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Why you should never let anyone in your shop alone...

  1. Sep 2, 2016 #1
    Hi Guys,

    The new amp I just finished started motorboating. You know, the kind of sound a old slow speed boat motor makes.

    I searched for hours trying to find the cause of the oscillations.

    A few day ago AT&T installed a new internet router in my shop. I was not there at the time. They hid it under my workbench. My wife was there but.....

    Even after I found the cause of the problem, it is hard to imagine that a device like that could cause a tube amp to act in that way.

    I learned two things with all this. One, don't assume you screwed something up when you know you did it right the first time. Two, don't let anyone in your shop when you are not there.

    What a true pain in the rear end this has been...lol


  2. jcsd
  3. Sep 2, 2016 #2


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    Was it RF detection of data bursts that made the pops? Or was it instability caused by the presence of RF?
    Looking at it from the positive side, AT&T have actually done you a favour by demonstrating that your amplifier can only be stable and reliable in isolation. Maybe you need to improve the shielding and RF bypass to reduce susceptibility to local EM interference.
  4. Sep 2, 2016 #3
    Hi Baluncore,

    I assume RF data bursts but I am not sure. I have seen tube amps pick up AM radio signals back in the day of lots of high power AM stations. When I moved the amp about six feet away from the router the issue stopped. I assume the signal was picked up directly by the tubes. It might be interesting to shield the tubes just to see.

    Tube electronics are notorious for doing strange things. One of my old analog tape recorders use to pick up radio transmissions.
  5. Sep 2, 2016 #4


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    Semiconductors are less predictable than tubes when it comes to picking up RF. The low frequency of motor-boating with tubes is often due to the high input impedance and low capacitance of the control grid. If you don't need all that high-Z and low-C then load the grid to stabilise the amplifier.
  6. Sep 3, 2016 #5


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    Interesting, although I've never heard of that description... :oldcool:

    Is this how motorboating sounds ?

    Lol... you might be lucky it doesn't sound like a Detroit ...[COLOR=#black]..[/COLOR]:oldwink:
  7. Sep 3, 2016 #6
    Hi OCR,

    In the world of guitar amp repair I guess there are many terms used to describe the sounds produced by guitar amps that are not very "scientific." The term "motor boating" is actually pretty descriptive of the sound produced by a amp that is experiencing positive feedback of some nature. I just looked at some of the video you posted. Motor boating is actually a pretty common issue. The Radiotron book he is referring is a good reference book for all things tube. I am sure a lot of things related to vacuum tubes seem a bit arcane in today's world.

  8. Sep 3, 2016 #7


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    Thanks, Billy ... :ok:
  9. Sep 9, 2016 #8


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    Never let anyone into your house. My parents are having their house painted. They still live in the house which I grew up in so I know the ins and outs of it very well.

    As we all know the landline telephone system has changed and evolved over the years. In our community we now have fiber coming to the house so the old network interface which is on the south side of the house now serves as a junction box. There is a new network interface on the north side. Why they didn't want to plow in the fiber to the same place is beyond me but that's the way it was done.

    Long story short the painter had the balls to get the phone company out since he did not think that old network interface looked right and he didn't want to paint around it. By the time it was all done the phone guy spent a couple hours there since I am sure he was scratching his head to figure out why there was a feed from the north side fiber interface to the south side back to the north side to tie into a security system that needs to be able to disconnect all of the phones in the house then back to the south side to tie into the old network interface.

    Why someone didn't do something more intelligent in the first place is beyond me. The fiber was the VERY last thing to be installed. While I guess it's ok to clean up the outside a bit, what gripes me and my folks is the fact that they could have just asked. Ended up that a upstairs bedroom phone is now dead. I would imagine this will be billed.

    I would have no problem cleaning up things on the outside. It would have been a perfect opportunity to straighten out the criss-cross mess that was there but I could have easily done it had the painter just asked. We have known this painter for many years very well but I am questioning whether he will do anything for my folks or me ever again.
    Last edited: Sep 9, 2016
  10. Sep 9, 2016 #9


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    Actually motorboating is a pretty common term in electronics (at least in practice, maybe not in textbooks). It is used to describe any low-frequency oscillation and I think has its own term because most oscillation problems are high frequency, so motorboating is special.

    I've seen two motorboating problems in my life (luckily they were never my problem). In both cases it was due to a input bipolar device acting as a mixer and mixing down a large AM signal.

    I think this is exactly what you're seeing, Planobilly. 802.11 wifi uses Quadrature Amplitude Modulation so your amp is almost certainly demodulating the WiFi signal. I would try Baluncore's suggestion as that is the most likely fix.
  11. Sep 9, 2016 #10
    Hi Guys,

    Just so everyone is clear about this issue. As soon as I found the cause of the RFI the solution was self evident, separate the source of the RFI from the amp. Also the amp was setting on my bench without the advantage of the normal shielding. The metal covers on the preamp tubes were removed and the metal shielding inside the wood case was not in place.

    While it is true that I have seen RFI with guitar amps in the past, that condition has only occurred very infrequently. The most frequent interference has been from very strong AM radio stations. All this direct experience has been over a period of more than 50 years. It is obvious that much has changed in the last 50 years with the proliferation of electronics devices capable of producing interference. Therefore the likelihood of experiencing such interference is more likely to occur in today's world and to some extent is mitigated by current FCC regulations.

    There is a considerable amount of wireless technology being used in the music business, both on stage and in the recording studios. Most of the issue I hear about relate to those wireless devices interfering with each other and not producing interference with the amplifiers. Well...whatever interference could be there in the amps is happening at frequencies that are not within human hearing range and is not producing effects that are noticeable to the musicians. Guitar amps are as much sound effects devices as they are sound reinforcement devices. With all the intentional and wide ranging changes introduced to the signal path by both the amp design and front end effects boxes so often used, it is sometimes difficult to hear low level noise of any kind.

    I assume as time goes on there may be a greater need for better shielding in guitar amps. It does not appear that manufactures in general are paying much attention to issues of better shielding. I assume that would be because they are not getting very many complaints. I am just guessing but I bet there are positive feedback issues caused by both external and internal conditions that are never noticed.

    Motorboating issues.
    Motorboating issues are fairly common. They are caused by positive feedback. They tend to be related to lead dress and grounding. They can be caused by improper decoupling and also by boards that have become conductive. They are often very hard to troubleshoot. It is now obvious to me that motorboating type sounds can be caused by RFI...something new for me.

    Telco installation issues.
    The why to all this is simple. In a word, money. All the telco and communication companies try to use the cheapest solution to install work that they think they can get away with. On balance they do a pretty good job and we have much better communication services today than we had not so long ago. Well..on the other hand, there are in fact some spectacular failures. The Samsung smart phone battery catching on fire and the recall comes to mind. It would be my luck that the phone fire would happen at the airport going through security...lol


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