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Fender Prosonic - NOISE

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  • Fender Prosonic - NOISE

    This amp came in with a bad input jack.. so I replaced it and that's fixed. Then I was checking voltages and there was -450mv on the grids of the 1st 2 preamp tubes. I read another thread that ended in replacing bad coupling caps so I replaced C9-12 and that grid issue is now fixed. However there is now a bad high freq oscillation AND crackle when the amp is turned up to a certain volume. This happens in both channels and is only if signal is being run through it and the amp is at a certain volume. Bias on power tubes is good. I tried new power tubes and the 1st 2 preamp tubes in the chain are new 12ax7lps. I will try other preamp tubes and see. If anyone can suggest what this might be I'm all ears.

    Oh I also retention all sockets and poked around the components w/ no issues there.
    Attached Files
    Last edited by lowell; 01-27-2009, 10:41 PM.

  • #2
    Negative half a volt?

    Look up "grid leak." In some older amps they relied upon it to bias the tube.
    Education is what you're left with after you have forgotten what you have learned.


    • #3
      First thought is the pre amp tubes. Check your new solder points then move some wires around in the preamp section. Sounds like oscillation perhaps from wire placement....or one or those new caps is bad. OR.....the shorting contact on that new jack is not doing it's job....


      • #4
        The shorting jack only "does its job" when there is nothing plugged in... right? I've replaced all preamp tubes now and the problem is still there. It's actually what I would call a "chirping" noise. Could this be parasitics? I just tried the preamp out and all is good! So def a power amp issue.
        Last edited by lowell; 01-28-2009, 06:44 PM.


        • #5
          Does the chirp volume increase when you turn up the volume? (not the master) Just want to be sure I'm reading it correctly. If so then your problem is in the circuit before the vol pot.


          • #6
            yes it does... but if there is a larger signal into the power amp, that could reveal instability in the PA. I don't know if that really narrows it down to the preamp... ya know? Also being that preamp amp sounds fine it's gotta be something in the PA. I've checked cathode grounds, ac on power supply caps, speaker jack ground is good. I'm at a loss.


            • #7
              Okay since I felt as though I'd narrowed it down to the power amp, I tried alligator clipping some grounds to different spots. When I did this from 1 power tube cathode to the OT secondary ground at the speaker jack it greatly if not completely, in certain settings, reduced the problem. This amp has those 1ohm cathode resistors with diodes in parallel for quick/easy biasing. I'm thinking maybe those were/are the problem. I checked resistance of the ground side of those to ground and it seemed ok to me.


              • #8
                Here's a little hint on those Prosonics: the crappy, underrated speaker impedance-select rotary switch contacts tend to burn, arc and develop resistance. Along with that comes a whole set of unpredictable issues. I'm not saying this is the case with yours, but I did see it in a few of them, and the first one (of course) drove me f***in' nuts!!! In addition, the switch is a real Bi-otch to change.
                John R. Frondelli
                dBm Pro Audio Services, New York, NY

                "Mediocre is the new 'Good' "


                • #9
                  hmmmm..... didn't check that... then again I was wrong this was a Super Sonic... but the Prosonic is the only schematic I could locate. thanks though I'll keep an eye out for that... yeah it drove me nuts!!!


                  • #10
                    Hmmm indeed.
                    The only repeated issue I've seen with the Pro Sonic is the propensity for the folded over component leads on the preamp board tend to cut into the traces that they are folded over onto.
                    It made for very intermittent crackling-popping & all kinds of weirdness. With the NFB I suppose it's possible that it could be affecting the power stage.

                    What I've taken to doing on these amps is to use an Exacto knife & bend those folded over leads that fold over traces other than their own. I haven't experienced this issue on the PI board yet, but I suppose it could be the same issue.

                    I'd say with the weird reactions you amp is showing, it might be worth a shot to fold those leads up. When you look at the board, there really aren't nearly as many as it may initially seem.

                    I'm thinking that getting the pwr tubes cathode to ground might be keeping weird feedback to the preamp stages out of it's path & that may be why it seems to reduce or change the oscillation.

                    Other than that it just sounds like one of those fart around with it until you get some reaction that gives you the clue you need (just like the cathode resistor thing) BTW, these amps are so new I wouldn't have expected coupling caps to have issues quite yet...glen


                    • #11
                      Glen thanks for that info. Yeah amps like this are a real nightmare and pain to diagnose. The folded over lead thing I did see and thought it was sketchy at best, however did not unfold them. Yeah the coupling cap change was definitely a shot in the dark, I figured they could be bad due to bad tube at some point or possible other failed component in that tube's circuit that maybe put stress on 'em. Only way to eliminate that possibiltiy was just to replace 'em.