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Help me track down 60 hz hum on Hammond A-100

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  • Help me track down 60 hz hum on Hammond A-100

    This is a 1963 a-100 going into a matching period 251 Leslie. The Leslie has been recapped and when hooked up to just AC via alligator clips is pretty quiet.

    The only thing non-stock in the whole rigs is a Bob Scleicher EIS solid state relay.

    One the years there is more and more 60hz hum coming from the organ...and yes I know the difference between it and its octave higher cousin 120.

    Even if I cancel all the presets and push in all the drawbars it is still there and is modulated by the expression pedal. It is loud enough that it causes Intermodulation distortion with played notes and while the organ still sounds OK it is not as good as many Hammonds I have owned or played.

    In following the wonderful Uncle Doug on Youtube videos, he posits that 60 hz hum is often caused by a preamptube which capacitively couples 60hz ac heater current to the grids/plates.

    Does anyone have experience with this and in particular which tubes in the preamp I might start looking to as culprits. FWIWE I have a bunch of other tube gear in the studio and it is all very quiet so I do not thing it is a mains problem.

    Thanks as always ,


  • #2
    If after cleaning and reseating the existing tubes and sockets there is still a persistent 60HZ hum there are a few things you can check besides the obvious power supply e-caps to mitigate these issues.Grounding is very important,roaded organs develop filter tray lid fastener loosening,preset panel loosening,check these for starters even if it isn't a road organ.
    If you have the Leslie on the bass side of the A100 move to the treble end of the organ.Helps keep the high impedance matching transformers away from the Leslie iron.
    Remove the doghouse cover on the expression pedal and blow compressed air through it,clean it of debris and make sure the green wire isn't wrapped around the axle.
    The most obvious hums are usually limited to but not exclusively; power supply caps and high impedance stages of amplification like the matching transformer etc.
    Seldom if ever is it a tube,maybe three in 50 years of doing this.Way more often I change Leslie tubes.
    Hope this helps you.
    Still working on the mall organ coffee table book for cork sniffers


    • #3
      Might sound a little strange to ask, but is the pilot lamp lit and of normal brightness? The reason I ask is that I've recently recapped and retubed an A102, and after work was done, fired it up and it had a distinct hum, nd the distortion you mention. I noted my pilot lamp will glow dimly or not at all, and I wrote it off as a bad bulb. The wires that run to the bulb are also tube heater circuit (6V) and one of the leads had a strand of wire poking out of the side of the stripped back wire, and when putting the steel cover on, I was grounding the heater circuit! Eyesight ain't what it used to be...
      Corrected my mistake and all is now well!


      • #4
        You’d expect a bad electrolytic in the 6x4 preamp power supply circuit to produce 120 Hz hum not 60 since it is a ‘full wave’ circuit, Well, unless one half of the 6X4 is really gone flat. Agree, not terribly common but a tube can develop heater to cathode leakage and inject some of the 60 Hz filament voltage into the cathode circuit. Most tube testers won’t show heater-cathode leakage as a ‘short’ unless really bad.

        Does the 60 Hz change level with soft/normal? If the hum varies in level with expression I’d just pull preamp tubes one at a time, starting with V4 and going backwards to the signal flow in the schematic as a guide, and see where the hum stops, to try to isolate the problem.




        • David Anderson
          David Anderson commented
          Editing a comment
          This is where my EICO 666 tube tester is very useful. It has a metered leakage test, not just a short lamp. It catches failing 6X4s and 6X5s.

      • #5
        To find the source of hum, you can either explore various guesses or speculations, or you can do it systematically. For example, is the hum entering the signal path prior to the signal reaching the preamp? If not, where, in the preamp, is it entering the signal path?

        Some small-signal tubes can introduce hum if they have fairly severe heater-cathode leakage. I keep plenty of spare tubes in stock, and I have a tube tester, so I can either confirm a tube problem or rule it out fairly quickly. Note that hum can also get into the signal path via the percussion input as well as the main vibrato and non-vibrato inputs.

        Diagnosis is a process of elimination. If you can determine where the problem is not coming from, you end up with a short list of potential sources which you then investigate in detail.
        I'm David. 'Dave' is someone else's name.


        • #6
          Thanks all for some good ideas as to where to start. Again this is 60hz hum, not 120. Yes the amount of hum is affected by the Normal/soft switch and the position of the expression pedal. Yes the leslie is on the table side of the instrument and there are no other amps or electrical devices nearby. I have a bunch of other 60's tube electronics in the room and they are all very quiet. I've got a bag full of 12ax7/au6 as well as a spare rectifier tube.. A couple of years ago I went through and reseated all the connections to the preset panels , matching transformers and grounds and have checked for ohms resistance between grounds on keybeds, preamp, power amp and reverb amp. Hoping to find some time this week to tried some tube substitutions. I'll post back when I find anything. Brian


          • #7
            treble side .......bloody auto correct


            • #8
              The AO-28 preamp ground comes from the MT via the shield of the wires which carry the A & B inputs. On a refurb I sand the metal tabs to ensure good contact with the AO-28 chassis. The last time I chased a hum problem, it seemed there was no magic fix, rather incremental improvement as I went through the connections.



              • PGR
                PGR commented
                Editing a comment
                My Amp on the B3 and A100 were the same lots of hum I found that the shielded wire from the amp created the 60hz hum . It was actually the long hanging wire and when i put my hand over it /near it then hum increased - I simply re arranged the shielded wire in the bundle enough to eliminate the hum

            • #9
              Originally posted by Jaim View Post
              The AO-28 preamp ground comes from the MT via the shield of the wires which carry the A & B inputs. On a refurb I sand the metal tabs to ensure good contact with the AO-28 chassis. The last time I chased a hum problem, it seemed there was no magic fix, rather incremental improvement as I went through the connections.

              Good advice. Thanks Jaim


              • #10
                Was the hum ever fixed. If so, what was the problem?
                Hammond RT-3, Estey circa 1903, Baldwin Acrosonic spinet piano, Fender Rhodes Mark I 73 stage piano.


                • #11
                  Still working on it. Now that I'm retired I have time to get into this. Also I think I was wrong about 60hz. I think it is more 120


                  • #12

                    You may very well have an organ that doesn't buzz, a booth that doesn't buzz. But that together they start to buzz.

                    So since the A100 has its own amplifier, you have to try it out on its own speaker.

                    As your 251 has been recaped/modified, do not rule out too quickly the possibility of a wiring error.

                    But the first thing to do is to check the AC power circuit that includes plugs, sockets, switch, connection kit ... The contacts and the insulation in this circuits must be perfect.



                    • #13
                      Have the can caps been replaced in the AO28? I have run into several clients with that hum... Changed the caps and nice and quiet..
                      I rebuild Hammonds and Leslies. Please visit my website. tbstonewheels.com It's still in the works but is up and running. My personal setup is a Model A # 2203 with Trek II percussion, 122RV and 2 31h's. One is fitted with two speed motors and the other a Hamptone. Also sporting a working Vibratone 30A # 326.


                      • #14
                        Thanks for all the thoughts folks. Re the 251, if I just send AC in (Using test leads and alligator clips) it is very quiet. If I pull the 5u4GB on the A0-39 it is also very quiet until I plug it back in. I have checked all the grounds between all the components of the organ and nothing is more than 2-3 ohm on my meter. I swapped in some brand new 6BQ5 and agin no difference. At this point I'm pretty sure it is the the filter caps are the likely culprit. the organ is a '63 so they are 57 years old. I think I am going to tackle this on my own but I have two questions for the group. #1, once If have the Preamp out are there which other caps and or other components would you replace. #2 I'm here in Vancouver Canada. Any locals recommend a parts supplier other than Lee's on Fraser. Finally, even though it may be cheaper to stuff individual caps to replace the multi cans I think I will go the multi can cap route. Am I correct in assuming that the CE caps are the route to go?. Thanks as always folks