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Prosonic suppression cap excision

Discussion in 'Amp Central Station' started by jondanger, Dec 29, 2018.

  1. jondanger

    jondanger Poster Extraordinaire

    Jan 27, 2011
    Charm City, MD
    All of you who love your Prosonic, but feel as though it is just a *little* too dark, listen up. I went hunting for a solution to this, and found a note, professed to be from Bruce Zinky himself, saying that the suppression cap on V1 is unnecessary, and clipping it out could put some more sparkle in the amp.

    http://www.blueguitar.org/new/schem/fender/modnotes/prosonic_notes.txt

    It’s in the “Improvements” section.

    Well, I clipped it, and I’m here to report that it has been a great success! It’s not in Blackface territory (and shouldn’t be), but it does have some more sparkle and zing. I even turned the bass up past 2! If you’re looking for a little more brightness, this is a super easy mod worth exploring. It would be super easy to put it on a switch too, if that’s your thing.

    Cheers!
     
  2. jondanger

    jondanger Poster Extraordinaire

    Jan 27, 2011
    Charm City, MD
    I should have been much more clear about what cap it was - cap C301 on the schematic. It’s a 5pf ceramic disc on V1 - the only one there.


    Sent from my iPhone using TDPRI forum mobile app
     
  3. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    That’s interesting that’s the result you get.

    The 5 pf C301 connects the plate of v1 to the grid of v1.




    That should be positive feedback, and the high capacitance should be passing high frequencies, so with the cap in I would think it would boost high frequencies.
     
  4. horseman308

    horseman308 Tele-Meister

    456
    May 7, 2016
    US
    Interesting. I'll have to have a look.... Thanks for the tip!

    Sent from my SM-G930P using Tapatalk
     
  5. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    It’s negative feedback, so the cap will roll off the high end.
     
  6. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    How would that be negative FB?


    The higher voltage in-phase signal from the plate is fed back to the grid.




    Negative feed back is when out of phase signal is fed back.
     
    Last edited: Dec 30, 2018
  7. Bob Arbogast

    Bob Arbogast Tele-Afflicted

    As the grid goes positive, plate current increases. As plate current increases, plate voltage goes negative (more voltage dropped across the plate load resistor). So it's negative feedback.
     
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  8. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Jondanger, did you remove the other cap on V1a? Zinky suggests removing that one as well? I have done neither. Perhaps that is where I can find that high harmonic overdrive content that is the only thing missing from these amps, ime.
     
  9. jondanger

    jondanger Poster Extraordinaire

    Jan 27, 2011
    Charm City, MD
    Is that cap on the tube socket or is it actually on the board? In mine the 5pf on V1b was the only one directly on the socket. I don’t have access to the schematic now, but I could take a look when I get home this evening.


    Sent from my iPhone using TDPRI forum mobile app
     
  10. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Jon, after reading the Zinky notes where he says to remove ‘all’ of those ceramic caps on the tube sockets, I went to the schematic...
    https://schematicheaven.net/fenderamps/prosonic.pdf

    The cap you have removed is on V1b. The schematic shows another cap, C7, connecting the plate and control grid on V1A, which is the second gain stage that is added in the Lead mode. That cap is a 6.8pfd. Maybe it is not there on your amp. I need to retube mine and will take note of what is there on mine, which is an ‘L’ serial number.
     
  11. jondanger

    jondanger Poster Extraordinaire

    Jan 27, 2011
    Charm City, MD
    Wally - mine is a L also, with a December 95 date IIRC, very early. Interested to hear what you find.


    Sent from my iPhone using TDPRI forum mobile app
     
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  12. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

    That’s right. Thanks


    The grid and the cathode are in phase.

    The plate and the cathode are out of phase.



    And the plate and the grid are out of phase.
     
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  13. jondanger

    jondanger Poster Extraordinaire

    Jan 27, 2011
    Charm City, MD
    I finally had time to open her up and locate C7. It was indeed on the board . . . and now it’s not. I think this one made a bigger difference than C103. The overdrive channel sounds more open and airy, which I prefer. It is maybe a little raspier, so if you hate that, it might not be for you. Here’s a photo of where it was:

    [​IMG]

    This is looking down the board from the preamp side toward the power section.

    So, end result I think is that I can recommend clipping both C103 and C7 out if you’re looking for a little more top end out of your Prosonic. Cool!
     
  14. jondanger

    jondanger Poster Extraordinaire

    Jan 27, 2011
    Charm City, MD
    I guess the next thing to look for is an effects loop mod that makes it work with pedals. Apparently these were designed for rack effects, and they don’t seem to get along with pedals in the loop.
     
  15. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    28
    Feb 22, 2009
    New York
    Are you going to hear 5pF capacitance in this instance? Seems like it'd mostly be dealing with ultrasonic frequencies, in which case you'd definitely want to keep those caps, as the high end cut will not be audible, but will stabilize the amp more.

    Calculate the cutoff frequencies before you go clipping caps because someone online said so.
     
    Wally likes this.
  16. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    28
    Feb 22, 2009
    New York
    You need a buffered FX loop for that. The loops 'designed for rack effects' tend to just be passive loops. The lack of buffer makes them less compatible with floor effects, as the amp tends to overdrive the units, plus you lose highs through the cable runs. There are some external FX loop buffers that should work, as building one into the amp may not be that viable.
     
    Wally likes this.
  17. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Remember that due to the Miller effect, Cg-a (grid to anode capacitance) is multiplied by the gain of the tube.
     
  18. jondanger

    jondanger Poster Extraordinaire

    Jan 27, 2011
    Charm City, MD
    I definitely feel as though I can hear a difference with both. I had the bass tone control down around 2, the mids around 3, and the treble at 7. Now I can run the EQ a lot closer to flat, which gives me more flexibility in different rooms.

    WRT the effects loop, I’m not too pressed to put pedals in the loop, but if there was something simple I might give it a shot. Seem to remember Brian P mentioning a 12AU7 in the effects loop driver?
     
  19. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Re: the removal of those caps. Since Bruce Zinky asks in his mod suggestions for the Pro Sonic something to the effect of ‘what was I thinking?’, one might have to wonder as well. Maybe he was thinking in terms of the high Gain/overdrive that these amps can do in those four tube gain stages for the Lead mode????
    I like what the stock version does. I have to retube soon and I might check out these two caps. I already know that I would like for them to be switchable IF their removal from the circuit would do something that I liked in some operational mode. I still would want them available because I do like what this amp can do. It works well with the bass rolled off....as most amps based on this type of topography can do. It has a lot of overdrive available.
    In comparison, the Super Champ that is based in the Bf topography but with added gain does a different thing...and all of that BF/Sf high end is there. It is a different kind of clean and overdrive in comparison to the Pro Sonic. It is good to have both kinds available, I suppose.
    I have a friend’s Soldano Hot Rod 50 sitting around. Interesting amp...very well built. And....it has topography that is similar to the PronSonic.....only it has 5 gain stages in the overdrive mode. It has the Warren Haynes factory mod, which lifts a bright cap only in the overdrive mode. I don’t care for the mod very much....it is too dark for me. Maybe with a bright insrument it would work? I have gotten to know the amp and like it better than I first did....I am running a 12 AY7 in V1. The stock 12AX7 in V1 had a ragged edge. The lower gain AY took that away yielding much smoother distortion and still an enormous amount of overdrive.
    back to the suppression cap mod...sometimes designers change things after thinking about it or when presented with a user’s request.
     
  20. jondanger

    jondanger Poster Extraordinaire

    Jan 27, 2011
    Charm City, MD
    Wally - making the cap mod switchable would be pretty easy. I suppose you could save space on the back panel by using a DPST and switching both at the same time, since they have a similar effect on the clean and lead modes. In general I prefer a brighter amp, so I didn’t feel as though this warranted drilling the chassis for me.

    I have considered adding a rotary impedance selector to take advantage of the multi-tap OT and build a matching 2X10 (or maybe even a 1X15?!) to set this on. At this stage in my life though, imagining the need for a 4X10 seems a little ambitious. All my friends and I have young kids, and we are playing our first show in about 18 months in February. I’ll be using the Prosonic, but I don’t expect to be able to wind it up to its full volume potential.

    If anyone knows a source for this surf/seafoam green lizard skin tolex, I would buy a couple yards just to have it on hand. Can’t locate this stuff anywhere.
     
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