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A-100 Reverb Thumping "Noise" - what to try next ?

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  • A-100 Reverb Thumping "Noise" - what to try next ?

    I've been off-air for a while working on restoring two A-100's.

    On one, I've had a few issues with very bad vibrato/chorus, which thankfully was cured by recapping the vibrato line and rebuilding the scanner. I also had a thumping noise (some call it motorboating, but it's more of a thump) on the reverb. This unfortunately has not gone away and I don't know what to try next. The funny thing is that the speed of the thump can be controlled by the reverb pot ... the more reverb, the quicker the thumping. But, when the pot is turned to off or maximum, the thumps stop. I can also add that the thumps do not start immediately when the organ is fired up but start around 15 mins later. Using any rocker switches do not affect the noise at all ... only the reverb control affects it.

    Can anyone point me in the right direction to fix this please ? Thanks.

    BTW, I've looked at all the videos posted on "motorboating" but none are dealing with what I have here.

  • #2
    IMHO they used a weird method for the reverb, but it works.

    The signal from the main amp to the main speakers is paralleled to the reverb amp. The signal passes through a couple of light bulbs that act as signal limiters, and feeds the reverb tank (or necklace). If comes back from the tank into the reverb amp. The amp's output feeds the speaker and reverb control, which are in series with each other (another weird thing).

    I would think that the issue is with the reverb amp going wacky when the load it sees changes as you move the reverb control. Here's what I'd do:

    First, make sure that the thumping is coming from the reverb speaker, and not the main speakers.

    Inspect the reverb springs and make sure that they are OK. "Exercise" the connections to make sure that they are making good contact (both hot and shield).

    Take the reverb amp's tubes out and put them back in a couple of times to clean the pins.

    Check the integrity of all the connections to the preamp and both poweramps.

    If all seems well, I'd probably replace all of the electrolytic caps in the reverb amp.


    Let us know how it goes.

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    • #3
      Make sure you don't have a microphonic tube. Tap tubes with the erase end of a pencil.

      Jim

      Comment


      • #4
        You don't note which version of the A-100 reverb amp you have, AO-35 or AO-44, but if you have a noise that changes speed with the reverb rheostat, you may have a low-frequency oscillation going on in the reverb amp. In particular, the AO-35's output stage, including the driver/phase inverter tube, depends on a number of electrolytic capacitors to work properly. There's a negative feedback loop around those amplification stages, and by changing the position of the reverb control, you are changing that feedback loop.

        If you have an AO-35, check C19.
        I'm David. 'Dave' is someone else's name.

        Comment


        • #5
          Originally posted by David Anderson View Post
          You don't note which version of the A-100 reverb amp you have, AO-35 or AO-44
          Hi David, just to answer you quickly here while I check out the other suggestions, the reverb amp is an AO-44. Thanks for your help.

          Comment


          • #6
            Originally posted by Tim_at_Jonas View Post
            First, make sure that the thumping is coming from the reverb speaker, and not the main speakers.
            Yes, the thumping is coming only from the reverb speaker.

            Originally posted by Tim_at_Jonas View Post
            Inspect the reverb springs and make sure that they are OK. "Exercise" the connections to make sure that they are making good contact (both hot and shield).
            I had inspected the reverb tank and springs during the restoration and everything seemed to be okay. I have a couple of reserve reverb tanks, so can change the unit out if all else fails. I've also "exercised" the connections many times.

            Originally posted by Tim_at_Jonas View Post
            Take the reverb amp's tubes out and put them back in a couple of times to clean the pins.
            Did this and also tapped each tube with the rubber end of a pencil as suggested by Jaim. Unfortunately, this did not help.

            Originally posted by Tim_at_Jonas View Post
            Check the integrity of all the connections to the preamp and both poweramps.
            All connections seem okay.

            Originally posted by Tim_at_Jonas View Post
            If all seems well, I'd probably replace all of the electrolytic caps in the reverb amp.
            This may be the last resort. Thanks for your help.

            Originally posted by Tim_at_Jonas View Post
            Let us know how it goes.
            Well, up to now the thumping is still there but today it appeared after 25 minutes of playing ...

            Comment


            • #7
              Originally posted by SONARtist View Post
              Hi David, just to answer you quickly here while I check out the other suggestions, the reverb amp is an AO-44. Thanks for your help.
              Is it a thumping with a regular time interval?

              There are, unfortunately, lots of things in an amp that can start misbehaving after they heat up. You might, just to see what happens, try swapping the 6GW8 tubes. In doing so, you're swapping four tube sections since each 6GW8 is an unusual (these days) triode and power pentode in the same envelope. That might tell us something. Does it have an effect or no effect?
              I'm David. 'Dave' is someone else's name.

              Comment


              • #8
                Originally posted by David Anderson View Post
                Is it a thumping with a regular time interval?
                Yes, it is regular and on deeper listening it sounds as if there are two frequencies involved; one constant and louder than the second quieter one, which seems to sweep through when turning the reverb rheostat and obviously affects the "thump" rate due to interaction. I made a 1 minute video recording and (if necessary) will try to find a way to make it public ...

                BTW, I could only change out one of the 6GW8 (equivalent here is ECL86) as I had a old and used spare. I will have to source some new tubes anyway. The change out did not help.

                Comment


                • #9
                  No one has mentioned the transistor in the AO44? Old and noisy,folks change them too.
                  And try to bring down/tame the startup peaks on those 6GW8's!
                  Can't remember the value,David Anderson suggestion!

                  Are there any fans or inductive loads on the circuit shared with the Hammond?

                  If this noise wasn't present before the refreshing,maybe a solder joint needs reflowing?
                  I consider myself an experienced solder penciller and have redone my own work and found out otherwise,hahahah!
                  If the thumping has a zipper type Q changing frequency? Change those PS caps.
                  Cathode bypass solder reflow maybe, but a 6GW8 swap didn't change anything so it's worth a shot!
                  Reflow the solder on your AO44 terminal strip,and your 'verb L-pad pot;and on your 'tone area' work under the hood.
                  I cleaned out the 100 ohm 'verb pot with compressed air and the spring contact survived.
                  It works well enough for something approaching 60 years of age.

                  6GW8? Get them while you can is my advice.
                  Still working on the mall organ coffee table book for cork sniffers

                  Comment


                  • #10
                    Originally posted by SONARtist View Post
                    BTW, I could only change out one of the 6GW8 (equivalent here is ECL86) as I had a old and used spare. I will have to source some new tubes anyway. The change out did not help.
                    Sorry, I wasn't clear. I meant remove the two 6GW8 tubes from the amp and put each one in the opposite socket from the one it was in.
                    I'm David. 'Dave' is someone else's name.

                    Comment


                    • #11
                      Originally posted by David Anderson View Post
                      Sorry, I wasn't clear. I meant remove the two 6GW8 tubes from the amp and put each one in the opposite socket from the one it was in.
                      Okay David, thanks ... I swapped the two tubes, powered up and waited until the noise started. But about 10 minutes in and before the thumping started, I got a few very loud squeals and then no reverb at all. The squeals were "new" to me and I got a bit of a shock (non-electrical . Switched the organ off and swapped the tubes back. Powered up again and have reverb ... and have been playing for over an hour and a half WITHOUT any noise. Those squeals must have done something ! Any ideas on what's going on ?

                      Comment


                      • #12
                        Originally posted by SONARtist View Post
                        Any ideas on what's going on ?
                        How clean are your tube pins? I use a brush with fine brass bristles to clean older miniature tube pins. At this point, the contact surfaces have been slowly oxidizing for >50 years.

                        You rarely see this problem with the older/larger octal tube types. Miniature 7/9 pin tubes have a smaller contact surface area.

                        But if you're getting different symptoms with the tubes switched around, then my experiment worked. If the problem was elsewhere in the circuitry, your symptoms would have remained the same. My reasoning is this: the circuit is not symmetrical. One triode section is used for voltage gain, while the other one is a phase inverter/driver. Making them perform different jobs tests whether or not it's a tube problem.
                        I'm David. 'Dave' is someone else's name.

                        Comment


                        • #13
                          Originally posted by Sweet Pete View Post
                          No one has mentioned the transistor in the AO44? Old and noisy,folks change them too.
                          Thanks for your help Pete. Yes, a friend mentioned the transistor but I am researching all avenues before pulling stuff out.

                          Originally posted by Sweet Pete View Post
                          Are there any fans or inductive loads on the circuit shared with the Hammond?
                          No

                          Originally posted by Sweet Pete View Post
                          If this noise wasn't present before the refreshing,maybe a solder joint needs reflowing?
                          It was present before and it was one of the things (amongst a helluva lot) to fix. I know that reflowing joints helps and this was done as a "first".

                          Originally posted by Sweet Pete View Post
                          I cleaned out the 100 ohm 'verb pot with compressed air and the spring contact survived.
                          Maybe I'll do that too ...

                          Originally posted by Sweet Pete View Post
                          6GW8? Get them while you can is my advice.
                          I agree and am already looking.

                          For now, the thumping noise has gone (see post to David). Question is: For how long ?

                          - - - Updated - - -
                          (wrongly posted line removed)

                          - - - Updated - - -

                          Originally posted by David Anderson View Post
                          How clean are your tube pins? I use a brush with fine brass bristles to clean older miniature tube pins. At this point, the contact surfaces have been slowly oxidizing for >50 years.

                          You rarely see this problem with the older/larger octal tube types. Miniature 7/9 pin tubes have a smaller contact surface area.
                          I've got one of those fine brass brushes and I'll clean the pins.

                          I am always indebted to your help David. Thanks for the explanation.
                          Ted

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                          • #14
                            Darn ! I spoke too soon ... it's back again today after only 5 minutes, so further searching (and re-soldering, and, and, and ...). Thanks guys.

                            Comment


                            • #15
                              Looks as though it would not take much to covert that amp to use EL84s, which are easier to get cheap.

                              Comment

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