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My worst idea ever? Greatest? Both? (12 stage pre)

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  • My worst idea ever? Greatest? Both? (12 stage pre)

    So, I built this amp for an old bandmate of mine, into an amp he didn't really like and had a taste for power caps: an ampeg vl-1002. I build him a sweet, sweet ass two channel thing that suits him perfect, i'll spare y'all details. I ended up with the VL, which is cool because i'll build a dumble or something brawny with the trannies, I think. BTW? Fav amp is my DR build, i just like building silly things in my spare time.

    So i'm cleaning my workshop last night and I'm moving the thing for the umpteenth time (duval's rule: the frequency with which an item needs to be moved is directly proportional to it's mass, times its volume) and I think, hey, this thing has potential.

    The trannies are already mounted, and one's prolly still good after all the abuse, and it has that nice driver tube hole over by the power tubes, switches... etc...

    oh yah, and 6 preamp tube holes.

    I'll put it out front- i've built a buncha unique and good amps, but i'm not a primarily mathematical builder, so this is a stretch. I'm gonna go order the parts and do this anyhow, but i'll ask to see if anybody's done something this foolish before. Luckily- no fear of failure or endless rework. Is there any reason, besides lead dress, that the typical 100k/1-2.2k/meg pot or divider stage and knockdown resistors couldn't continue to be scaled up to, say, 12 stages, or 11 with a cf to drive the tone stack? I'm thinking of just propagating the entire front of the amp with gain knobs. Then, I can start tapering the fool thing back down so the range through them actually is functional. If it never works, well, I can break it down and build a multichannel back into it or use it to prototype stuff.

    I guess I am asking for some starting points on the coupling caps, really the only spendy part in the thing to start messing with. I'm betting the largest in the whole thing is .022, but even that seems large as a starting point. I'm anticipating theres going to be very minimal cathode bypass in this pre, and a cold stage, so that'll perhaps come later. I'm never going to ask any stage to do much so I don't have to throw it away and I suspect that adds to the compressed feel of typical ultra gain amps.

    (sug)jest away! There is no goal here but a dumb amount of gain, but with more feel than the triple rec/krank space. The newest 5150, iirc, has 7. This is five louder!

  • #2
    With that many dual triodes to work with I would just do one of everything: cascode input, parallel triode stage, cathode follower, coupla common cathode gain stages, and maybe a couple of triodes wired as diodes to provide clipping/limiting. All 12AU7's, no cathode bypass, maybe some diode or LED cathode biased stages.

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    • #3
      I think you'll end up minimizing the DB of each stage to get the amp under some control. And at that point there will be more series resistance overall in the signal chain resulting in a ton of hiss noise that is avoidable by reducing the number of stages.

      Of course we'll never know what kind of extreme high gain can happen if we don't build one. Could be at least interesting.

      To my thinking a better idea would be to build a more typical high gain preamp, a dedicated clean preamp with a seperate tone stack, include an effects loop and reverb. If you still have a tube left over add a trem.

      Chuck
      "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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      • #4
        My amp has 6 preamp stages, and then a PI. Of coure, one can get lots of preamp overdrive from fewr stages, but I wanted to try using lots of preamp stages, and over driving each one quite mildly, attenuating the signal from the previous stages using split loads, and seeing how it sounds. I use .047u coupling cap after the first stage, but from then on use .1u coipling capacitors with 220k grid resistor, and I dont have any apparent problems with blocking distortion. The firt few stages I over drive kinda lightly, but the 6th stage I over drive kinda hard to get the overdrive sound that I want. But that hard overdriving gives it an edgy sound, so I'm thinking about adding in another preamp valve so I can get it as saturated as I want using mild overdrive all the way. The clean tone (low volume after the first preamp stage) is really sweet. So, I'd like to see how you project turns out.
        A

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        • #5
          Chuck, thats the kind of thing I was worried about, was s/n artifacts. I was planning to use my favorite rn65 plate resistors to minimize hiss. Perhaps this is one of the reasons noise gates have shown up in this sort of rig- that might be a fun tech to work with, too, but it doesn't really seem like anybody's really got that right, ever! Active gain lowering like a radio might be neat... I may also step up to the 1w dale rn70 for the plates and do 1/2w 65s for the rest. Those resistors are ridiculously awesome- robust, stable, quiet!

          You're right, there's an empirical imperative here. Must... Adventure... Forth! Here's my plan B, incidentally. Clean Channel: fender (2 stage) non reverb, then that stages up to a d-inspired thing. Thats 2 dual triodes. That leaves 8 triodes for high high gain, a la slo+ or 5150+. A loop might be cool here! I'm totally pimped on the belden digital reverb, which should be cool for something like this.

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          • #6
            you've seen this right?>



            IME you don't need so many stages to get gut wrenching dist. and sustain, even the 5 stage 5150 has one set up for unity gain. check the Ax84 LeadII with 3-4 tubes or even the (2 tube) Ax84 Ubergain pre

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            • #7
              Re: Carvin Quad-X. The only thing I can think of when I look at that schematic is "To me, quantity IS quality!"

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              • #8
                It's fun to see how every other stage is throwing almost the entire signal away! That kinda proves the point made in the above posts!

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                • #9
                  Originally posted by octal View Post
                  Re: Carvin Quad-X. The only thing I can think of when I look at that schematic is "To me, quantity IS quality!"
                  Companies still markets their amps as "having an extra tube for more gain" or some such...people (men?) are often too eager to go down the "more is better" road witness various porn actors...and this guy's amp:

                  The Big Ass Guitar Amplifier | ken-gilbert.com
                  nice guy...crazy amp...

                  Hmmm never noticed the (Al foil?) cap "crazy hats"...

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                  • #10
                    Well, tedmich, that carvin schematic certainly took the wind out of my sails. I guess I can than you for saving me a benjamin in parts!

                    what to do with the monster is the question. I found it a nice place where I won't have to move it until I figure it out!

                    thanks guys-

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                    • #11
                      Originally posted by tedmich View Post
                      Hmmm never noticed the (Al foil?) cap "crazy hats"...
                      Someone over at the slo forum was talking about getting shocked from the cap ends. Makes me wonder if its tape to insolate the ends.
                      -Mike

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                      • #12
                        Originally posted by defaced View Post
                        Someone over at the slo forum was talking about getting shocked from the cap ends. Makes me wonder if its tape to insolate the ends.
                        Ken said specifically that the tape over the caps is the result of those caps floating at high voltage. It's out there on the forum or his website somewhere.

                        jamie

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                        • #13
                          Originally posted by imaradiostar View Post
                          Ken said specifically that the tape over the caps is the result of those caps floating at high voltage. It's out there on the forum or his website somewhere.

                          jamie
                          I find that a little odd, check these series caps rated at 550uf 1800VDC that Apexjr sells for $35
                          just an Al rail on the ends....

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                          • #14
                            Yeah, it does seem odd. I was just repeating what I thought I'd read.

                            jamie

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                            • #15
                              no, you're right about those caps and why the tape is there.

                              the issue is not their voltage capacity, which is 500vdc--it's the fact that they're stacked in a full wave voltage doubler supply, which means the "ground" of the "top" caps is at B+/2. in this case, 305vdc.

                              trust me, i only had to brush my palm on them once before the tape got added.

                              -ken

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