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  • can someone explain this?

    My brain just sits there idle when i try and figure certain things out. This is one of those things. What is the difference between adding a series R at the input of a gain pot or the output. I know at the input it changes the pot's value and will reduce gain. But at the wiper what does it do? Does it just act as a grid stopper and not alter the way the pot works ?

    second question.....can altering a stage cause the previous stage to become gainier? Or does it only affect anything AFTER the change? IE:, if i changed something at the 1st stage of the second AX7 that causes the preamp to have more gain, is it definatly after that point or could it be causing one of the V1 stages to have more gain?

  • #2
    My two cents worth

    If its in series with the wiper, it'll just limit the effective overall resistance to the current going through the pot, like say a 50k pot with a 39k resistor will be 89K when the pot is maxed out and 39K when it isn't.

    If its in series with the output its the same.

    (I should add that I'm assuming the pot is wired as a variable resistor in the two above examples)

    It its in series with the ground, the whole thing'll be a voltage divider.
    Last edited by tubeswell; 06-27-2008, 10:53 PM. Reason: Stating my assumptions
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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    • #3
      Thoughts on the 2nd question...

      The signal will be different if you change stages after the first stage, because you are increasing gain in the signal path as you move through the stages. At the end of the path before the output tube grid(s), you have the product of all the gain in earlier stages, which equals more voltage-swing than earlier stages. Therefore this may introduce more impedance into the circuit (assuming that each gain stage has its own resistance - so you are effectively adding resistance when you add stages) which would affect the impedance following the first stage would it not? Therefore affecting the 'load' that the first stage is driving into, therefore affecting the gain of the first stage? I guess the only way to tell for sure is to monitor the voltage swing after the first stage coupling cap when you flick the switch to kick in the extra gain stages (assuming you are going to have these in a separate channel)?

      PS Somebody's bound to correct me and then we can both learn something ;-)
      Building a better world (one tube amp at a time)

      "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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      • #4
        A resistor in series with the top of your pot principly has a gain limiting effect. While you've increased the resistance of the load (which would tend to increase the gain of the stage) you've limited the range of travel of your wiper so it can no longer reach the top of your voltage divider.

        A resistor on the wiper has principly a tonal effect. It's most effective on the bottom portion of the pot's travel. A volume pot provides a constant load resistance to the previous stage but a variable grid resistance to the following stage. As the grid resistance drops low frequency content is increasingly attenuated. When you see large value resistors placed on the wiper the goal is not to attenuate the signal so much as to set the minimum following stage grid resistance with the aim of reducing the tonal variations seen across the pots travel.

        Changes to any one stage's grid circuitry may have consequences on the previous stage's load and in this manner could easily effect the previous stage's gain.

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        • #5
          This is really strage. A voltage divider made up of 2 resistors should do exactly the same thing as a pot set to where the values are the same, right? I had a set VD after the second stage instead of a pot. But i decided to put a pot there instead. So i used a 1 meg pot. The previous VD was a 120k to ground with a 560k in series from the previous stage and the signal to the next stage taken from between the 2. That would equal about a 700k pot turned down quite a ways. But with a 1 meg pot which really isn't that different, i cannot get nearly the same result. If i turn it to where i'm getting the same amount of gain as i did with the set VD, i get bad blocking distortion. If i turn it down to where that goes away, i'm left with a lot less gain then i had with the set VD. Doesn't seem to make sense because once it's down to where i have the same amount to ground as the set VD, (120k) then the only difference is the series value would now be about 820k instead of 560k. Thats not enough to make such a big difference.

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          • #6
            That is not the only difference. I bet those two resistors didn't have wires running to and from the panel. Wires that could be right next to something they shouldn't be.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              I'm not sure what you mean Enzo, but the dividers are soldered right to the pots, and the same wires are there as were before. I had a 2nd gain pot originally but replaced that with that divider which i sent to the next grid via the same shielded cable that was there when i first had a second gain pot in place.

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              • #8
                Oh. I thought you had added a pot somewhere on the panel.

                680k versus 1M. Well, 120k up from the bottom of 1M is a lot lower than 120k up from the bottom of 680k. If the signal entering the divider was 10v, 120k/1M yields 1.2v, while 120k/680k yields 1.8v That's a pretty large difference. How does it sound at 180k on the 1M pot?
                Education is what you're left with after you have forgotten what you have learned.

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                • #9
                  I did ! I guess i wasn't clear but lemmie try again. I had 2 gain pots at first. The gain was overboard and blocking distortion was bad. Even turning them down or one way down still didn't sound right. I removed one gain pot and simply put a 100k to ground at that point. That wasn't great either, tho better. Then i put in that 120k/560k VD. Not bad. Blocking was not there at that point. But still the amp wasn't right. Then i put the 2nd pot back in and added VD's at each pot so it is now....

                  v1a out>>>gain pot>>>330k/330k VD>>>V1b>>>gain pot2>>>330k/330k VD>>>v2a. And from there it's typical marshall last stage and cathode follower. I gotta tell you, this amp is begining to scare me. I have never gotten such swirling rich harmonics with higher gain settings from any amp ever. Use a clean boost and theres zero blocking yet it becomes so complex it's scary. I think i may be done !
                  I will be adding pair of passive loop jacks after the treble pot and possibly reverb down the road. But as far as tone tweaking, i can't see it getting much better. there are things i'd like to be improved such as retaining more of that killin tone at higher volumes. But then every marshall i've owned was the same. I never liked cranked amp tone anyways unless the amp is tweaked just right. Otherwise output distortion sounds like #@$% to me.

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                  • #10
                    Originally posted by daz View Post
                    I did ! I guess i wasn't clear but lemmie try again. I had 2 gain pots at first. The gain was overboard and blocking distortion was bad. Even turning them down or one way down still didn't sound right. I removed one gain pot and simply put a 100k to ground at that point. That wasn't great either, tho better. Then i put in that 120k/560k VD. Not bad. Blocking was not there at that point. But still the amp wasn't right. Then i put the 2nd pot back in and added VD's at each pot so it is now....

                    v1a out>>>gain pot>>>330k/330k VD>>>V1b>>>gain pot2>>>330k/330k VD>>>v2a. And from there it's typical marshall last stage and cathode follower. I gotta tell you, this amp is begining to scare me. I have never gotten such swirling rich harmonics with higher gain settings from any amp ever. Use a clean boost and theres zero blocking yet it becomes so complex it's scary. I think i may be done !
                    I will be adding pair of passive loop jacks after the treble pot and possibly reverb down the road. But as far as tone tweaking, i can't see it getting much better. there are things i'd like to be improved such as retaining more of that killin tone at higher volumes. But then every marshall i've owned was the same. I never liked cranked amp tone anyways unless the amp is tweaked just right. Otherwise output distortion sounds like #@$% to me.
                    In the high gain amps I have built I have found that some sort of fixed voltage divider at the output of each gain stage is really helpful at controlling blocking distortion and allowing the complexity of tone to build up from stage to stage. Just basically agreeing with your observation.

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                    • #11
                      Yes, the load on the output of a tube stage will affect its gain. If it was previously driving a 1M gain pot, and you plunked a 100k voltage divider on there, the load is 10 times heavier, and by the time you take the tube's own Rp into account, the gain and maximum output, measured at the plate, will be cut by about 30%.

                      If you thought of the voltage divider as part of the input circuit of the next stage, then yes, it is affecting the gain of the previous stage.

                      Also remember that pots have a wide resistance tolerance, maybe 20%. So if you replaced a 1M pot with a 1M 1% resistor, things could change by up to 20%.

                      For more information on VDs see Billy Connolly: http://www.scotland.com/forums/scotl...html#post11744
                      "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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