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Conjunctive/corrective filter in Magnatone

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  • Conjunctive/corrective filter in Magnatone

    Hi. Got a Magnatone 197-3-v. It has: 1 ea- 6v6, 6sj7, 5y3. 261 vdc at the 6v6 plate & about 14vdc across the OT. There was a fried .01 cap across the OT primary.
    Is it OK to just replace the cap with another 600v or should I put a resistor in series with it like most of the filter examples that I have seen?
    I have listened to it with the .01 straight across the primary and also tried 1k and 5k ohm resistors in series with the cap. All three sound surprisingly similar with the speaker that I have connected. Without the filter it is ice pick like.

    Second question. Using a resistor in series, why do people recommend big watt resistors? With the volume max and pounding a power chord I get about 180vac across the xfmr (cap + resistor) but only read about 3 vac across the resistor itself (this doesn't make sense to me yet...still thinking). Maybe me & my fluke can't read power chord frequencies properly across an OT primary?
    An explanation would be great.
    Thanks!

    edit: missed this good info earlier when searching-
    http://music-electronics-forum.com/t13097/
    Last edited by Tonewood; 03-02-2014, 10:58 PM. Reason: add info

  • #2
    You'll want the cap in to be about 1kV. It's because of voltage spikes that can be better than double the HV. Just do it. Same for the high resistor wattage. Generally speaking, higher wattage resistors are also higher voltage resistors. The general rule for that sort of circuit is to have the resistor at 1.4X the primary impedance. I don't remember where I read that. It seems like the sort of parameter that would be dependent on many other factors to idealize, but just value the resistor at 5k (or 4.7k standard value) and be done. Mouser use to sell 11W "high energy" resistors with an impressive voltage rating. I've used them for this circuit in 35W amps without issue. Whatever resistor you choose, look up the specs for the voltage rating. IMHO .01 is an enormous capacitor for a shunt filter on the OT primary. I might experiment with smaller values for tonal reasons.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

    Comment


    • #3
      There are two "schools" for dimensioning those components. The first one tries to entirely linearize the rising inductance of the speaker load. Such filter values will easily affect response at the audible band. As this may already have hinted, such design relies a lot on properties of a specific loudspeaker load. R is dimensioned to be equal to loudspeaker's voice coil resistance (this is a bit lower than the nominal impedance and usual resistor torelarances are good enough for match). C is then calculated from L/R^2. Obviously measuring tools or datasheets are needed to figure out speaker's inductance. The second school of design doesn't aim for such a linear action or close loudspeaker design dependence. The values are picked so that R is at least about on the same scale as loudspeaker's nominal impedance. Values twice or three times higher or lower are still considered to be in the ballpark. Cutoff point is set much higher with C: the inductance of the total load may increase notably towards higher frequencies but well above audible range the filter starts to have effect and stabilises the amplifier.

      In OT coupled amps this "zobel" circuit can be either in the secondary side or in the primary side but naturally impedance needs to be scaled accordingly. At the primary side it will be subject to much higher voltage swings. In secondary side to much higher current swings.

      Naturally you need to heed currents and the nature of them in this circuit. Under normal conditions not much high frequency signal is shunted through the filter and not much power is dissipated in it. The filter with lower fc will naturally effect already at frequencies where inductance of the speaker considerably rises so under normal operation it will be subject to passing more current that the latter type filter design. At higher frequencies the impedance of the shunt capacitor decreases, as usual. Ultimately this would introduce loads that look like direct short circuits at higher impedances, therefore the series resistor to limit higher current flow. So, at extremely high frequencies this circuit is practically asked to dissipate almost the entire output power of the amplifier. Faults resulting to oscillation, for example, are usual causes why these components get toasted. Obviously it may be too excessive to "overbuild" this filter but at least you wish to ensure that if it fails it does so in a manner that imposes least harm to the amp.

      Comment


      • #4
        Originally posted by Chuck H View Post
        IMHO .01 is an enormous capacitor for a shunt filter on the OT primary. I might experiment with smaller values for tonal reasons.
        Thanks for the info Chuck. 0.01 does look large compared to what people currently talk about. The RCA manual uses .05 in their example! I'll pick up some 1kV caps & do some more testing with .001-.005.

        Comment


        • #5
          The larger caps always seem to be used in series with a resistor value greater than the primary impedance, reducing the influence. A straight .01 shunt is a dead short to frequencies above 2k to 4k (depending on primary impedance). That's pretty crazy and, it seems to me, possibly even bad for the tubes. Certainly as audible as covering the cabinet with a packing quilt! If you're going with a series resistor larger values might perform better depending on the nature of the OT. Or so I've read. Teemuk gave a rundown that goes partly over my head. My own experience with these circuits is where I derive any knowledge of the audible effect. A scope is a good tool for dialing in good values because you can see if any spiking needs to be dealt with and then experiment to find accurate correction. That's from a functional perspective. If an amp is harsh there's no reason I can see not to use larger capacitance and/or or lower resistance in this circuit for tone shaping. Tubes handle playing into a lower load at HF pretty well.
          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "If you're not interested in opinions and the experience of others, why even start a thread?
          You can't just expect consent." Helmholtz

          Comment


          • #6
            Teemuk, thanks for the details. I'm still trying to digest it all. I'm going for the version that is in the audio range to get rid of the Ice Pick in the ear problem.
            This amp came with a lap steel (haven't strung it up yet, needs tuner buttons) that I'm guessing is as bright as my Strat.

            Comment

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