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Humming Univox U 155

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  • #1

    Humming Univox U 155

    I am fooling with a Univox U-155 amp that hums somewhat all the time but more at either extremes of the volume pot. The tone control also seems to work backwards (darker tones are at the clockwise end and vice versa). I have replaced both of the pots (1M vol, 500K tone) and all of the components that are wired between them (220K resistors, 500PF cap and .002 cap). These components are wired between the pots, soldered directly to the pot connections as original. I have traced out the circuit and it matches the schematic exactly as far as I can tell. Is the schematic wrong or is there some sort of ground problem here?Univox_U-155R_GUITAR_AMP.pdf Could anyone suggest a change I could make to the tonestack that would improve the tone and lessen the hum?

    Normally I would try different tubes to see if I could lessen some hum but the tube complement includes some real oddballs which I have no spares of - 6CA4 recto, 6AQ5 x2, 6AV6, 6AN8, 6AT6.

    Would it be a good idea to twist the heater wires up and over the tubes ala Fender to lessen the hum?
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  • #2
    Amp Hum

    Check for Vac ripple on the B+ rail.
    If it is excessive, your filter capacitors should be changed.

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    • #3
      Will Check...

      JP Bass - Thanks for the reply. I have replaced all of the electros in this amp already. The old Univox's have Atlas paper and oil caps for EVERY capacitor in the amp, and so I replaced them all. I used F&T electros for the larger values/voltages, and used a mish-mash of electros and mylar orange drop types to replace the other values. Perhaps I should check my solder joints at the big electros...otherwise this amp has really woken up, the tremelo and reverb work now.The tremelo is the type that modulates the bias to affect the signal.

      The grounding scheme might be improved, the pots are grounded to the front panel and are also grounded by connections to a ground point on the chassis. I wonder if I could eliminate some of the connections to reduce ground loops.

      I have access to a scope and would love this opportunity to try it out to trouble shoot this amp. How would I go about doing that?

      Thanks - Earache

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      • #4
        More specific question - how can I use a scope to check for VAC ripple on the B+ rail? Or do I just use a meter?

        I suppose that one of the new F + T caps I put in could be faulty. Has anyone had any trouble with these caps?

        I have removed the perceived ground loops at the pots on the front panel. Now, every pot that had a wire going to ground has that pot lead soldered to it's body. All of the pots had no fiber washers when I got the amp so I assume they were not meant to be isolated from the chassis. This didn't help with the problem of hum in the vol and tone pots however....

        If a pot (or other component) is meant to be isolated from the chassis, how is that shown on a schematic? Is there a specific symbol or would it just be a written notation?

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        • #5
          Bump - Check B+ For Vac

          Hi

          I checked the B+ line for VAC and found 4.95 VAC at the output of the rectifier tube and 3.3VAC after the 20uf cap (at ground). There was also .67 VAC after the 1K 10W resister.

          Where should I see and not see AC current? If I see some, how much is too much?
          Where else should I check for VAC ripple?

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          • #6
            I also measured 217VAC at pin one of one 6AQ5 output tube and 124VAC at pin one of the other. I swapped the tubes around and got the same readings. What gives?

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            • #7
              Bumpin' for a little help!

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              • #8
                Originally posted by earache View Post
                I also measured 217VAC at pin one of one 6AQ5 output tube and 124VAC at pin one of the other. I swapped the tubes around and got the same readings. What gives?
                Ummm... Pin 1 on a 6AQ5 is the #1 grid.
                Any Vac is suspect.

                [Quote=I checked the B+ line for VAC and found 4.95 VAC at the output of the rectifier tube and 3.3VAC after the 20uf cap (at ground). There was also .67 VAC after the 1K 10W resister.]
                These voltages look good.

                You have to find where the ripple is coming from on 6AQ5 pin #1.
                I am assuming this pin was tested with the vibrato OFF.
                What is the Vdc measurement.
                What is the Vdc measurement of the 6AQ5 cathode pin #2?
                Link: 6AQ5 datasheet: http://www.mif.pg.gda.pl/homepages/f...93/6/6AQ5A.pdf

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