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Designing push pull stages - many questions!

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  • Designing push pull stages - many questions!

    I'm looking to finally add a tube power section to a pre-amp I built a while ago to make a complete amplifier. The catch is that it runs off a 48v 100W power supply. I can't really just rip-off designs at this voltage so I want to ask a couple questions, mainly concerning class A/B etc.

    I'm probably going to use a couple of 6080 twin triodes, because of the HUUUUUGE current capability at low voltages, plus they're like 5 bucks. I want to run them in push-pull but in class A (and not A/B - I don't even come close to the dissipation limit to warrant this; or am I missing something here?) However I'm a little confused about how to actually determine the power output of a push-pull stage, and what class has to do with the actual power output.

    I have a load-line attached, and am sort of wondering, since my bias point (blue dot) is clearly not in the centre of the load-line, it would go into grid current limiting earlier than cutoff. How does this affect the class of the amplifier and the power output? Would a different load-line or going class AB net me more output power (which is my main concern).

    I know that when a tube is in cutoff it essentially changes the load that the other 'on' valve will see, but what happens when a tube is in grid current limiting? Any help would be appreciated
    Attached Files

  • #2
    First of all, in a push pull class A tube power amp, the static dissipation of each side is roughly equal to the power output (assuming you have the correct load). Your blue dot is roughly 50V at 50mA, that's 2.5W. Assuming both halves of each 6080 on each side of the push pull, your output would be less than 5W. This assumes you can drive the tubes to complete saturation where the voltage drop across the tube is near zero. Good luck with that. The problem with using triodes in an output stage is that the saturation voltage is high.

    With a 48V supply a conventional totem pole solid state power amp driving a 4 ohm load would only make 72W (RMS) if it could swing to the rail.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      What OT will you use ? You must know to choose a bias point, actually, you shoul try to find an OT according to the usable voltage The Valve Wizard -Single Ended
      You need higher voltages (your load line assumes 100 volts on the plate, and it's not enough)
      Last edited by kleuck; 10-05-2011, 10:09 PM.

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      • #4
        With only a 48V rail, go SS.
        Even if you could build a tube design with some very oddball tube, it will sound nothing like a "regular" tube stage, which makes the exercise futile.
        Juan Manuel Fahey

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        • #5
          It could be an interesting exercise to build this with a center tapped 220 volt to 70ish volt power transformer used as an output transformer. Of course I agree with the above- it won't sound like a traditional guitar amp.

          Build it! Prove us wrong! It's an interesting tube!

          jamie

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          • #6
            I don't have enough knowledge to help you figure out how to fix your dilemma, but why don't you consider just building a separate tube power amp? I know it costs more as you'd have to buy another power transformer (and that's the more expensive one), but you can use "normal" tubes and you won't have to worry about the problems of finding a tube that can generate the amount of power that you want at such low voltages.

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            • #7
              Kevin O'Connor has an interesting design he calls "The Hood Amp". It's a class A transistor design from the 60s. I built a similar design when I was in high school. It sounded good but got too hot! It will run fine on a single 48V rail. It only has four transistors. You can find that design in the book Tonnes of Tone. If you really want to learn about tube power amps, get his book Principles of Power.

              Tube Amp Kits, Tube Amp Books, Tube Amplifiers by London Power

              There is an odd design called the Futterman. It uses six (or more) 6080s directly connected to a 16 ohm load. It's in Principles of Power.
              WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
              REMEMBER: Everybody knows that smokin' ain't allowed in school !

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              • #8
                This project is only a cheapie. I'm gonna use a scavenged power transformer for the OT, and I already have the sockets/tubes on hand. I kind of only expect to produce a couple watts of power, but it'll be an interesting couples of watts to say the least...

                If it turns out OK, I might bump up the voltage and give OTL a crack, but for a 64 ohm load. Without the output tranny it might sound too harsh for guitar use (from the lack of high freq rolloff). And the bass might turn to mush because of the stupidly large coupling caps you need to use (I don't think a speaker would appreciate 1 amp of idle current going through it!).

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                • #9
                  I once built a pedal with push-pull mosfets and a tiny transformer and it sounded pretty good. Build it and see how it sounds!

                  jamie

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                  • #10
                    A couple of things just for your info:
                    First:
                    For a Class AB solid state guitar power amp you can get a fair amount of "tubiness" by running bipolar transistor (e.g. 2SC5200) on one side and a mosfet (eg IRFP9240) on the other side of the push pull. The unequal gm on the push and pull sides (bipolar have higher gm than mosfets) imparts some good old 2H distortion for tube like sound.
                    Also:
                    6080 were specifically NOT recommended for push pull due to the difficulty of balancing the two triode sections, many data sheets carried that caution.
                    Cheers,
                    Ian

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                    • #11
                      Sorry to disagree, but in a classic SS amp design, any difference between the bipolar and the Mosfet side transconductances will be crushed by the huge NFB such amps use, rendering any such difference microscopic.
                      Consider this: for *ages* the state of the art was the "quasi complementary" architecture, because of lack of suitable complementary devices.
                      Many such amps are still built today.
                      Can't imagine a larger unsymmetry .... much higher than what's suggested above, yet such amps easily achieved less than 1% distortion.
                      My personal opinion is that (in SS amps) it pays more to first generate distortion, assymetry, whatever , and then faithfully re-amplify it with a conventional amp.
                      Juan Manuel Fahey

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