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Stereo guitar amp trouble shooting

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  • #16
    Thanks very much for the nice comments and all the trouble shooting advice
    as far as replacements go it seems the Hammond 290EX doesn't quite have the heater current capacity I need and I'd probably be better of with the 290FX which is what they are selling for twin reverbs.

    It's good to have less worries about the transformers capacity, it will probably run cooler but it will add a couple lbs which is a big consideration in this build.
    I've built smaller amps(a couple vibrolux reverbs and deluxe reverbs) recently so I wouldn't have to carry my lovely sounding but 80 lbs super reverb around to smaller shows.

    The motivation(besides fun and curiousity)behind this amp was to have a stereo rig in one comparatively light amp instead of carrying two almost as heavy amps

    I was also happy to make use of a 2x12 combo cabinet I made about 8 years back that has been kicking around under-utilized for all that time. In it's current format the amp weighs about 44lbs.

    These pictures show it roughed into the cabinet before final modifications.

    I have since made a new back panel and completed some more of the reconfiguring of the cabinet.
    It had been used for one of my vibrolux's previously and a sherwood S-1000 conversion before that so the cut outs for the amp face plate and the back panel were wrong.
    It also now has a speaker extension plug for the right channel so I can use one of my mini leslies with it.

    It has 2 x 12" speakers one Eminence Little Texas (Clean, really efficient, and LIGHT! 4lbs!!) and a 60s approx. 20 watt Rola organ speaker which is nice sounding too.
    Attached Files
    Last edited by Magic Potion; 04-20-2018, 10:24 PM. Reason: Tsunami


    • #17
      I was monkeying around and i discovered a couple of other unsual things which I was wondering if someone could tell me if they were related to my likely faulty PT

      When I removed my PT from it's original chassis I had made a wiring map, but then lost it (duh) so I had inadvertently shrink-wrapped the heater centertap and created an artificial one using 2 100 ohm resistors to ground which worked fine

      in light of recent exploration around the PT
      I was checking two wires that I had shrinkwrapped and set aside. One had no resistance to the heater wires. A centertap??. I measured the AC between this wire and each filament and it was half the heater voltage. BINGO!

      The other wire had 3.5 ohms to one primary and about .7 to the other. the primaries have about 3.4 ohms between them. I think this other 'primary' is the 110 volt tap which I have since checked by measuring between it and the black primary, yup 110 volts)

      I decided to hook up the newly discovered real heater centertap
      i disconnect my artificial one and hooked up the proper one then turned my amp pilot light wouldn't come on
      I flipped the switch a few times, checked the power and tried it a few more times,no pilot light... then it worked..then it didn't. weird!

      I left the switch on to see if it was just the pilot light. the tubes heated up quite bright and the amp came on buzzing louder and ringing like it was a reverb tank starting to feedback
      Everything was turned down though.
      I tapped a tube and it was hugely microphonic and rang like a bell, I tried the next..and then all of them.They all rang like bells!! I then turned the amp off. I should've taken a heater reading but I felt like i was just meddling and something was very wrong, so I disconnected the heater centertap and hooked up the artificial one again. The amp worked as before and so did the pilot light!

      Is this part of the PT problem or symptomatic of something else

      I also think it's weird that my primaries seem to have polarity. I have a grey, a black and a blue one(110 tap)
      Is it because there's the extra primary tap?

      The main primaries are the black and grey ones

      When I reversed those ones while testing the AC between the chassis and wall ground before I was able to drop the AC voltage on the chassis by half??
      What was that indicating??

      I included the part of the hammond M2 organ schematic that has the power supply. I think the AO-14 amp this PT came out of was from an M2 organ
      Attached Files


      • #18
        Originally posted by Magic Potion View Post
        ...I also think it's weird that my primaries seem to have polarity. I have a grey, a black and a blue one(110 tap)
        Is it because there's the extra primary tap?

        The main primaries are the black and grey ones

        When I reversed those ones while testing the AC between the chassis and wall ground before I was able to drop the AC voltage on the chassis by half??
        What was that indicating??...
        I think that may indicate that the part of the primary winding where the bad insulation lies is closer to one end of the winding than the other.
        To be expected really, it's more likely to be that way than if it was exactly in the middle of the winding.
        And it may be a further indication that the issue is bad insulation, rather than capacitive coupling etc.

        I can't think what might be going on with the apparent CT on the heater winding

        Really though, I would get that PT outta there, and sling it into the ecycling dumpster
        My band:-


        • #19
          I don't think that was a heater center tap, you said it had no resistance to the heater wires. (assume you meant 'open')
          Perhaps it was a ground shield or something. That might give the AC reading to the heaters like you got while the artificial tap was connected.
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


          • #20
            In the absence of an insulation tester you can check earth leakage by inserting a 1500 ohm resistor shunted with a 0.15uf capacitor in the mains earth connection. Measure the voltage across the resistor and using ohms law calculate the current. It should be less than 0.5mA with the mains checked with both polarities.


            • #21
              1) what Country are you writing from?
              2) whatīs mains voltage where you live? Show what outlet are you using.
              3) you have a gross wiring/grounding/both error which may be potentially deadly, Iīll suggest some measurements but only after you answer these questions.
              Juan Manuel Fahey


              • #22
                Originally posted by Magic Potion View Post
                So I put a ground lift on the AC plug at the power bar the amp is plugged into (a properly grounded 3 prong power bar)
                and measured between the power bars earth ground and the amps chassis and I get 235 volts AC
                turn the amp off, voltage drops, turn it back voltage returns along with loud evil buzz.
                I find this very very worrying.
                You didnīt answer (I hope you are still alive) but now guess you live in a 120V mains Country; then how the heck can you have 235V on the chassis?.
                Even if you have 127V !!!!!!
                Voltage between chassis and mains Ground MUST be ZERO.
                Juan Manuel Fahey


                • #23
                  Originally posted by J M Fahey View Post
                  Voltage between chassis and mains Ground MUST be ZERO.
                  Good catch on the verbiage here. Indeed, since the mains ground should be physically connected to the chassis, even with the actual ground wire lifted there should still be virtually zero volts with this measurement if there's a million volts on the chassis. Confusing.
                  "Take two placebos, works twice as well." Enzo

                  "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A


                  • #24
                    Apologies I used the wrong terminology, see posts 2-7 . The reading I was talking about was taken between the chassis and the wall socket ground, with a ground lift on the power chord going to the amp. I live in Canada and yes 120 VAC and still alive. I've done this test as suggested above, post 2, several times and get the same result. I have a new twin reverb type PT on the way from Hammond.
                    Thanks for the concern.

                    Embarrassingly, it turns out my excitement with the heater centertap must have been something shorting, likely from me moving things about to hook up the centertap. Further testing between the heater tap and each heater wire showed it to actually be the heater centertap (1/2 heater voltage between the centertap and each heater) and when tested again and connected to ground it was functioning perfectly.

                    The amp still works and sounds great, I was playing it today.