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Mixed Bias designs, anybody?

Discussion in 'Amp Central Station' started by CoolBlueGlow, Mar 5, 2012.

  1. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Yep, got that...thx dgr888!

    cbg
     
  2. dgr888

    dgr888 Tele-Meister

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    By the way Mesa used 34's and 6l6's in their simulclass circuit and egnater uses 6v6's and 84's that can be blended or used in pairs in one of his amps. So it's not a new idea..You can get some ideas flowing by checking out those schematics to get your juices flowing and by all means dig in and get after it. It's nice to see someone trying to accomplish something different....
     
    Last edited: Mar 8, 2012
  3. teleamp

    teleamp Friend of Leo's

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    I think the London Power Studio has option of mixing power tubes and bias type and can even be run in SE operation.
     
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  5. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Thanks guys and gals, Yes - I've seen the Power Studio. Didn't know about the Egnater.

    Dang, there's a lot of good amps out there these days!

    CBG
     
  6. StephaninMelb

    StephaninMelb Banned

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    http://www.egnateramps.com/manuals/Rebel30Head.pdf

    Yes Egnater Rebel 30 Head uses mixed pairs of EL84 and 6V6 with mix control on front panel. Output power runs @ 30W with 50/50 blend of both tube types. With control fully over to single tube type side maximum power output is 20W. Power can also be set on an individual channel basis from 1W to 30W. This interesting statement is in the Rebel 30 manual about power tube tone, since they are one of the few amps that run concurrently two power tube types:

     

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  7. celeste

    celeste Friend of Leo's

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    CBG, no need to apologize, in fact you gained some major cool points for knowing what a celeste is. Usually when I tell people "unlike my musical counterpart, I can be both sharp and flat simultaneously" I get a blank stare and maybe a blink or two. Too many layers of knowledge and too many double meanings.

    So, back to the question at hand. You want something weird, whacky and wonderful. Three of my favorite W words. How about 4 of your 6X8's, pentodes wired as parallel push-pull, maybe 6 to get the plate impedance down to a manageable level. A quick swag for reflected impedance is 26K for 4 and 18 for 6. One pair is fixed bias with 250v on the anode, control the current with judicious use of negative bias, deep class Ab or even pure B. The other has 250v on the plate as well, but burn up 50-100v in a cathode resistor, bias it with lots of current, deep class A. Two triodes for a LTPPI, the others for a preamp You won't need much gain as the power tubes will be so easy to drive. The class A side, see if you can get away with direct coupling them to the PI. A hammond 125 series universal OT will get you the need impedances.

    Other wilderness, you can also get your impedance by using a 16 ohm speaker on a 4 ohm tap of a 6.6K to 8k OT. If you get one with UL taps, use them to supply the anode voltage to the LTPPI. If you are using a UL OT and 6 output tubes, run the third pair in UL. If don't have a UL tap, try running the screens from a high voltage with a large screen resistor, connect the LPTPI plates to the screens.
     
  8. StephaninMelb

    StephaninMelb Banned

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    Fortin Amps Denmark NATAS four power tubes have individual bias and individual test points on the back panel.

    Take a look at this interior shot. Talk about quality components. Take a look at the output transformer!
     

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  9. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Hi Celeste,

    haha, yeah, I get it...I GET it. :)

    re: 6X8 monster. Yeah, NOW you're talking. Sadly, the 6X8's are gone to donors...BUT I was already playing with a 10 (!) 12BH7 PP parallel design running an ultra low ratio OPT. NO mains tranny either, series heaters and a diode ladder for the B+ PS. interstage transformer for PI, bridging tranny for the input isolation. Front end was a 5719A for gain and a 6U10 for the other three stages. It is weird and wonderful.

    Thanks for the great ideas. THIS is getting the weirdness juices flowing. Now, OUT TO THE LAB!

    :)

    CBG
     
  10. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    p.s. LOVED the UL idea. Had not thought of that in quite that way!
     
  11. StephaninMelb

    StephaninMelb Banned

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    Nice guitar tube design feature

    Diezel VH4 from Germany

    The Diezel VH4 is high-gain tube guitar amp popular with metal groups like Metallica, Guns & Roses, Nine Inch Nails, etc. It will run 2 pairs of mixed power tube types...

    Valves: Four 6L6 power valves (Accepts 6L6, EL34, 5881, 6550, KT66, etc)
     

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  12. jefrs

    jefrs Doctor of Teleocity

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    I can be a bit pedantic at times. :oops:
     
  13. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Ha! We LOVE that about you jefrs :)
     
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