Can I use ground switch for switchable negative feedback without adding noise?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 17, 2019.

  1. itsGiusto

    itsGiusto Tele-Meister

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    I have a silverface princeton reverb, and I was thinking of putting the now defunct ground switch to use. It would be cool to have the NFB resistor be switchable, but is this possible without adding tons of noise? The ground switch is smack in the middle of the power section, so I can imagine that there might be a significant amount of noise picked up by the wires if I run them through that switch.

    One way I thought of to get around this is to switch the location of the ground switch with the power switch, to put it in a less noisy section. Would this work?
     
  2. tubeswell

    tubeswell Friend of Leo's

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    Yes. Have it so the switch-close position shorts the 47R resistor in the global NFB loop. This will reference the (PI) driver stage’s cathode directly to ground (killing the NFB).
     
  3. wabashslim

    wabashslim Tele-Holic

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    On my '67 PR I use the ground switch to open the 2.7K feedback resistor line from the OT. Noise is not an issue, I used a twisted pair, that may have helped. The effect is not exactly stunning but it works, a bit.
     
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  4. itsGiusto

    itsGiusto Tele-Meister

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    Is there a better mod that you would suggest instead?
    There reason I thought of making the NFB switchable is because I once tried a modern Fender reissue of the PR, and it gets some really cool overdrive, and mine does not. I think the NFB is one of the main differences between that circuit and the vintage silverface.
     
  5. wabashslim

    wabashslim Tele-Holic

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    Those modern amps have overdrive circuits built in, the NFB in our amps actually doesn't do that much - you'd be surprised. With it switched out I get just a tad more treble & upper mids, which boosts the volume a smidge which in turn allows a touch more distortion but very little - about as much as turning up the volume 1 notch at most. And IMO those old Princetons' distortion comes on pretty ratty, it's not a smooth glide into compressed heavenly hair. There are other changes that can be made to the PI wiring that are widely published, and they will smooth out the overdrive a tad but I returned all that stuff to stock config and use my PR for good Fender cleans and use pedals, or better yet, other amps for the OD. Leo designed those amps to be as clean as possible, ironic that they reached their pinnacle just as dirty sounds came into vogue.
     
  6. tubelectron

    tubelectron Tele-Meister

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    Yes, indeed : a Stand-By switch . It's what I did on my 70s SF PR. ;)

    -tbln
     
  7. itsGiusto

    itsGiusto Tele-Meister

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    I made the NFB switchable in my 5F1, and I feel like it did exactly what you said, changing it to be more or less like "a smooth glide into compressed heavenly hair" or more clean with an abrupt transition to ratty, depending on which setting I used. I felt like the NFB has a great deal of control over whether it sounds/responds like a tweed or blackface amp. What values of NFB were you switching between?

    The modern reissues of the PR do not have overdrive circuits built in, they're almost exactly the same as the blackface and silverface PRs, with the custom 68 having some minor modifications. See the schematics here:
    https://guitarampblog.wordpress.com/2017/01/26/fender-65-princeton-reverb-schematic-vs-68/
     
  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I built a Princeton Reverb earlier this year and did a 3-way NFB switch. It made a huge difference in the brightness and breakup. I did stock 2.7k, Custom 5.6k, and then those two combined. I really liked the added treble of 5.6k, and it was noticeably louder. Worth doing for sure.

    Sent from my XT1585 using Tapatalk
     
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  9. Wally

    Wally Telefied Ad Free Member

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    That NFB loop in the PR is already providing less phase cancellation than some other Fender NFB loops. Doubling the resistance of that top resistor would make the amp a bit livelier, but more resistance there would take the amp to a different level.
    I like to use a pot in series with the top resistor. If one used a 25K pot, one would witness a very noticeable difference at the maximum resistance...and have a range of choices as o what one wanted.
     
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  10. Paul G.

    Paul G. Friend of Leo's

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    Why?
     
  11. Jorgen83

    Jorgen83 TDPRI Member

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    Well I added one to my 5e3, and the difference is night and day. To be fair I changed the resistor value so it’s more prominent. I don’t like it as much as the stock non feedback tweed sound though.
     
  12. tubelectron

    tubelectron Tele-Meister

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    Well, the Princeton Reverb do not have one, and here in Europe the GND switch is useless, so it's interesting to replace it for a stand-by feature that you find on other bigger Fender models, like the Deluxe-Reverb (which uses 6V6GTA also).

    I find the SBY SW very useful on stage or in studio, while pre-heating, pausing or preparing to play. And it's good for the tube's life expectancy...

    But it's me, OK ? :D

    -tbln
     
  13. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    <in before this turns into a standby switch thread...>

    I have a negative feedback on a SFPR and do find that's useful, especially when trying to keep volumes down. The change it noticeable, but I haven't flipped the switch too many times since I put it in.
     
  14. itsGiusto

    itsGiusto Tele-Meister

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    Before (or after) this becomes an NFB thread... The main thing I wanted to find out about was how to prevent noise in this solution, since the ground switch is in noisy territory, surrounded by power lines.

    I just swapped the ground switch location with the power switch, so hopefully that'll work. The power switch was right next to the speaker jacks, so I won't have to run the wire too far to add switching options for NFB. But I'll have to wait until my new NFB resistors come in the mail before I can actually try it out.

    Unfortunately, the other mod I thought about doing was to add a mid-pot in place of the AC receptacle-out, cause I never use the receptacle (who does?). But once again, I don't know how I can do this if there's so much noise in that general area of the amp, and I'm worried it'll be too noisy to use.
     
  15. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I haven't run into a noise issue no matter how I implemented the NFB switch. I've done two plain wires, shielded (if you do use a shield, make sure to only ground one end), and both have been fine.

    Mid pot also works fine without noise issues - I ran plain wire for that one as well.

    The real answer is - just try it. If you have to deal with noise after, then you'll know, but I doubt you will.
     
  16. itsGiusto

    itsGiusto Tele-Meister

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    What sort of shielded wire do you use?
     
  17. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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  18. itsGiusto

    itsGiusto Tele-Meister

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    Hmm, well I have wire like this:
    https://www.tubesandmore.com/products/wire-22-awg-stranded-tin-braid

    Will this do? It doesn't have an insulated exterior around the shielding, the braid is the outside.
     
  19. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I wouldn't - you'd risk shorting something out as it lays across the other components.

    I really think you'll be fine with any wire, even it's not shielded.
     
  20. Wally

    Wally Telefied Ad Free Member

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    There is no reason to have a standby switch except if one wants to change a speaker connection or to pull a tube. There is NO harm that can be done to a tube in these guitar amps by applying full voltage before they are warm. There can be no cathode stripping at the voltage levels these amps deal in. Cathode stripping is a concern IF the voltages are up around 10K. When I first encountered this question about standby switches and their usefulness, I did some searches and got to some engineering papers on the subject. Lord Valve discusses it in his excellent work.
    If there is a standby switch, then one should use it sparingly since there is a harmful effect when the amp is left in standby for long periods. This causes cathode poisoning which does cause a gradual deterioration of the tube life.

    Sorry for the quick hijack.....
     
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