Prosonic questions

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by DanD, Jan 9, 2008.

  1. DanD

    DanD Member

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    Is there a mod that can be done to raise the frequencies of the treble and mid-range on these amps?

    Am I correct, that these use an active tone circuit?
    Could it be as easy as increasing the midrange cap value, and decreaseing the treble cap value?
    or
    Would it be better to just pull the celestions and put in a brighter speaker?

    Your input is appreciated,
    DanD
     
  2. AdmiralB

    AdmiralB Silver Supporting Member

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    It doesn't have active tone. It's pretty much a Boogie Mark II with the tone stack moved back just before the PI, and no NFB.

    The tone stack is pretty much BF Fender, but there is a resistor in parallel with the bass pot that limits it to about '4' on the Fender scale. There are also numerous high-end rolloff caps.

    I think these respond really well to tweaking, but you have to be willing to experiment.
     
  3. DanD

    DanD Member

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    Thanks AdmiralB,
    So the active tone stack is a myth. That's good cause the amp is just about there when the treble is dimed.
    Which cap(s)would you recommend?

    DanD
     
  4. Dana

    Dana Guest

    I love the Fender Prosonic. I get more compliments on my tone when I use it.
     
  5. AdmiralB

    AdmiralB Silver Supporting Member

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    Take out R20 - this will restore the full range of the bass control.

    Take out either R2 or C1 - these are in series, and are a treble shunt after the first gain stage.

    You can get fancier from there if you like. Since there's no NFB, there's no presence as such - you could add an NFB circuit with a fixed (or trimmable) presence boost.

    One thing I like is that there's an unused half-triode. I configure it as a DC-coupled cathode follower to drive the tone stack. Do that, and change the tone stack values to Marshall, and you've got a pretty mean amp.

    If you have the head, you can run EL34s if you beef up the screen resistors (and tweak the bias accordingly). I don't know if the combo has enough filament capacity, since it's got two extra triodes.
     
  6. DanD

    DanD Member

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    Thanks again AdmiralB,
    I'll give the R2 and C1 a try.

    DanD
     
  7. DanD

    DanD Member

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    AdmiralB,

    I gave the R2 and C1 mod a try.
    It increased the overall response of the amp.
    It's sounding like a good fender amp now. Like taking the blanket off.

    I did notice that on a certain note the amp would give a sound like tearing paper.
    I thought maybe it was a tube so I tried tapping on each with a chopstick and didn't get any noise.
    I was using a BugleBoy in v1, so I put in a Phillips. The gain increased and the sound got worse. Still only on that one note. All other notes sound fine.
    I guess I could just not play in that key:)
    Any other suggestions?

    DanD
     
  8. AdmiralB

    AdmiralB Silver Supporting Member

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    Sounds like a resonance. Is it a combo?

    Everything tight (speaker jacks, etc.)?
     
  9. DanD

    DanD Member

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    Yes it is a combo.
    I never thought that it could be something loose.
    Got my work cut out for me.

    DanD
     
  10. phsyconoodler

    phsyconoodler Member

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    When you alter the circuit you may induce an oscillation that can sound like a torn speaker.There are some mods on the net if you google prosonic/zinky mods.Seems he has a few 'fixes' for his amp design.One of which is replacing the output transformer with one of his rather pricey units.I have tried some of those mods and they did improve overall tone.
    There are some snubbing caps on the preamp tubes that should be removed as well.
     
  11. AdmiralB

    AdmiralB Silver Supporting Member

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    If that were the case it wouldn't be only with one input note.

    I've added a LOT of gain to Prosonics and never had oscillation issues.
     
  12. phsyconoodler

    phsyconoodler Member

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    Added a LOT of gain to a prosonic?How much more gain would one actually need in an amp that has too much already?
    An oscillation can occur on one note by the way.Put it on a scope and watch the frequency spike on that one note.Not saying that's what's happenning here,but it can and does happen.5F6A bassman's are famous for that.Originally they didn't have grid stoppers and with higher voltages they can oscillate on certain notes.Pop in the grid stoppers and they stop.
     
  13. DanD

    DanD Member

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    My amp has a small 5pf ceramic cap on v1b between pin 6 and 7.

    The schematic shows a small cap between pin 1 and 2 on V1a, my amp does not have a cap on the tube pins at that location. Possibly on the board?

    Would those be the snubbing caps?

    DanD
     
  14. AdmiralB

    AdmiralB Silver Supporting Member

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    I guess different people have different notions of how much is too much. The Prosonic doesn't have *that* much gain - compared to a lot of other amps - due to the way the second gain control is implemented.

    I've changed that in the past to me a simple interstage voltage divider, like a Mesa Mark II-IV lead drive control. It does allow for a LOT more gain, if desired.

    I've certainly not seen everything, but every amp I've fiddled with that oscillated on a single note, or a range or notes only, did so due to issues that had nothing to do with overall gain.

    If grid stoppers on power tubes quell oscillation, it ain't gain related.
     
  15. DanD

    DanD Member

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    AdmiralB, Phsychonoodler,

    I took both of your suggestions.
    When I fired it up it sounded great for a couple days then the thing started getting microphonic.
    So...I undid the mods, and the microphonics were still there.
    So I pulled pre-amp tubes starting w/v1.
    When I pulled v4 (12at7) the microphics stopped.
    I continued to v5-v7 and no changes. Nice and quiet.

    I replaced the bad (telefunken 12at7) with a (sylvania bp 12at7) and the amp worked fine.
    I redid the mod1 removal of resistor capacitor series circuit from v1 to gnd. Sounded better.
    Redid the series circuit to the bass pot. Smoothed out the bass response.
    Redid the parallel circuit removing 5pf cap between input and high voltage in. Increased the brightness a bit.

    I took the amp to the gig. Plugged it in used it all night long. Sounded spectacular. I only used the clean channel (forgot the footswitch).
    I daisy chained it to my 5f6a clone and switched between the two for certain songs, and ran both when it was my turn to sing.
    What a combination.

    I thank you both for your help.

    Best regards,
    DanD
     
  16. brownerthanu

    brownerthanu Member

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    Hi all. Sorry to resurrect this ancient thread, but I have a question. I'm new to amp modding, but not new to electronics, and I did the 3 mods mentioned here. I clipped the 5pf capacitor on v1 which gave me more chime, desoldered R2 which added to responsiveness and increased treble, and desoldered R20 which made the bass knob more useful. Now, however, the amp is significantly more noisy, with a noticeable hum. This happened after my removal of R2 and R20.

    I understand that by removing treble bleeds I'm going to be able hear high-frequency buzz better, but I hear an increase in the entire spectrum. Some of the hum changes if I rotate my guitar, as if it's coming from an RF source into the pickups. I have noiseless pickups in my Strat, though, and have never heard hum like this before. I seems as if the RF source might be the amp itself. Did I inadvertently create an RF antenna? My Prosonic is a '95 and seems to have the original filter capacitors in it. Would changing them reduce this hum?

    I'm extremely happy with the tonal changes from these mods. I'd love to reduce the additional noise, though. Any thoughts about how to do this?
     

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