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Design using variac to lower the power.

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  • Design using variac to lower the power.

    Is there any amp on the market using variac to control output power?

  • #2
    No, because a variac is in the AC line, prior to the PT & subsequently affects ALL secondary PT voltages, not just B+ voltage, low heater voltages sound awful. It's a bad idea.

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    • #3
      Yeah, if you could get around the low heater voltage, it "might" be worth a shot.

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      • #4
        There are already simple and MUCH cheaper B+ "power scaler" circuits on the market.

        It would be simple enough to run a separate B+ transformer in an amp to get around the heater problem. Or separate heater transformer maybe.

        I have a number of commercial variable B+ supplies here in the shop, which use a variac in front of the transformer. They are actually intended for use in electrophoresis, but 0-500v or 0-1000v metered for voltage and current sounds like B+ to me. Just the thing to power tube experiments.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Check out the Blankenship " Variplex".
          I don't know the amp, but it does incorporate a Variac.

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          • #6
            From Enzo:
            There are already simple and MUCH cheaper B+ "power scaler" circuits on the market.
            Just to try the simplest thing first: Try using a large R and a decoupling cap in series with the supply to the PI. Carvin did this many years ago and it's been copied a few times since and it does an excellent job. No it isn't a perfect emulation of a cranked amp, but pretty damn good.
            My rants, products, services and incoherent babblings on my blog.

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            • #7
              Actually I modified a Twin in 1978 with variac only on the power section and used it for gigging. I even designed a cascading channel on one and use a switch pedal to switch channels. I don't think anyone did that yet at the time. But I since left music all together and had a full career as an electronic engineer and manager of engineering. It is only lately after I retired that I kind of look back into all these.

              Any other way they vary the power other than variac? What is PI? Putting a series resistor with the power section require a high power resistor and generate a lot of heat. there got to be a better way than these two!!! I did the variac trick over 30 years ago before I even learn electronics, there got to be a better way.

              I know Mesa Boogie has so called Simucast that switch tubes pairs in and out. And there is triode mode that connect the grid 2 with the plate. Don't they have anything better than all these? No offense, but I expect they should have made leaps and bounds by now.

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              • #8
                Any other way they vary the power other than variac? What is PI? Putting a series resistor with the power section require a high power resistor and generate a lot of heat.
                The PI is the phase inverter tube. It draws a few mils, you can choke it down with a 2W resistor that won't even get warm. The idea being to reduce the drive to the power tubes and still get the entire preamp through driver driven hard. Like I say, it isn't perfect, but it's pretty damn good.

                There are other approaches, like using a current regulator on the supply to the power amp.
                My rants, products, services and incoherent babblings on my blog.

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                • #9
                  Thanks

                  The current limiting might be a good idea!!!

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                  • #10
                    Its pretty easy to implement variable voltage regulation into an amp. I made a 20W cathode biased amp with a VVR circuit which allows the output to be scaled down to about 1W. I only applied it to the plates and screens but you can easily add the PI and whole preamp if you wish. All it required was a MOSFET and a few small common components. Above 20W, 350V it gets more complicated, fixed bias also adds to the part count. Merlin Blencowes power supply book has a section on this if you're interested.

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                    • #11
                      Thanks, I am going to order the book.

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                      • #12
                        google "Dirty Boy" amp.
                        they used a variac in this model, quite successfully. when they still made them that is.
                        they used several xformers to make the amp, so it was probably cost prohibitive to manufacture.
                        from the clips I have heard/seen on youtube, it sounds really good. cool design.

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                        • #13
                          Matamp also made a model with a small variac built in. It had two power transformers, so the heaters always stayed at 6.3V.

                          Here is my own take on the "power scaling" concept.
                          http://scopeboy.com/scopeblog/wp-con...etteHybrid.pdf
                          scopeblog Ninja Corvette Hybrid
                          "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                          • #14
                            Originally posted by Steve Conner View Post
                            Matamp also made a model with a small variac built in. It had two power transformers, so the heaters always stayed at 6.3V.

                            Here is my own take on the "power scaling" concept.
                            http://scopeboy.com/scopeblog/wp-con...etteHybrid.pdf
                            scopeblog Ninja Corvette Hybrid
                            I don't quite follow the transistor amp stage of Q4 and Q5 because I don't quite follow the switch. How is the switch work, how does it slide on the contacts?

                            I follow the MOSFET Q3 changing the screen grid voltage, does that lower the power when you lower the screen grid voltage?

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                            • #15
                              The switch connects the little 3w tube amp to the speaker. Either through an attenuator giving 0.3W, or directly, or through the transistor stage which increases it to 30w.

                              The screen grid voltage also modulates the output power. So you have a fine power adjustment as well as the three 10dB steps.

                              The circuit isn't perfect, but it gives a lot of bang per buck spent on components and per pound carried to the gig. The transistor part has a natural current limit and needs no other protection.
                              "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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