High-pitched Whine on Startup and Shutdown

Discussion in 'The Workbench' started by Chris-in-LA, Jun 16, 2019.

  1. Chris-in-LA

    Chris-in-LA Well-Known Member

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    My 2204 Mojotone clone has been experiencing some problems. It sounds great overall but it makes funny whines and clicks when the main power is switched on or when everything is shut down. It started life with an aluminum chassis but I recently upgraded to a steel chassis. The steel chassis has a different design and I had to update the voltage and ohm selector switches to Mojotone's proprietary switches. But i've had some noises with both types of switches.

    https://www.mojotone.com/amp-parts/Switches_1/Mojotone-Impedance-Selector-Switch

    Right now the problem is the whining that mainly comes on startup or shutdown. Although, when I first brought it into the house and plugged it in the whine was continuous but faint. I'm thinking that it may have something to do with the NFB. The presence control seems to have very little to no effect and the amp really shrieked when I accidentally turned the ohm selector to 4ohm and plugged in a 16ohm speaker.

    I did post questions about this problem on the Building the Classics thread but I'm not getting much response over there.

    http://www.marshallforum.com/threads/upgrading-a-2204-build.107838/

    The NFB is hooked to the 4ohm output from the OT. The presence knob makes a slight static noise when it is turned. Also, the static noise can be heard even when the volume is completely off.

    I was initially thinking that the whine might be some type oscillation but I haven't reversed any of the usual (pin 3 or pin 5) wires at the power tubes.

    I'm trying to get some ideas of what may be going on and how to fix this before I start blindly switching things.
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Reverse the pin 3 connections to the output transformer primary.
     
  3. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I will try that.
     
  4. 67Mopar

    67Mopar Well-Known Member

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    Why would you go from an aluminum chassis to steel? I would consider this a downgrade.
     
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  5. ampmadscientist

    ampmadscientist Well-Known Member

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    For one thing: electromagnetic compatibility (corrosion and oxidization)
    Second: strength / durability.
    The reason amps were built on aluminum was to cheap out and save money, as much as possible.
    Since then most manufacturers have abandoned aluminum.
    Since 1971 the electrical safety codes were changed prohibiting ground connections based on dissimilar metals. (copper / tin / steel connected to aluminum with screws or rivets is now considered unreliable and unsafe. Before 1971 this was a common but very poor choice.)
     
  6. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Because of what AmpMad says.
     
  7. 67Mopar

    67Mopar Well-Known Member

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    Unless he's planing on tossing his amp down a stairwell, aluminum is just fine. Aluminum is a superior conductor compared to steel. Aside from is superior conductivity, its ability to release heat at a much faster rate makes it the preferred material. Nothing destroys components, especially electrolytic capacitors, faster than heat. As long as there's no mating of copper to aluminum, using the proper fasteners/terminals, all is well. It's not like he's going to be storing his amplifier on his back porch. Most amp chassis' are chromate plated, which makes dissimilar metals a non-issue.

    https://www.mojotone.com/amp-parts/ChassisCapPans_1/Undrilled-Aluminum-Chassis
    https://www.tubesandmore.com/products/chassis-marshall-style-18-watt-tmb
     
    Last edited: Jun 17, 2019
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    I would consider aluminum a poor choice for several reasons.
    Then I'm not sure how capacitors in Fender amps with steel chassis still work since the mid 1950s, and continue to work.

    "As long as there's no mating of copper to aluminum, using the proper fasteners/terminals, all is well."

    Grounding by dissimilar metals has been prohibited for some time now...
    Using the aluminum chassis as a ground path for the circuit is also considered bad practice, and completely avoided in new designs...
     
  9. 67Mopar

    67Mopar Well-Known Member

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    Why would anyone build an amplifier that uses the chassis as a current carrying conductor? I can see two prong vintage amps, but nothing that uses a three prong hot/neutral/ground. Properly welded steel terminals [to aluminum chassis] would be the way to go, IMO.

    I was once "bit" by a '72 Marshall when my forearm touched the chassis. I was also hit with current when I touched the power switch of a Boogie Mark IIC+. It took days to shake that off.

    As I recall, the issue with aluminum wiring in homes was about current. I believe they were using #14 w/15 amp circuits, which needed to be #12. #1O w/2O amp circuits... NEC made the code change before I entered the field.
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    "Why would anyone build an amplifier that uses the chassis as a current carrying conductor?"

    All the amps you guys build use the chassis as a current carrying conductor.
    All plexi clones, all the other amps, the amp you built, etc all use the chassis as a current carrying conductor.
    This was a common practice back in the day when tube amps ruled the galaxy.
    You guys copied these amps (everything), including the chassis as a circuit path.

    "Properly welded steel terminals [to aluminum chassis] would be the way to go, IMO...."

    Yes that is exactly right. Any method which excludes oxygen to prevent oxidization of the aluminum. TIG, MIG, would be the proper way to attach ground.
    In other words, a method which can not fail due to corrosion.

    "As I recall, the issue with aluminum wiring in homes was about current."

    Aluminum wiring in residential applications is banned / illegal.

    The issue is oxidization of aluminum wire connections...
    which causes current to heat the ends of the wires (where the corrosion occurs).

    "NEC made the code change before I entered the field."

    Most of the major code changes were in 1971.
     
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  11. ampmadscientist

    ampmadscientist Well-Known Member

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    You can use an aluminum chassis,
    as long as all the grounds are connected in a way which can never fail due to oxide of the aluminum.

    Example: TIG weld steel ground lugs to the chassis. Solder the grounds to the steel lugs.

    Steel Chassis:
    We still solder all the ground lugs to the steel.
    This method has been used by Fender, etc...
    But soldering is a lot easier for me, compared to TIG welding. I'm sticking to the steel chassis.
     
  12. 67Mopar

    67Mopar Well-Known Member

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    Are you saying that the return and ground are shared via the chassis?
    At what point is the return [neutral] connected to the chassis in my Marshall Silver Jubilee?
     
    Last edited: Jun 17, 2019
  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I would only use the metal housing/chassis as part of the faraday cage, nothing else.

    The metal chassis has been used in grounding schemes but I consider that bad practice.
     
  14. 67Mopar

    67Mopar Well-Known Member

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    You would rather have a chassis with no grounding? :shrug:
     
  15. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Basic electronics is used. The metal chassis would be used as a Faraday cage therefore it is grounded for noise but only for that purpose. All electronic's grounds/ground bus would be self-contained ultimately tying to mains/power return ground.
     
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  16. ampmadscientist

    ampmadscientist Well-Known Member

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    I'ts not the AC on the chassis, it's DC that uses the chassis as a circuit path. And so it has been since the beginning of tube amps.
     
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  17. ampmadscientist

    ampmadscientist Well-Known Member

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    The chassis is connected to mains ground (safety ground).
    The DC circuit doesn't need to use the chassis as a path for current. (but it does anyway)
    The DC can be referenced to the chassis, without using the chassis as the circuit path.

    And so this is how newer amps are manufactured: the circuit path is no longer the chassis.
    It's a safer way to build.
     
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  18. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Just letting you know that switching the wires fixed the problem.
     
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  19. 67Mopar

    67Mopar Well-Known Member

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    When I use my Foxrox Flanger with my AC15, the effect on/off switch pops. No other pedal I own does this. Grounding issue?
     
  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    That sounds like a pedal problem. I suspect it does the same with other amplifier setups. :scratch:
     
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