Old shool octal build

Discussion in 'Amp Tech Center' started by tobyk, Jun 8, 2021.

  1. tobyk

    tobyk TDPRI Member

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    Hi all, would really appreciate if you good people took a look at my schematic for a build I'm planning. I want a hot rodded, old school early 50's amp, all octal (6SJ7+6SL7 with the ability to try the 6SK7 in place of the pentode to explore non-linearity). When cranked however, I'm hoping for some Marshall-esque tones (thus the .68 uF cap in V2).
    Also, a grid leak/cathode bias switch in the input stage, I'd like to see what difference it makes.
    Do you see any mistakes in the drawing? Anything you would have done differently?
    Thanks!
     

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  2. Jon Snell

    Jon Snell Tele-Holic

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    First major issue is the DC coupled anode to grid of the output valve.
    Second is the choice of 680k for the grid leak. 6L6 is maximum 470k as is 6V6.
    Third is an unstable 6SL7 stage, first valve, using 90° feedback.
     
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  3. tobyk

    tobyk TDPRI Member

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    Thank you! Could the 90° feedback problem have been remedied with a resistor left of the connection, and if so, of what value?
    Well, I decided on a more normal gain control anyway. Does this look better? Are the dropping resistors and filter caps looking ok?
     

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  4. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    ^ Yeah, I was curious what the direct wire around the first 6SL7 stage was supposed to do.

    The tone control will only roll off highs (just like your guitar) so I think it might be disappointing in use. Consider using the one that Fender did in the 5F2A, which can brighten the sound as well.

    The 2.2k/0.68uF will trim off a little bass, but you could accomplish the same thing by using smaller coupling caps. Crunch the numbers to find your Fc for each coupling cap/grid-leak resistor along the way.

    Subscribing to watch this thing take shape!
     
  5. tobyk

    tobyk TDPRI Member

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    The idea was that when the gain control was set low or to zero, half a 6SL7 (V2) would be bypassed.

    I’m used to these tone controls, so no problem. But one thing I’d like perhaps is a mid control that is able to add mids, as well as subtract. Does anyone have a good circuit for that?
     
  6. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Problem is, any time the gain control isn't turned all the way down, that triode is going to have the output from its plate on its own grid. You can imagine how that could get you into trouble. ;)

    If you're out to do a variable boost, or mix two channels, that's much easier to do.

    Using an inductor would be the old-school way to boost/cut frequencies around a particular band. Lots of ideas in the classic Radiotron Designer's Handbook. But something like the Framus midrange control just uses resistors and capacitors, so it might be a little more approachable. Twisting a knob takes you from flat to a nice-looking scoop. A guitar amp with a truly flat response sounds pretty midrangey anyway, so a mild scoop would probably do the job.

    Have you dinked around with the Tone Stack Calculator yet? :cool:
     
  7. tobyk

    tobyk TDPRI Member

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    Thanks for the link, will check it out. I think I’ll stay in the 50’s though, and not have a full tonestack. It’ll be quite lossy, too. The Framus mid control I think only cuts mids, but could perhaps be nice anyway. Problem is, I only have place for three knobs in the chassis..
     
  8. tobyk

    tobyk TDPRI Member

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    But it should be possible to dial in or bypass gain from an extra stage just the way you dial in reverb, without taking it from another channel, no?
     
  9. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Tone Stack Calculator has a lot of tone circuit simulations that aren't "tone stacks", too. If you're clever, you can do a lot to change the voice of the amp with one knob. :cool:

    Well, look at how reverb is implemented in Fender amps -- they're not trying to "bypass" anything completely, they're carefully mixing from two stages into a third. I get what you're trying to do! Here's a thread where someone tried replacing the reverb circuit with a couple of cascaded triodes for more gain, but as one might expect, he had issues with the output of the OD section feeding back into its input: https://el34world.com/Forum/index.php?topic=16416.0 If you are careful with the mix resistors, you might be able to make something like that work, but...

    I think a truly vintage-approved way to do this might be to look at some old amp schematics where they had independent "instrument" and "microphone" channels, like the Gibson EH-150. The microphones of the day had a lot less output, so the "microphone" channel featured a bunch more gain. You could implement a blend control between the two channels, and then drive both channels from the same input, so that it's all real clean from the outside. Maybe use the 6SJ7 for one input, and a 6SL7 triode for the other?
     
  10. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Another thought: if you're hoping for the "Volume" control to act as a master volume, you might get better results by moving it later in the chain. Substitute the 6V6 grid-leak resistor with a 250K or 500K pot, and then you'll have one more preamp stage to beat up before knocking down the volume again. :)
     
  11. tobyk

    tobyk TDPRI Member

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    Thanks for understanding me! Yes, I’d rather have it old school basic, with no Gain or Master Volume controls, which feels too 70’s for this amp. Maybe I can just call both knobs Volume..
    I thought about having two combined channels and that is certainly more vintage correct, but in order to get a gain boost they need to be cascaded I guess. Just blending them won’t do much in terms of gain, right?
    If I were to cascade two channels I guess it could be done at the input jack, like in this thread:
    https://www.tdpri.com/threads/thoughts-on-a-single-ended-valco-inspired-build.876185/

    The reverb mod link was interesting and in line with what I had in mind.
    I was thinking that a reverb dwell control cirquit, connecting before the volume, like Gibson Falcon/Scout has it, perhaps would work as a dialed-in gain stage as well (please see Falcon schematic).

    3BCBDD7F-F86C-4365-9D3D-E01F8846A0B0.jpeg
     
  12. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Another idea I found while searching: check out the "fatness" control in reply #11 of this thread: https://el34world.com/Forum/index.php?topic=18208.0

    Run two triodes off the same plate resistor at the input, but put a volume control on the grid of one of them so you can dial it in or out, then give them differing cathode resistor/caps for thin and fat tones to mix.
     
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  13. tobyk

    tobyk TDPRI Member

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    Thanks @ThermionicScott , interesting ideas there as well!
    Ok, so I took another shot at the dial-in gain circuit – what's the verdict? :) Is it better to just have it like in version 2 above, or does this version 3 have anything going for it? Would feedback still be a problem here? ver3.png
    @Jon Snell
     
  14. Jon Snell

    Jon Snell Tele-Holic

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    Your circuit is the same as the first one and are trying to apply 180º phase shift across the tone control and have forgotten to add a grid leak resistor to V2B.

    If you study Version 2, where do you get the notion of 'dial in gain' from I wonder. V2 is a standard amplifier with a reverb unit and tremelo, nothing more.
     
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  15. tobyk

    tobyk TDPRI Member

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    I didn't mean the Falcon schematic was Version 2. I roughly copied the reverb circuit from Gibson Falcon and applied it as a gain stage instead, in my latest schematic.. If it works in the Falcon, why not in my amp? Well, I guess I'll leave that idea and do an ordinary, linear gain structure!
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Inkedver3_LI.jpg
     
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  17. Jon Snell

    Jon Snell Tele-Holic

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    Why not try it and find out. It will be a good learning curve for you and simple to rework if it doesn't do as you expect.
     
  18. tobyk

    tobyk TDPRI Member

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    Thanks – got it. So why is the Gibson reverb circuit working (apparently not out of phase)?
    EDIT: I guess I need another gain stage after V2A, to switch phase again. I have an odd number (1) of stages in this parallell circuit, whereas Gibson has an even number (4), reverb transformer included.
     
    Last edited: Jun 11, 2021 at 2:44 PM
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  19. tobyk

    tobyk TDPRI Member

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    Yes, but I think it would be easier to learn this theory first before building..
     
  20. mrriggs

    mrriggs Tele-Meister

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    There is no phase through a reverb. The signal is being converted to mechanical motion, transmitted through a couple jiggly springs, then converted back to an electric signal. The signal you get back is a big wad of random, chaotic, short delays. Some small portion may, for an instant, be in or out of phase with the clean signal but the vast majority will be between a perfect 0° or 180° and constantly moving and changing.
     
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