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  • #31
    Originally posted by Helmholtz View Post
    What is the DCR of the driver coil? This acts just like the 470R as an extra series resistor in this highly damped series resonant circuit. (The resonant frequency could be varied by using a different coupling cap.)
    About 60 ohms, quite small.
    Experience is something you get, just after you really needed it.

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    • #32
      Originally posted by nickb View Post
      About 60 ohms, quite small.
      O.K., thanks
      - Own Opinions Only -

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      • #33
        Originally posted by nickb View Post
        These are not relevant results as
        1) You are showing the plate voltage not tank current
        2) You are driving it into clipping

        Back down the drive to keep in in the linear region, add a small resistor on the ground side of the tank and monitor the voltage across that.


        I see the same if I crank it into clipping, but that is not how we are operating, or at least shouldn't be. it Even then there is no significant difference with and without the 470 ohm.



        [ATTACH=CONFIG]50161[/ATTACH]


        A Norton equivalent gives the same result. Mr Thevinen was great relieved to learn of that Since we are using idealized sources here the frequency response is different from when driving with the tube, but the result is the same.

        [ATTACH=CONFIG]50160[/ATTACH]

        I find this pretty compelling evidence that any difference between the two is negligible. For simplicity, I’ll probably omit the resistor.
        If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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        • #34
          What’s the formula to calculate the resonant frequency in this case? I figure in order to bring the resonant requency even down to (about) 121Hz, the coupling cap would need to be 10uF. Firstly, Im not sure thats right. Secondly, bringing the resonant frequency down to twice the mains frequency is probably not a good ideal?
          If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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          • #35
            It's the usual f= 1/(2 x pi x sqrt( L x C)).
            Roughly speaking, there's no point in being too precise as inductance is a rather variable quantity in practice and especially so with a tank or so I should think, with 1uf we're at 460Hz, so 10uF get us to the sqrt(1/10) of this i.e. 145Hz, so you were about right . I really don't see the merit in going bigger than 1uf as you don't change things much. OTOH you don't want to go any lower capacitance.

            Then there's the 82K resistor. It's not really doing anything other then adding a path for cap leakage current to ground with no tank. If the leakage were high enough to be dangerous then the 82K isn't doing to help. So, you might consider a film cap rather than an electrolytic for safety reasons.

            If you were to make it lower, even 10k won't change the frequency response much but will provide a bit more anti-zap protection. Lower it a bit more and you can get some useful HF attenuation e.g 2.7K gets you a corner frequency of about R/2/pi/L = 3.6Khz. Something else to play with, huh?
            Last edited by nickb; 08-23-2018, 07:20 PM.
            Experience is something you get, just after you really needed it.

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            • #36
              Originally posted by SoulFetish View Post
              What’s the formula to calculate the resonant frequency in this case? I figure in order to bring the resonant requency even down to (about) 121Hz, the coupling cap would need to be 10uF. Firstly, Im not sure thats right. Secondly, bringing the resonant frequency down to twice the mains frequency is probably not a good ideal?
              As usual the formula for the resonant frequency is 1/fres= 2*pi*(L*C)^0.5. With L=120mH and C=10µ you get fres=459Hz. Because of the strong damping by the 15k series resistance you see no resonance peak but rather a bandpass response with a flat top. Increasing the coupling cap's value by a factor of 10 would lower the fres by around a factor of 3.
              If you look at the current frequency response of Nick's simulation (blue curve), the actual lower corner frequency is already as low as ca. 100Hz. I don't think more bass drive is desirable.

              Edit:
              Didn't see Nick's post above. As usual, composing a reply in a foreign language takes me a little longer .
              Last edited by Helmholtz; 08-23-2018, 07:06 PM.
              - Own Opinions Only -

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              • #37
                Originally posted by nickb View Post
                It's the usual f= 1/(2 x pi x sqrt( L x C).
                Roughly speaking, there's no point in being too precise as inductance is a rather variable quantity in practice and especially so with a tank or so I should think, with 1uf we're at 460Hz, so 10uF get us to the sqrt(1/10) of this i.e. 145Hz, so you were about right . I really don't see the merit in going bigger than 1uf as you don't change things much. OTOH you don't want to go any lower.


                Then there's the 82K resistor. It's not really doing anything other then adding a path for cap leakage current to ground with no tank. If the leakage were high enough to be dangerous then the 82K isn't doing to help. So, you might consider a film cap rather than an electrolytic for safety reasons.

                If you were to make it lower, even 10k won't change the frequency response much but will provide a bit more anti-zap protection. Lower it a bit more and you can get some useful HF attenuation e.g 2.7K gets you a corner frequency of about R/2/pi/L = 3.6Khz. Something else to play with, huh?
                Yeah, not a bad idea!
                But I don’t even use electrolytics is the power supply.
                If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                • #38
                  Originally posted by SoulFetish View Post
                  But I don’t even use electrolytics is the power supply.
                  I know, you crazy..........

                  But, I think maybe you are a perfectionist. That's a hard road to walk.
                  Experience is something you get, just after you really needed it.

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                  • #39
                    I think it's time to give it a listening and evaluate different options.
                    - Own Opinions Only -

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                    • #40
                      Originally posted by Helmholtz View Post
                      I think it's time to give it a listening and evaluate different options.
                      I’m scared.



                      Originally posted by nickb View Post
                      I know, you crazy..........
                      You’re probably right, the evidence is overwhelming.
                      If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

                      Comment


                      • #41
                        Originally posted by nickb'505379
                        Since we are using idealized sources here the frequency response is different from when driving with the tube, but the result is the same.
                        Why did you omit the resonant/coupling cap in the basic circuits L3/L4?
                        Last edited by Boss; 10-17-2018, 10:22 AM. Reason: removed PHP tags (why were they there?) from quote
                        - Own Opinions Only -

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                        • #42
                          Originally posted by Helmholtz View Post
                          Originally posted by nickb'505379
                          Since we are using idealized sources here the frequency response is different from when driving with the tube, but the result is the same.
                          Why did you omit the resonant/coupling cap in the basic circuits L3/L4?
                          I er....er... left them out deliberately to see if you were paying attention? Well done, you were.

                          Sorry about that. Here it is corrected.
                          Attached Files
                          Last edited by Boss; 10-17-2018, 10:21 AM. Reason: removed PHP tags (why were they there?) from quoted quote
                          Experience is something you get, just after you really needed it.

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                          • #43
                            Thanks a lot. Obviously the resonant bandpass is far wider - reaches down lower - than I expected. And the bass drop in the complete circuit starts at a much higher frequency than the 3dB frequency of the cathode circuit (25Hz).
                            - Own Opinions Only -

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                            • #44
                              Originally posted by nickb View Post
                              These are not relevant results as
                              1) You are showing the plate voltage not tank current
                              2) You are driving it into clipping
                              Here is the same setup with the same control grid input at 1KHz. 1V/Div., 5.7Vpp, 2.85Vp, 2.85Vp/641r = 4.5mAp
                              Attached Files

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                              • #45
                                Originally posted by 66 Kicks View Post
                                Here is the same setup with the same control grid input at 1KHz. 1V/Div., 5.7Vpp, 2.85Vp, 2.85Vp/641r = 4.5mAp

                                You are looking the the plate voltage and this is what is misleading you.

                                The tube produces lots of 2nd harmonic and the tank load is reactive. So, when you drive around the resonant frequency you see little plate voltage swing at the fundamental due to the low impedance at resonant load. However the 2nd harmonic is now much bigger and so you see a very distorted waveform at the plate.

                                If you look at the current in the tank you will see it's just fine. Use as small a resistor as you can in series with the tank to ground so as not to disturb things too much.
                                Experience is something you get, just after you really needed it.

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