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  • Thoughts on this one

    Spent a good bit o' time on this one... any thoughts? Only main circuit here... no PS yet but you can use your imagination there.

    New 12 Watt.pdf
    Hey you... Yeah you kid... Ya wanna buy some "Magic Beans"?

  • #2
    For the over-drive channel, I don't think you have enough gain to clip those led's.. How about's you use V1a to drive V1b on the over-drive channel, and then raise the plate resistor on V1b to 270K ? I would also remove the first volume control. I think it becomes too redundant with the vol control already used on the instrument. . . Some folks seem to miss that one. . . .

    -g
    ______________________________________
    Gary Moore
    Moore Amplifiication
    mooreamps@hotmail.com

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    • #3
      Originally posted by mooreamps View Post
      For the over-drive channel, I don't think you have enough gain to clip those led's.. How about's you use V1a to drive V1b on the over-drive channel, and then raise the plate resistor on V1b to 270K ? I would also remove the first volume control. I think it becomes too redundant with the vol control already used on the instrument. . . Some folks seem to miss that one. . . .

      -g
      Thanks much! Two questions tho... (1) How much gain do I need to od led's & might I be better off using 1N4148's or germanium 1N60's? & (2) Could you show me an example how one might go about using V1a to drive V1b? I also have a question regarding transformers & chokes as well... such as choosing between a Tweed Champ/Princeton repl. PT & a Weber Tweed Deluxe/Marsh 18 Watt repl. PT w/310-0-310 & 270-0-270 secondary taps & a BF Princeton OT (about 12-15w) or 18 Watt OT (16-18w) OT... what mA rating for supply choke assuming SS rectified & 32/32/16/16 filter caps & how many Henrys (something in the 1-9 range I know but how do they affect ripple, etc. kinda a mysetry to me still?) Any good explanations would be appreciated.
      Hey you... Yeah you kid... Ya wanna buy some "Magic Beans"?

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      • #4
        The second (master?) volume pot is connected to the lower phase inverter resistor instead of ground. You don't have any screen resistors drawn for the output tubes.

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        • #5
          ?????

          Thanks! I wasn't sure if I could do that with the 2nd stage vol or not... guess you answered that question & fixed I think. Also decided to take out the diodes... moore was saying wouldn't have enough gain to drive them (1.7v inv. p-p I believe) so go with what I know should work. No screen resistors... take their juice from 2nd filter tap right after coil similar to Tweed Deluxe design (5E3 I think) Thinking on going PS wise Weber Tweed Deluxe/18 Watt trans & use 270-0-270 tap, SS rect., dropping resistor(/), then 1/2 JJ 32/32 can, coil (1-9H, ???mA), 1/2 JJ 32/32 can then parallel taps, 10K/3w to 1/2 F&T can & an 18 Watt repl. OT... Pretty sure should be soild but should I add some 1K screen resistors?

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          • #6
            Originally posted by octal View Post
            The second (master?) volume pot is connected to the lower phase inverter resistor instead of ground. You don't have any screen resistors drawn for the output tubes.
            It's a boot-strapped volume control and it should work fine (more transparent than a regular volume control). Kevin O'Connor recommends using a series resistor between the pot wiper and the PI input though.

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            • #7
              One serious problem with the schematic is the lack of resistors to mix the channels. As drawn, the channels will be highly interactive (probably in a bad way). E.g. turning down the tone control on the clean channel will mute the sound on the OD channel.

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              • #8
                Originally posted by Howlin' Mad Mac View Post
                Pretty sure should be soild but should I add some 1K screen resistors?

                [ATTACH]13409[/ATTACH]
                Yes. Also add the mix resistors and add a 100 pF bypass on the OD gain control... You'll want to keep some sparkle in that channel..

                -g
                ______________________________________
                Gary Moore
                Moore Amplifiication
                mooreamps@hotmail.com

                Comment


                • #9
                  Hmmmm... mix resistors? Sounds simple enough but could you perhaps point me towards an example? I'm trying to picture some sort of summing network here but it's just not coming to me. As for the two channels, I wasn't really thing of one being so much of an OD channel, rather the more "traditional channel" providing a single-coil friendly Nashville kind of sound & the "bright channel" cooperating well with humbuckers with a hint of "bluesy" saturation. If more gain is desired a boost or good OD pedal could be used. That said, yes I did err on the schematic... the 100K resistor proceeding the "bright channel" volume pot should be wired between the wiper & ground, not inline as drawn. I'll fix this immediately & good catch! Also any thoughts on suppy filter choke specs (closer to 5 or 10H? how much current... 40mA? 70mA? more?) would really be of great help to me.
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                  • #10
                    Since u have two triodes in the preamp, and u obviously want enuff preamp gain for some dirt, why go this way with two low gain channels? The LED/diode clipping effect can be added to any stage even if u cascade the two triodes.
                    But if u want to try, and that's good...I prefer the first circuit with the LEDs. Plenty gain to clip those. Tho I'd add a diode to one of them unless u want really fuzzy dist, or perhaps a variable resistance so u can adjust the the way one LED clips compared to the other.
                    U need mixing resistors out of both channels, if not the signal will be attenuated by the non-resistor pot when it is turned down.
                    The last volume control going into the PI follower needs to go to gnd, and both input AND output needs to be AC-coupled. The PI then needs a grid resistor going to the cathode resistor.

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                    • #11
                      I haven't made up my other mind yet about a clipping circuit... if I do end up incorporating one I'll tweak it a bit, swap one side for a si/ge signal pair to make it asymetrical, perhaps some resistance on each side to soften it up a bit... Only time will tell but anyway take a look at these changes & tell me what you think... in blue fixed inaccurate drawing of bootstrapped volume, in red I'm thinking should work to keep one channel from affecting the other, & in gold is reference to AC coupling on the input as well as the output of the PI. Thoughts anyone?

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                      • #12
                        Originally posted by Howlin' Mad Mac View Post
                        Hmmmm... mix resistors? Sounds simple enough but could you perhaps point me towards an example?
                        Look at the 270k mixing resistors (one on each channel) after the V1 stage(s) in the 5F6A.
                        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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                        • #13
                          Latest Revision with Power Supply

                          Here's my latest revision with power supply shown as well... PT output is 540v (270-0-270) @ 50mA so expecting to see 381v following bridge & OT specs say 8.4K @ 16 ohm/9.2K @ 8 ohm/10.8 @ 4 ohm. Chomping at the bit to start punching chassis holes, etc. so look it over & let me have the good, the bad & the ugly.

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                          • #14
                            Oh, I think it's a nice circuit. Meanwhile back inside my "laboratory" heh, heh, heh... I've decided to make my 813 project into a metal amplifier... More to come...








                            -g
                            ______________________________________
                            Gary Moore
                            Moore Amplifiication
                            mooreamps@hotmail.com

                            Comment


                            • #15
                              Originally posted by Howlin' Mad Mac View Post
                              Here's my latest revision with power supply shown as well... PT output is 540v (270-0-270) @ 50mA so expecting to see 381v following bridge & OT specs say 8.4K @ 16 ohm/9.2K @ 8 ohm/10.8 @ 4 ohm. Chomping at the bit to start punching chassis holes, etc. so look it over & let me have the good, the bad & the ugly.

                              [ATTACH]13437[/ATTACH]
                              Oops... read PT specs wrong... 150mA (big difference) so plz change choke specs on schematic to 3H/125mA from 4H/40ma. Thanks
                              Last edited by Howlin' Mad Mac; 04-06-2011, 10:55 PM. Reason: typo
                              Hey you... Yeah you kid... Ya wanna buy some "Magic Beans"?

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