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1958 Gibsonette (GA-8) Amp: early breakup

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  • 1958 Gibsonette (GA-8) Amp: early breakup

    Hallo everybody!
    Few days ago I found an old 1958 Gibsonette amp, the amp seems to be all original and is definetly in near mint conditions; tubes are a couple of 1958 RCA 6V6GT, one 1958 RCA 5Y3 and one 12AX7 Elexctroharmonix :-(
    the fact is that this amp has almost no clean!! i can get a clean tone (almost clean) up to volume 2, after that level the amp starts to breakup, at 6 the amp is useless.
    Is there anyone who had played this kind of amp? is this very early breakup behaviour normal for such amp or do i have to start troubleshooting?
    thanks to you all!!

    BTW: i use a gold top les paul with two underwound (10%) P90s

  • #2
    There is a very high likelihood that your 55 year old capacitors are dried out and leaky.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      Ciao Enzo! (italian name...)
      i've took a look at the schematics http://www.webphix.com/schematic%20h...gibsonette.pdf if i mistake not there should be only 4 EL caps (10, 10, 10 and 20 uF), i will start replacing them... what about tubes? is ther any chanche that a worn out tube could result in lack of clean sound?
      thanks!!

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      • #4
        Most likely the amp needs to have its tubes replaced.

        The fact that the amp appears to be in mint condition doesn't mean the tubes have not worn out. Old tubes may have mythical status, but they do not have eternal life. I had a 20 year old Bassman head once, the amp was very well kept and in excellent condition but it still had the factory original RCA 6L6's, power output before clipping was a mere 6 watts or so. A fresh pair of Sovtek 6L6's fixed that.

        Edit: And, yes, the electrolytic caps might need replacing too.
        Last edited by flyingdutchman; 11-08-2013, 10:54 AM.

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        • #5
          Actually the electrolytics are probably dried out, but I also suspect the coupling caps - probably paper from that era - are leaky. I see two 0.022uf coupling caps. You mention four e-caps, the filters on your B+, but no not forget there are also two used as cathode bypass caps.


          An easy test for coupling cap leakage: with the amp running, measure for DC voltage at the top of the volume control. There should be no DC there. If there is DC, it is coming through the coupling cap. Same at the power tubes, if there is any positive DC voltage on the grids - pin 5 - it is coming through the coupling cap.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Thank you all!
            you are giving me precious hints!! i guess i will start by replacing tubes and than i'll see what happens...

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            • #7
              Updates: I've tried to change the power tubes, I've installed a couple of brand new 6V6GT (tung-sol) and what happened is that the sound (with volume at 2,5) cleaned up a bit (no clean sound yet! it was like using a tube screamer instead of a fuzz!) and the output volume increased significantly.
              I've not tried to change the preamp tube because it was the only "new" (I'd better say "modern") tube of the set, but I think I will try to change it as well...just to be sure.
              I was wondering wheteher, before changing capacitors, it could be useful to change also the rectifier tube, do you think it might impact the clean sound? I have a 5U4GB, a GZ34 and a solid state rectifier (octal), which one could I try without damaging the amp/tubes?

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              • #8
                None of those rectifiers are a suitable replacement for a 5Y3; they would likely cause an increase in voltage supply in the amp, possibly causing damage.
                Try to find a tested old production 5Y3; current production 5Y3 may result in excessive voltages also.

                However, it would be best for your amp to be checked for leaky coupling caps, as it's a common fault from that era, fits with the symptoms and may result in damage to the transformers.

                Pete
                My band:- http://www.youtube.com/user/RedwingBand

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                • #9
                  Thank you Pete!
                  I'll definetly go for Coupling caps change, i've got a few Mallory 150 0,022uF caps that might fit the purpouse. I will also try changing the cathode bypass caps also, 20uF @ 50V would be fine or a higher voltage rating is needed?

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                  • #10
                    20uF 50V would be fine for the cathode bypass; when fitting, ensure some separation between the cathode resistor and capacitor, as heat from the resistor will be bad for the cap.
                    Are you able to verify the voltages noted on the schematic, eg 1.5V at the 1st cathode, 16V at the power tube cathodes?
                    Pete
                    My band:- http://www.youtube.com/user/RedwingBand

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                    • #11
                      Originally posted by pdf64 View Post
                      20uF 50V would be fine for the cathode bypass; when fitting, ensure some separation between the cathode resistor and capacitor, as heat from the resistor will be bad for the cap.
                      Are you able to verify the voltages noted on the schematic, eg 1.5V at the 1st cathode, 16V at the power tube cathodes?
                      Pete
                      Yes, i can measure tensions, but i also have RCL meter to test caps

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                      • #12
                        Please do the voltage related tests on the amp as it operates. Your RCL meter, unless it is a professional bench unit, probably does not test caps at working voltages. if it is battery powered then it certainly cannot put 400 volts on a cap.

                        A cap might measure close to its value, but your meter likely uses just a colt or two to test the cap. The cap may test fine at a volt or two but leak like an open window at 200 volts.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #13
                          well...i've replaced the two 0,022uF coupling caps, they were a couple of Bmblebee, i've replaced them wit a couple of mallory 150 (630 V rating) but it brought non improvements at all.
                          i've took a closer look to components and i've seen that the two cathode bypass caps are the original ones, and they are marked as "dry electrolytic", the other electrolytic caps (10 and 20uF filters) seems to be younger than the amp, they're actually sprague atom caps (blue) so i believe they might be fine; i haven't checked the voltage readings yet...

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                          • #14
                            "Dry electrolytic" isn't quite true according to my old Mallory capacitor book; it merely means that these are significantly less "wet" than their predecessors. They still rely on moisture in the paste inside, so they still will dry up.

                            Enzo is the expert, he is steering you in the right direction. Take those voltage readings, both DC as well as AC ripple on the B+.

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