perfect.

    But without looking, don't diodes have minimal voltage drop?
    The B+ would be whatever the PT puts out wouldn't it?
    That's why I was thinking about staying with the 5Y3 that ran with that RCA transformer.
     
  20. NSCS

    NSCS Premium Member

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    It is only 4 tubes, aside from the optional tube rec of course. 2x 12ax7 and 2x EL84
     

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