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 Well-Known 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...
     
    Exotic, mAx___, EndGame00 and 5 others 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?
     
  9. EdHopper

    EdHopper Member

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    This is a 6G6 - B. The pre-amp tube trick sounds familiar. I think I had this exact same symptom for this amp years ago and it was indeed solved by changing out one of the pre-amp tubes. I think I put in a lower gain tube and it solved the problem.

    As far as the comment about the dangers of an 8 ohm load on this amp, I've used this amp with an 8 ohm load ever since I acquired it in 1987 and never had a problem. The only problems I have had with this amp (other than the high pitch noise) is I put some tubes in it once that just got super orange and burnt out - I am assuming they wanted a difference bias or where just crap tubes.
     
  10. EdHopper

    EdHopper Member

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    I just tried a lower gain tube in the V1 position and it did not stop the noise. I've been switching the pre-amp tubes around, substituting different values such as a 12AY7 or a 12AT7 or a 12AU7 in these positions but no luck so far. As far as photos of the amp go, that will take some time as I have re-installed the chassis in the box.
     
  11. Trapland

    Trapland Running Into Walls Silver Supporting Member

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    Seems obvious but ampmadscientist made a good point about making sure the amp is in its cab and shielded. That one has caught me before.

    I’ve used Powerbrakes with 4 ohm Fenders before, Twin, Bassman and Bandmasters. It seems like blackface Fenders are really forgiving, especially when you look at how few have failed trannies. I never had a problem but always clenched my teeth a little. Maybe it’s just your amp that really hates it.

    Maybe a tube change will play better with the powerbrake? BF bassmans have an extra gain stage compared with other BF amps, then if you have a 12ax7 in a 12at7 slot, the gain is really up there, maybe too much for that group of tubes. That plus maybe not enough negative feedback makes a real shredder but SUPER sensitive to high frequency oscillation.

    I’m not telling you anything anyone else did, just saying it my way. My last BF bassman got sold for another Marshall. Solved my powerbrake worries.
     
  12. EdHopper

    EdHopper Member

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    I'm wondering if the output transformer has other taps which can be used - maybe ones that will work better with an 8 ohm load.
    Or I could simulate a 4 ohm load by making an in-line resistor box which would have 8 ohm resistors in parallel. Using this device, the Bassman would "see" 4 ohms but the signal would continue on to the power break. I don't care about wasting power - in fact, that's what I want to do is soak out some of the power. All of this being said, this is only one "shot in the dark" solution to my problem.
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    The amp, attenuator, and speaker must be a matched impedance. There is no exception to this.
    Do not assume anything until it has been verified, verify everything.

    1. Start by plugging in speaker cable to speaker cabinet;
    Connect ohm meter to end of speaker cable, what is the reading? (should read about 6.2 ohms for 8ohm speaker)

    4 ohm speaker will read about 3.2 ohms.
    16 ohm speaker will read about 12.6 ohms...

    2. Connect speaker to attenuator output,
    Connect speaker cable to attenuator input, connect ohm meter to end of speaker cable, what is the reading of attenuator input?

    The amp is SET to 8 ohms?
    The attenuator IS an 8 ohm attenuator? (you are sure)?
    The speaker IS an 8 ohm speaker? (you have verified beyond question) ?
    ALL the speaker cables are NON-shielded wire? (you are sure) ?

    Is the amp located far away from the attenuator? Try moving the attenuator.
     
  14. mickeydg5

    mickeydg5 Well-Known Member

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    I know its loud but is there high pitch with no attenuator?
     
  15. EdHopper

    EdHopper Member

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    I used the same amp head with a Tom Scholtz Power Soak for attenuation and did not get the high pitch sound. I wonder why.
     
  16. neikeel

    neikeel Well-Known Member

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    Did you have the impedance matched?
     
  17. Trapland

    Trapland Running Into Walls Silver Supporting Member

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    I would and do. I’ve used Powerbrakes on blackface twin reverbs since the 90s and NEVER had a failure, not even a tube. I still do. But I only mismatch old Fenders and only up one step never down.

    @EdHopper Make sure the squealing isn’t just because your guitar isn’t in too close proximity to the amp. Also a poorly shielded cable, perhaps wrapped near the OT could have that effect. Most old Marshalls will do the high occupation thing and the attenuator is unlikely the problem. Your amp is just cranked. I’ll bet you get the same squeal without the attenuator if you are too close (and cranked). There’s footage of Hendrix causing the same effect while standing close. It’s probably not good, but just move away. Just thoughts from another Powerbrake user who sees the exact same thing.
     
  18. Trapland

    Trapland Running Into Walls Silver Supporting Member

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    The Marshall Powerbrake reads about 1 ohm at DC. It only provides nominal impedance at AC audio frequencies.
    This is correct.
     
  19. Trapland

    Trapland Running Into Walls Silver Supporting Member

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    One last for @EdHopper . Again, you DO have the Bassman in its shielded cabinet right? It took me a minute to figure that one out one time.
     

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