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Please tell me what current amp this TEISCO CHECKMATE 20 is closest to....

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  • #16
    Hi Mick!

    Are you SURE that it's a voltage doubler? I researched Google and I thought not....but I don't know what I'm talking about, so.....

    Click image for larger version

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    Also, Mick, Olddawg, Justin, & RJB, I owned a Epi VJr. v3 for a while, and it just didn't make it for me, sonically, so I sold it. I also have SS amp (Fender M-80) and an effects pedal, and I am also not satisfied with that either. I'm not going to enter the tube vs. SS argument, b/c I don't really have an opinion.

    I've kind of been bitten by the tube bug... and I've already suffered for many years with the "buy a fixer upper" bug for MANY years, and it usually worked out for me.
    Last edited by aaron; 10-17-2017, 09:34 AM.

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    • #17
      Thanks to JM for answering my questions....

      No, I haven't bought it yet. The owner just had a baby right after he posted the ad, so he's up to his ears in diapers, sleep deprivation, and who knows what else, so no rush there.

      and yes, I know that $100 is the STARTING price!

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      • #18
        thought I'd share this:...













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        • #19
          Originally posted by aaron View Post
          Are you SURE that it's a voltage doubler?
          Yes it's a doubler, the two diodes and two caps are a clue.

          I usually draw it as below so it looks like two stacked half wave rectifiers.

          Click image for larger version

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          Last edited by Dave H; 10-17-2017, 03:29 PM. Reason: added schematic

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          • #20
            Originally posted by Mick Bailey View Post
            ......I hate to say this and will be burned alive, but at low volume I much prefer SS amps with a multi-FX pedal......
            I got a chuckle out of that. Why? Because I was thinking the same thing and didn't post it because I didn't want to be burned alive.
            That said, I agree. IMO, it's much easier to get bedroom volume out of a small solid state amp. I've got some little Fender thingy in my bedroom for learning songs. I forget the model, but it's one of those little things you might get with a guitar beginner package. It has minimal modelling built in and sounds great at low volumes.
            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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            • #21
              We don't burn people here. It spoils the soup.
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


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              • #22
                You mean something like ?:
                Juan Manuel Fahey

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                • #23
                  Thanks, Dave. Your drawing is clearly a doubler. Which is why I was confused from the schematic.

                  Really confused now.........

                  So does that mean that the power transformer is only putting out about 200V? (I've read the amp delivers 400+ volts to the tubes, so 400/2=200).

                  that doesn't make sense to me b/c the amp was made in Japan for the European market.....where 220-240V comes out of the wall....why would you use a transformer to step up voltage from the wall, if you're going to use a voltage doubler to step up voltage to 400V???

                  Wall@220-240VAC>>>Power Transfomer>>>Voltage double>>>400VAC. ???????

                  Here's the schematic, for any who can help...


                  Checkmate 20 Teisco Schematic.pdf
                  Last edited by aaron; 10-18-2017, 06:43 AM.

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                  • #24
                    The schematic is clearly a voltage doubler, too.

                    A voltage doubler does not actually double the input voltage. It doubles the peak value of each half of the waveform and unloaded gives a little over 2.8x the RMS input voltage (1.414 x 2).

                    There are many reasons to use a doubler;

                    1. The transformer can be an off-the-shelf one used in a different sector and therefore more economical to source.
                    2. The insulation requirements are not as stringent - the lower voltage means less insulation stress. This can give lower cost and greater reliability.
                    3. Each diode 'sees' only half the peak inverse voltage (PIV) that a regular full-wave rectifier does and gives greater reliability, more so in the early days of SS rectifiers.
                    4. Each capacitor can be rated at a lower voltage = lower cost.
                    5. For a guitar amp the sag better emulates tube-rectifier compression. Doubler circuits experience greater voltage drop under load than a regular full-wave circuit.

                    Don't think about the amp in today's context - you have to wind back to the time that the amp was originally designed. There are lots of things that were done then that you wouldn't necessarily do now. Having said that, I use doublers quite a bit in builds. A 230v>120v stepdown transformer will give 350v on my 240v mains using a doubler - just nice for a 2xEL84 amp and hardly any significant increase in parts count.

                    EDIT: as per J.M. Fahey's comment below - I'm referring here to a transformer with isolated windings - NOT an autotransformer which has no isolation and leaves the secondary connected to the live incoming mains.
                    Last edited by Mick Bailey; 10-19-2017, 11:28 AM. Reason: Safety note.

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                    • #25
                      Thanks so much Mick! The light is starting to go in my head!
                      I think I need a voltage doubler for my brain!

                      So what do you think is happening in the Teisco Checkmate 20?

                      I'm still confused about what the transformer is doing! Since wall voltage is 220-240VAC, it's going to be multiplied by the voltage doubler to about what the valves/tubes want to see, right?

                      1. Wall sends 220-240VAC to the transformer....How many volts are coming out?
                      2. Transformer sends THOSE volts to the double.........How many volts are coming out then?

                      I hope you see what's confusing me........

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                      • #26
                        Originally posted by aaron View Post
                        1. Wall sends 220-240VAC to the transformer....How many volts are coming out?
                        2. Transformer sends THOSE volts to the double.........How many volts are coming out then?
                        Mick explained it in the post above.

                        1. It depends on the pri:sec turns ratio of the transformer. If it's designed to be an isolating 240:120 volt power transformer then when you put 240V AC in you get 120V AC out which is isolated from the 240V mains voltage.

                        2. In the above example the transformer is sending 120V AC rms to the doubler. The doubler converts that 120V rms to a DC voltage of 2 x the peak AC voltage. The peak voltage of a sine wave is 1.414 x the rms voltage so the output of the doubler is 2*120*1.414 = 340V DC.
                        Last edited by Dave H; 10-19-2017, 01:17 PM. Reason: transformer clarification

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                        • #27
                          Please Mick Bailey and Dave H edit your posts to make it **VERY** clear that the
                          230v>120v stepdown transformer
                          you respectively mention and comment on without further clarification is NOT the same as the common available anywhere **autotransformer** 230v>120v stepdown transformer by any means.
                          Please make clear you are referring to a separate/mutually insulated windings transformers.
                          Sadly the 230v>120v rating and the stepdown transformer label by themselves strongly hint at the autotransformer type.

                          I would have used the "Power Transformer" label which canīt be confused with the mains adapting one.

                          As confirmation, please look at what does Mr Google aswer when asked the exact keywords 230v>120v stepdown transformer

                          https://www.google.com.ar/search?q=2...O5Lj8AfG6Z2IAg

                          The words stepdown transformer and just nice for a 2xEL84 amp should not be together in the same phrase.
                          Juan Manuel Fahey

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                          • #28
                            aaron
                            You have mentioned several times the 240v in and then tried to equate that to the DC supply circuit. They are not related. When you design a circuit, you determine what voltages you want to end up with for your circuit. Might be 400vDC for B+ and maybe 6vAC for heaters. SO you decide what AC voltage do I need to make the 400vDC. Whatever voltage that might be, I can make a transformer to provide it. I then have to consider the mains voltage from the wall, and make the transformer to use that.

                            If I have a voltage doubler, and want 400vDC, I might look at it as 200v twice. To make 200vDC, I need about 140vAC. SO I need to make a transformer that has 140v output and 6v output. I can make such a transformer with 120v primary or with 240v primary. There is no direct connection THROUGH the transformer, SO the DC circuit only knows it has 140vAC to work with, and has no idea whether the wall outlet was 120v or 240v.
                            Education is what you're left with after you have forgotten what you have learned.

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                            • #29
                              Originally posted by Enzo View Post
                              aaron
                              You have mentioned several times the 240v in and then tried to equate that to the DC supply circuit. They are not related. When you design a circuit, you determine what voltages you want to end up with for your circuit. Might be 400vDC for B+ and maybe 6vAC for heaters. SO you decide what AC voltage do I need to make the 400vDC. Whatever voltage that might be, I can make a transformer to provide it. I then have to consider the mains voltage from the wall, and make the transformer to use that.

                              If I have a voltage doubler, and want 400vDC, I might look at it as 200v twice. To make 200vDC, I need about 140vAC. SO I need to make a transformer that has 140v output and 6v output. I can make such a transformer with 120v primary or with 240v primary. There is no direct connection THROUGH the transformer, SO the DC circuit only knows it has 140vAC to work with, and has no idea whether the wall outlet was 120v or 240v.
                              Ahah! thanks to everybody, I'm starting to see the light! My first assumption was (after seeing all the Fender schematics where 120VAC>>340VAC, e.g.) is that the transformer that greets the wall VAC supply always steps up the voltage.

                              My second assumption was that diodes only convert AC to DC. (I didn't know about voltage doublers).

                              So I concluded that the PT would step up the voltage from 220VAC to 400VAC, and the diode(s) would convert AC to DC.

                              My mind has been blown by you telling me that the power transformer is actually stepping down the voltage and that the voltage doubler is the one doing the step up job. Dayum!
                              Last edited by aaron; 10-19-2017, 10:36 AM.

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                              • #30
                                Originally posted by Mick Bailey View Post
                                I use doublers quite a bit in builds. A 230v>120v stepdown transformer will give 350v on my 240v mains using a doubler - just nice for a 2xEL84 amp and hardly any significant increase in parts count.
                                You know, I didn't understand what you said the first time I read it. Now I can see what you wrote. Sometimes my mind ignores things I don't comprehend, and just keeps reading.....

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