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deluxe reverb amp with extra gain stage on chanel 1 to make it more like channel 2

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  • #31
    with changes that order, would you measure up everything, voltages and re-calculate the bias? How far can you go without risk of toasting the tube?
    You can't go wrong using one of the examples from the chart that suits your B+.

    Only increasing the plate resistor will increase gain but not plate current or tube dissipation. But it will cause off-center biasing resulting in reduced clean headroom and changed clipping behaviour. To avoid this the cathode resistor should adjusted as well.
    The chart takes care of this.
    Last edited by Helmholtz; 08-22-2019, 03:42 PM.
    - Own Opinions Only -

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    • #32
      Originally posted by Helmholtz View Post
      You can't go wrong using one of the examples from the chart that suits your B+.

      Only increasing the plate resistor will increase gain but not plate current or tube dissipation. But it will cause off-center biasing resulting in reduced clean headroom and changed clipping behaviour. To avoid this the cathode resistor should adjusted as well.
      The chart takes care of this.
      This is really important. It's easy enough to just bump the plate resistor value for gain. A lot of "mods" do this. I never liked the sound of replacing stock 100k resistors with higher value resistors. Then when I got beyond rote modifications and began to learn more about design I discovered that it was the bias change that I didn't like and not the gain or impedance changes. I've since graduated to where I do sometimes use higher value plate loads than the ubiquitous 100k and with no tonal detriment when biased properly.
      "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

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      • #33
        Cool, thanks. Ive read about some amps purposely reducing clean headroom to get 'unbalanced' clipping. it has a particular kind of sound? Until then, I always thought clipping meant both top and bottom of the wave.

        If you use a higher plate load, then re-bias, that means changing out the cathode resistor for some other values as well?
        The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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        • #34
          Originally posted by mikepukmel View Post
          If you use a higher plate load, then re-bias, that means changing out the cathode resistor for some other values as well?
          Yes. You can use the load line to calculate, or just do what I do. Use values previously demonstrated as good sounding in classic designs. There are examples by many major manufacturers, but the most obvious would be the AC30 using 220k loads and a shared 1.5k cathode resistor for two triodes. Similar to Fender BF amps using 100k loads and a shared 820 ohm cathode resistor. So for a single stage that doesn't share the cathode circuit you'd need a 3k resistor. Not a standard value. 2.7k or 3.3k would work fine. With 2.7k seemingly being the preferred choice with a 220k plate load among designs I've seen. These are also the values used by Fender on some models.
          "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

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          • #35
            3K is a standard value for 5% parts as is 750. Fender used 10% parts for the most part.
            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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            • #36
              Originally posted by loudthud View Post
              3K is a standard value for 5% parts as is 750. Fender used 10% parts for the most part.
              Yes. Thank you. 3k is available. I was considering what a person might have on hand or what would be easiest to source. When I design I usually go with the standard values for 10% parts in case I ever run across the need for an immediate repair and have to get a resistor at Fry's or something. So that's just my personal bias on the matter.
              "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


              • #37
                In a single tube stage clipping is never completely symmetrical, rather positive and negative output signal peaks are clipped differently : different mechanisms, different shape, different harmonics produced. Depending on biasing one or the other appears first. Center biasing ensures that positive and negative clipping starts at similar levels.
                - Own Opinions Only -

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                • #38
                  There are examples by many major manufacturers, but the most obvious would be the AC30 using 220k loads and a shared 1.5k cathode resistor for two triodes. Similar to Fender BF amps using 100k loads and a shared 820 ohm cathode resistor. So for a single stage that doesn't share the cathode circuit you'd need a 3k resistor. Not a standard value. 2.7k or 3.3k would work fine. With 2.7k seemingly being the preferred choice with a 220k plate load among designs I've seen. These are also the values used by Fender on some models.
                  In many (if not most) cases classic amp manufacturers (especially Leo Fender) just went with the tube manufacturers' recommendations/examples as they can be considered best practice.
                  No wonder the chart I posted lists the 100k/1.5k and 220k/2.7k combos.
                  - Own Opinions Only -

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                  • #39
                    Originally posted by Helmholtz View Post
                    No wonder the chart I posted lists the 100k/1.5k and 220k/2.7k combos.
                    Yep. And the "standard value" thing is probably why Vox used a 1.5k resistor for the shared cathode. Because 1.35k isn't a standard value
                    "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

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