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SV20 to JTM45 ?

Discussion in 'The Workbench' started by Mcentee2, May 18, 2021.

  1. Mcentee2

    Mcentee2 Active Member

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    I love my SV20, it does everything I need, except it's not two of them, Lol!!

    I was looking for a JTM style amp to run in parallel and/or different flavour, but can't find any "small wattage" versions around.

    I'm seriously thinking of getting another SV20 and modding it's preamp to JTM specs.

    I know the JTM is tube rectified, and the plexi/sv is ss, and the power section is fixed vs cathode, but these sort of balance each other get some compression (bloom?) from the sv power section akin to the tube rec of the JTM - not perfect but good enough methinks.

    Pretty sure kt66 can run in the sv too if that added true JTM flavour, I have successfully had 6ca7 and is 6v6 running fine with great aural results, but given fundamentally a modded sv isn't a spot on JTM anyway, close enough is fine :(

    So I'll concentrate on the pre PI bits.

    A couple of notes from my previous inspection of the sv:

    High Treble Bright cap is 4n7

    Split v1 cathode

    V1b 820r with 330uF (Channel 2, Normal)
    V1a 820r with 0.68uF (Channel 1, High Treble)

    Next on my list is to take the sv apart Gain and trace some more.

    Anyone got any gotchas or guidance before I dive into this?
     
  2. Pete Farrington

    Pete Farrington Active Member

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    A JTM45 power amp has about the greatest degree of feedback around it of any valve guitar amp, which may be seen as being significant to its tone and response.
    It’s unknown whether other amps could accommodate that much feedback without oscillating, as it largely depends on the OT spec, eg phase shift at 50kHz.
     
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  3. Mcentee2

    Mcentee2 Active Member

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    Interesting, I wasn't going to look at the PI or power side of things, but yes, the feedback R is different to be sure.
     
  4. _Steve

    _Steve Well-Known Member

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    I tried KT66s in my SV20 and thought it sounded terrible. I suspect the bias resistor is not enough to get in the range of the KT66s. So you might want to consider that before you splash out on really expensive KT66s.

    I think your best bang-for-buck would be the shared cathode and the mixer/bright caps mods, and sticking with EL34s.

    I know the bright cap is on the daughter-board, and I suspect the mixer bypass cap is on there too making them two an easy mod.

    You'll probably be OK reducing the NFB resistor a bit, but as @Pete Farrington points out the JTM45 has and extreme amount (it runs off the 16ohm tap vs the 8) you might not be able to reach. You can just alligator clip in another resistor in parallel to test and see if it works or not.

    You could also maybe consider the tonestack slope resistor and whatever-the-250/500pF-cap-is-called. I assume they are stock Marshall (33K/500pF) in the SV20, whereas the JTM45 uses Fender Bassman values (56k/250pF). I think these could be on the daughter board too. I'd do anything you can to avoid lifting that main board! :)

    Good luck!
     
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  5. _Steve

    _Steve Well-Known Member

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    This is interesting and I'd never heard of this before. Anywhere I can go to learn more about that?
     
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  6. Pete Farrington

    Pete Farrington Active Member

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    Try reading up on ‘classical control theory’. I found it very enlightening but the concepts can be tricky to get your head around.
    Due to rarely actually undertaking an analysis after completing the module, Ive forgotten pretty much all the detail after all the years :rolleyes:
    But for the few months that I could use Laplace Transforms, and convert between time and frequency domains, I felt like I’d lifted the lid and was able to see and understand the mechanisms by which amps truly worked.
    Whatever, the above was one of the practical take aways that have stuck with me. It’s why hifi amps, which have higher spec OTs, can accommodate higher degrees of NFB than guitar amps.
    And it’s why very poor spec OTs can cause instability even with regular guitar amp degrees of NFB, eg https://www.thegearpage.net/board/i...hase-inverter-question.2100866/#post-29599598
     
    Last edited: May 19, 2021
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  7. JM5010

    JM5010 Active Member

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    I have used as low as 33k >> 25k >> 0.68uF coming off the 8ohm tap with great results. I'm guessing that the 16ohm tap with an smaller NFB resistor = more oscillation
     
  8. JM5010

    JM5010 Active Member

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    I have been considering modding an SV20, are the boards really that flimsy?
     
  9. _Steve

    _Steve Well-Known Member

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    Ah Laplace Transformations... I vaguely remember a lecture about them 20 odd years ago and not understanding them even then :p
     
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  10. _Steve

    _Steve Well-Known Member

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    Yeah they're not great. Traces are very/small close too. Guess it depends on your level of experience dealing with PCBs. Mine is very low.
     
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