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Peavey 5150 Channel Mod

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  • Peavey 5150 Channel Mod

    I have the 5150 combo. I love the Rhythm channel, hate the Lead channel. I was thinking of modding the amp so that when I hit the channel switch I get the same tone, just louder, for a lead boost. Enzo previously explained the relay/switching scheme to me so I know that the three double pole relays K1, K2, and K3 take care of the channel switching functions. Looking at the schematic I see that this amp design does not actually have 2 discrete channels, but rather the circuit is essentially "tweaked" when the relays engage. So I was thinking it would be relatively easy to bypass the relays, but with one exception. K1B switches between the two "Post" volume pots and I would leave that one as is giving me my volume boost option. My question is, is this schematic consistent in terms of how it's showing the relays default positions? If you look at K1A it shows the amp being in Rhythm mode. But looking at K3A it appears to me that that one is in Lead mode since it is providing an additional gain stage that would be bypassed in the other position. Some of the other ones I'm not sure which position would logically correspond to which channel.

    peavey_5150_combo.pdf

  • #2
    Unless otherwise noted, relays are always drawn in their "normal" position. That means with the coil not energized. Normal does NOT mean the state usually encountered. In Other words all relays are drawn as OFF, whereas in operation, whenever K1 and K2 are OFF, K3 is ON, and vice versa. K1 and K3 are never both off or both on at once in the working amp., but schematically, they are drawn as OFF in all cases.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Thanks, Enzo, it makes sense now.

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      • #4
        Originally posted by Enzo View Post
        Unless otherwise noted, relays are always drawn in their "normal" position. That means with the coil not energized. Normal does NOT mean the state usually encountered. In Other words all relays are drawn as OFF, whereas in operation, whenever K1 and K2 are OFF, K3 is ON, and vice versa. K1 and K3 are never both off or both on at once in the working amp., but schematically, they are drawn as OFF in all cases.
        So I'm playing around with this idea and figuring out how far I want to take it. So far I bypassed K3A (eliminating the extra gain stage) and I like the result of that. I'm considering making that mod switchable.

        Anyway, I was hoping you could give me some insight into the thinking behind K3B. From what I can tell in Rhythm mode some of the reverb input signal is dumped to ground through the 10K resistor. In Lead mode that signal is restored to the reverb input and we have the 33K resistor to ground just before the tone stack. Can you explain the logic of this switching?

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        • #5
          REverb has nothing to do with it really. Remember there is only one channel through this amp, to get more than one "channel" they click things in and out to alter gain and signal level.

          K3b? The fairly hot signal from V5b finds some voltage division with R17,18,32. then into the grid of V5a. If I recall, K3 is OFF for clean channel. So its contacts are set as drawn. R33 and R122 mix the reverb into that grid as well. Leaving V5a, 470k R19 feeds the tone stack. K3b brings 33k R5 to ground from that point. That forms a 470k/33k voltage divider that drops your signal level there substantially.

          Turn K3 ON and now that R5 disappears, and R121 is added to the V5a grid circuit. That forms a voltage divider parallel with R32., dropping signal into the tube - which of course has plenty of gain to give.

          The same hot signal feeds the reverb from before all this gain moulding, to the reverb return signal is within this voltage divider to keep it about the same relative to the signal.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Originally posted by Enzo View Post
            REverb has nothing to do with it really. Remember there is only one channel through this amp, to get more than one "channel" they click things in and out to alter gain and signal level.

            K3b? The fairly hot signal from V5b finds some voltage division with R17,18,32. then into the grid of V5a. If I recall, K3 is OFF for clean channel. So its contacts are set as drawn. R33 and R122 mix the reverb into that grid as well. Leaving V5a, 470k R19 feeds the tone stack. K3b brings 33k R5 to ground from that point. That forms a 470k/33k voltage divider that drops your signal level there substantially.

            Turn K3 ON and now that R5 disappears, and R121 is added to the V5a grid circuit. That forms a voltage divider parallel with R32., dropping signal into the tube - which of course has plenty of gain to give.

            The same hot signal feeds the reverb from before all this gain moulding, to the reverb return signal is within this voltage divider to keep it about the same relative to the signal.
            Oops, I had the the reverb input and output mixed up.

            Voltage dividers, right, I haven't trained myself yet to think in terms of voltage dividers when looking at resistors in a circuit.

            Actually, I believe K3 is ON in clean mode. Remember, you explained how K3 is ON when K1 and K2 are OFF and vice versa. In clean mode K1 and K2 are OFF. In the drawing K1A is shown connecting R24 to the Rhythm Pre pot wiper.

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            • #7
              Well, sorry, but the voltage division described still happens. I just inverted which one was in charge, so to speak. Hey, I'm old... One diminishes the input to V5a and the other its output
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                Originally posted by Enzo View Post
                Well, sorry, but the voltage division described still happens. I just inverted which one was in charge, so to speak. Hey, I'm old... One diminishes the input to V5a and the other its output
                Yep, all good. I appreciate the detailed explanation!

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