Amp Gives High Pitch Sound When Loaded

Discussion in 'Let's Talk Vintage' started by EdHopper, Nov 7, 2017.

  1. EdHopper

    EdHopper Member

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    I put my amp output into a Marshall Power Brake (attenuator). The amp sounds great normally but it's very loud and to get the "sweet spot" of tone and volume, I use the Power Brake on amps like my 1971 Marshall 50 watt head (which have no Master Volume). I'm not having any problems with the Marshall 50 watt head but with my 64 Fender Bassman, when ever I turn the dial down on the attenuator to damp down the output, I hear a high pitch sound (not super loud but annoying as all heck). The Fender wants a 4 ohm load as this was the value of the cabinet that originally came with those 64 Bassmans. The Marshall Power Brake only has two setting: 8 ohm and 16 ohm. I'm sure I've used the Fender with the Power Brake before with success but I am guessing since I changed out the tubes and tweaked the bias to the optimal value for those tube, something - possibly a pre-amp tube, is causing the problem?
     
    Last edited: Nov 9, 2017
  2. BadgerO

    BadgerO Active Member

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    Here's a wild uneducated guess you should probably ignore; sometimes old transformers scream under full load as they physically vibrate (annoying and alarming like a mild pain in the balls). However, it can be easy fixed by having it re-shellaced (at least that's what we do here in Iran, we just dip them in shellac). As they age the shellac coating on the transformer disintegrates. Maybe since you have that power brake you're pushing the Bassman harder than you usually would, making those transformers work harder.
    But as you said, moving tubes around is usually the first step. If it's not a constant 100 to 120Hz, I usually rule out caps.
     
  3. Marshallhead

    Marshallhead Well-Known Member

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    As a first step, I would not use an 8 or 16 ohm attenuator on an amp expecting 4 ohms, particularly a vintage piece with irreplaceable ironwork.
     
  4. stickyfinger

    stickyfinger Active Member

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    Assuming were not hearing oscillation out the speaker when the amp is dimed (you would simply need the 5k6 grid stoppers on the octal sockets) its the transformers making noise under load. This is quite common even among some new production transformers
     
  5. johnfv

    johnfv Well-Known Member

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    ...Gives High Pitch Sound When Loaded

    I had a girlfriend that would to that ;)

    Sorry, couldn't resist. Carry on gents...
     
    MarshallDog and ampmadscientist like this.
  6. john l

    john l Well-Known Member

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    Is this an AA864 or a 6g6-b? Does the ringing increase and decrease in intensity and frequency with volume adjustment? Can you get it to go away when you turn down your treble control? I can’t account for why it worked before but if what I mentioned applies then your dealing with oscillation.
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    The preamp tube can cause that, so change it and try again. Usually: it's the first preamp tube V1 that goes microphonic or oscillates.

    Bad switch contacts on input jacks. The contacts can oxidize and the switch won't close any more. This will cause oscillation.
    (when guitar cable is UN-plugged, the switch contacts are supposed to close) BUT if contacts are not closing: this will cause oscillations.

    Metal shield in amp cabinet: the open end of the amp chassis needs to be covered. If the metal shield is missing, it can oscillate.

    The Bassman: was made for 8 ohm or 4 ohm load. BUT the design of bassman in 1964 had some bugs.
    (post pictures of inside)
    There are several variations in the actual wiring of these amps. (they are not all like the schematic shows, there can be differences)
    You "might" have the one with the bugs...which can be corrected.
    So...let's see some pics.

    The bug: is in the coupling to the input of the phase inverter.
    If you have this bug, we need to get you on the road to fixing it.
     
  8. john l

    john l Well-Known Member

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    While I’ve never seen an issue at the PI entrance. I agree that there are some consistent inconsistencies with these things lol. Im somewhat of a bassman nut. I’m curious what are you referring to AMS?
     

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