THD Univalve / amp hums

Discussion in 'Amps and Cabs' started by cochese, Apr 19, 2016.

  1. cochese

    cochese Supporting Member

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    I used to own one of these years back and regretfully sold it and just picked up one used. When I plugged it in to my cab the amp has a very pronounced power supply hum. It's like a low B flat on the A string and quite annoying. I think I would have remembered this. I swapped all the tubes with known good ones from my other amps and it still hums. My tech took a quick look at it and felt it was fairly noisy for such a low wattage amp. He also noticed that one of the screws on the power transformer was not the same which is a little suspect. Anyone?
     
  2. Vibroluxman

    Vibroluxman Member

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  3. stephen sawall

    stephen sawall Member

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    I don't have that problem with my Bivalve.
     
  4. tiktok

    tiktok Supporting Member

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    Neither my Univalve nor my Bivalve have this issue.
     
  5. trisonic

    trisonic Member

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    Useful replies so far!!!

    Probably the transformer - I suggest you find a competent Tech.

    Best, Pete.
     
  6. Ed DeGenaro

    Ed DeGenaro Supporting Member

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    Well they actually all have a pronounced power supply hum... What happens when you torque down on the x firmer shield. Or is that a newer version without shields.
    Also what happens in lov
     
  7. cochese

    cochese Supporting Member

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    Hi Ed. I used to own one and just don't remember this amount of hum. What is the X firmer shield? transformer? Is that done underneath the chassis? The hum is very evident with the amp in standby mode and louder than any of my other 6 amps including the two cathode biased amps. When the amp is off standby in play mode the hum is quite pronounced through the speaker. My tech looked at it and nothing seems to be wrong and he thinks the transformer is just unusually noisy for a low powered amp. He looked at the schematic on line and if I recall he said Andy used a fairly high value of "filtering?" or something to reduce hum compared to other low wattage single ended amps.

    With the cage off the amp when it is switched to play mode you can hear an additional metallic sort of rattling from the top of the power transformer. Low volume mode doesn't really change the hum. The noise reduction circuit will make it a bit quieter when engaged but I find I prefer the amp without that circuit.
     
    Last edited: Apr 20, 2016
  8. Ed DeGenaro

    Ed DeGenaro Supporting Member

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    Hmmm... The transformer should have a cover around it it's an earlier model. That amplifyre the hum.
    What happens if you rotate V1 and V2 hum in the speaker change?
     
  9. cochese

    cochese Supporting Member

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    It's 400 something serial #. Yes there is a black cover around both transformers. I tried all kinds of tube swaps. Amp sounds great but I can't deal with that level of hum.
     
  10. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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    What you're hearing is dynamo hum. (Pete, do I win for most useful response?)
     
    trisonic likes this.
  11. cochese

    cochese Supporting Member

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    This is my "noisiest" amp compared to the Univalve. Both amps set up with same speaker and mic. First amp has master all the way up preamp all the way off, and is on, about 8 seconds later the Univalve is on in high power with the volume at 0. Nothing plugged into either amp. I double checked to make sure if the amps were noisier or the same with a guitar plugged into the inputs with the guitar shut off and there was no difference with either. It's a pretty big difference although obviously magnified for the recording. It's about 12-15dB.

     
  12. RussB

    RussB low rent hobbyist Silver Supporting Member

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    I hear ya
     
  13. support

    support Member

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    Do you have any russian preamp tubes in there?
    EH, Sovtek, Mullard RI, Tung Sol RI, etc...
     
  14. cochese

    cochese Supporting Member

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    Hey DAVE.

    I tried everything. Some new HG Rubys NOS JJ's 12ay7 12au7. My first thought was a bad tube. Also tried a Winged C EL34 and a couple of 6550's. I really think it's something with the Power tranny. Anyway that's what my tech thinks. Maybe Ed will have another suggestion. I appreciate everyone's input.

    By the way the Univalve works great with the Power Station. The reason I sold my original was I liked the sound but it was impractical for gigs the Power Station changed that.

    I don't get the bad rap the Univalve gets around here. It's got some really cool tones and is super responsive.
     
  15. trisonic

    trisonic Member

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    I like them too. I think that the people that dislike them so much just bought the wrong amp. No amp is liked by everyone. My favorite power tube is the EL37.

    Good luck, Pete.
     
  16. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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    When I bought mine I definitely did buy the wrong amp.
     
    Last edited: Apr 21, 2016
  17. support

    support Member

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    What happens when you activate the master and turn it all the way down?

    And can you be more specific about what is mechanical noise - what you dont hear through the speaker - and what is electrical noise - what is coming out of the speaker?

    What Ed means by shield is that the power and output transformers used to have metal covers over the top kinda like an old Ampeg. They can sometimes make the mechanical hum a little louder but doesnt mean it makes what comes out of the speaker louder.

    The hum on your clip sounds like it has a lot of upper harmonic content rather that normal power supply ripple which sounds deeper and darker. That could be a loose internal ground or something up with the filament supply. I've never heard of a bad power transformer in one of those. Seems unlikely...
     
  18. cochese

    cochese Supporting Member

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    The Univalve doesn't have a real master it has the built in Hot Plate which is really a rheostat that is like the 16dB continuously variable control on the real Hot Plate. Not my favorite feature as the full on Hot Plate just works better (sorry Ed). I'm assuming that is between the final output right before the output jack. When engaged and turned low it does diminish the hum the same as putting a real Hot Plater after the amp would. The rheostat on the amp is a bit funky and one of the other issues. It doesn't seem to work consistently and seems to stick at full on or off. It doesn't seem to make the amp go completely silent like on my standalone Hot Plate at that setting. My tech said it might need to be cleaned.

    If you listen to the transformer when amp is in standby there is more hum than any of my other amps. When taken off standby you can hear almost a slight buzz (possible rattle) coming from the top of the transformer. I don't have a picture and have to leave for a gig today (yay). This is a picture I found of another amp but it's how this amp looks from the top. It seems I would have to take the transformer out from the bottom to access it or maybe I'm missing something. Once again I appreciate all the help and advice.

    [​IMG]
     
  19. Guinness Lad

    Guinness Lad Silver Supporting Member

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    Is there any loose or suspect ground connections?

    I know with the Mesa TA-15 it will hum in non-master mode, but once you engage the master the hum goes away. I just run mine with the master always engaged.

    Sorry to hear wish I knew more.
     
  20. support

    support Member

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    I know how the master works on this amp. That's why I asked.

    You were a little more specific about what is mechanical noise - what don't hear through the speaker. You mean you're mainly hearing this buzzing just from the top of the amp? Is that what your recording represents?
    Be more specific about what is coming out of the speaker. Be sure to isolate the speaker well away from the amp so you don't confuse the two.

    Right now I'm guessing the amp is more or less OK and you're just hearing a loose lamination on the power transformer. The sound you recorded is typical of that symptom. In that case I'd ignore it.
    If a power tube ever shorted in this amp, the power tube cathode capacitor could have blown. That could cause an increase in low frequency hum, but so far you're not describing that specific sound.
     

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