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  • #31
    Originally posted by onelife View Post

    It could be a filter capacitor in the power supply.

    Originally posted by Cornholio Farquarth III View Post

    That's where I'd place my vote.
    I don't know if Youtube Uncle Doug is an amp expert but he sure sounds like one.
    This is what he says about filter capacitor noise:

    60 Hz hum ----> tube
    120 Hz hum ----> filter capacitors

    In my case I have a 60 Hz hum, but as I mentioned above, the hum starts as soon as the amp is powered on,
    before the tubes have had a chance to warm up. Whether they can add noise before they can conduct sound, I don't know.

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    • #32
      After looking at the chassis pics I can tell you exactly where the hum is coming from.

      That amp is set up like my bassman is. It uses a Bias/Hum adjustment for balancing tube bias.

      What you do is tweak the bias pot till you hear minimal hum. That's it. Turn the standby on and off a few times to make sure the hum remains very low. If you cant get rid of the hum completely try a different set of power tubes. This is really common with these style amps. How low you can get the hum is purely based on having a good matching set of tubes and tweaking the balance.

      Allot of people hate the design because it assumes tubes are all the same and the fixed bias of the one tube is in an ideal range. Many in fact modify the amps to put in separate the bias of the two tubes and set each tube separately. I never bothered myself. I've used the amp since 67 and it only takes seconds to balance the two tubes using the bias pot and the most that might happen is the pot may move a little due to vibrations in transport or the two may vary as the tubes get some hours on them. you simply give it a little tweak and you're done.

      I do like the fact they mounted that adjustment in the back. On the Bassman they mount it between the transformers which makes it nearly impossible to tweak with the head mounted in the cab. You can tweak that one from the back with a small - screwdriver Once the tubes are broken in you shouldn't have to mess with it any more. You may hear a little hum when voltage surges occur but that's normal.

      What I do, instead of just using my ears is connect an AC meter with a low voltage setting of 2Vac to the speaker jack, no input, volume turned completely down and then adjust the pot till I get the lowest AC voltage. After that when I turn the standby switch on and off, I may get a slight surge where the hum might be a little louder, but after the surge it should be unnoticeable.

      Hum on this kind of bias balance amp is fairly normal. The type of tubes makes a big difference on how low you can get the hum. I've run Groove tubes and had good results. I used JJ's recently and the hum level would come and go so I went to Electro Harmonix and there's no hum at all (plus low hiss levels. If the tubes aren't well matched you may have to live with a minimal amont of hum till you change the tubes.

      The other possibility are the 470 ohm screen resistors aren't well matched. For awhile I was running a high impedance on my Bassman and I kept having the tubes blow and take those screen resistors out. I eventually put in a set of military grade screen resistors with 1% tolerance and got a perfect balance between the tubes. The ones in the picture look like they have a silver band which indicates a 20% tolerance. That means you can have as much as 80 ohms variance per resistor, especially when they get hot. I'd lift one end and measure each to see if they are a close match. If they're only off a few ohms I wouldn't worry and simply focus on the tubes and the bias balance.

      You did mention about shielding inside the amp case. Whet the old fenders like mine used was steel screen stapled to the cab casing. When you installed the head the chassis would press against it grounding the screen making a faraday cage. It works very well grounding the open side of the chassis. The only thing that might be better is copper foil. It has to be stapled in place and flat as possible. You don't want any rough edges extending inside the chassis causing a possible short.

      Don't use aluminum foil or aluminum flashing tape. Aluminum sucks for blocking hum AC hum passes right through it. It will work if its 1/4" or more but paper thin copper foil will work twice as well.
      Last edited by WRGKMC; 05-17-2017, 03:32 PM.

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      • #33
        Thanks again WRG for the tips!


        Originally posted by WRGKMC View Post
        After looking at the chassis pics I can tell you exactly where the hum is coming from.

        That amp is set up like my bassman is. It uses a Bias/Hum adjustment for balancing tube bias.

        What you do is tweak the bias pot till you hear minimal hum. That's it. Turn the standby on and off a few times to make sure the hum remains very low. If you cant get rid of the hum completely try a different set of power tubes. This is really common with these style amps. How low you can get the hum is purely based on having a good matching set of tubes and tweaking the balance.
        I got up the courage to adjust the bias/hum pot a couple of weeks ago. I didn't know if I could make adjustments with the amp powered on, so not taking a chance I would turn the pot slightly and then power up and test, power down, and repeat...
        It was a bit tedious, but in the end, going through the complete turn, the hum noise went down just marginally, if at all.

        What I do, instead of just using my ears is connect an AC meter with a low voltage setting of 2Vac to the speaker jack, no input, volume turned completely down and then adjust the pot till I get the lowest AC voltage. After that when I turn the standby switch on and off, I may get a slight surge where the hum might be a little louder, but after the surge it should be unnoticeable.
        How do you get at the speaker jack leads to do the test? Can you just use a speaker cable plugged into the jack and gain access to the leads on the other end of the cable? Can you use a metal screw driver to adjust the bias pot while the amp is on?

        Hum on this kind of bias balance amp is fairly normal. The type of tubes makes a big difference on how low you can get the hum. I've run Groove tubes and had good results. I used JJ's recently and the hum level would come and go so I went to Electro Harmonix and there's no hum at all (plus low hiss levels. If the tubes aren't well matched you may have to live with a minimal amont of hum till you change the tubes...

        ...You did mention about shielding inside the amp case. Whet the old fenders like mine used was steel screen stapled to the cab casing.
        I don't really know what to do next to tell you the truth.
        On the one hand, this is a brand new amp with a lifetime warranty.
        I could certainly swap some tubes to see what happens but... that would be an extra investment on my part that would be covered by the builder.
        And I wouldn't want to mess with shielding the cab either. This would be his job to do.

        But as I mentioned just before, just getting the amp to the builder and back is going to cost someone at least $200.
        So I think you can see the conundrum I am in.

        Don't use aluminum foil or aluminum flashing tape. Aluminum sucks for blocking hum AC hum passes right through it. It will work if its 1/4" or more but paper thin copper foil will work twice as well.
        Speaking of aluminum flashing tape...
        When I was separating my Fender Princeton II combo into head and speaker cab a couple of years ago I knew I was supposed to shield the top of the cab.
        I had already bought some window screening to use as the shielding but decided to use aluminum flashing tape because I looked at my Fender Vibroverb cab and saw a sheet of metal tape stuck to the top of the cabs. Granted, it was a different tape than what I was using, but it seemed like it would be easier to install.

        Well fast forward... My Princeton II also has a hum/hiss now on par with the new amp.
        Before I split the amp into head and speaker cab it was dead quiet. It had a metal sheet at the top of the cab.
        So maybe that's where I should be concentrating on?




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        • #34
          Originally posted by Jazzer2020 View Post
          ...
          How do you get at the speaker jack leads to do the test? Can you just use a speaker cable plugged into the jack and gain access to the leads on the other end of the cable? Can you use a metal screw driver to adjust the bias pot while the amp is on? ...
          leave the speaker plugged into the amplifier - connect the leads of your voltmeter to the terminals of the speaker

          yes you can use a metal screwdriver while the amplifier is switched on - if you have a non-metallic tool then use that
          As a human being, you come with the whole range of inner possibilities
          from the deepest hell to the highest states.

          It is up to you which one you choose to explore
          .

          Comment


          • #35
            The minimum hum setting should be close to the center of the bias pot rotation. You wont hurt anything going to either extreme but I wouldn't leave it there If the hum is worse. There's a null point between two slightly different kinds of hum which you need to find. If you're having to turn it to one extreme or the other it may indicate one tube is weaker then the other. I'd swap the two around and try again. If you still cant get a minimum hum buy a new matched set.

            I would suggest a set of EH over some others. I tried Ruby and JJ's and they didn't come close to sounding good. The EH sound like the old RCA tubes I used to use. Groove tubes can be good too. They are a bit tricky buying however. They come in low, (clean) Medium (Normal) and High (Hot) bias. I bought a set before Fender bought the company but I cant remember what range they were in and the site doesn't tell you what they recommend for different amp types. Not that it matter that much on this type of amp because the bias is fixed on one tube and the pot simply balanced the second tube to the first.

            You also mentioned using screen over the amp head. Normal door screen is nylon, not metal so it would have no shielding capabilities. Some other types may contain aluminum or other metals that simply don't shield well. If you go to a good hardware store they may have a section with screen on reels. The one store by me has this. They sell Nylon, Steel and Brass screen. The brass would work best.

            Like I said, aluminum sucks for shielding AC hum. If you're using it, get something better. You can buy thin galvanized steel at hardware stores, cut it to size, drill holes and screw it to the inside of the cab. That will knock out the hum quite well. Copper foil tape can be purchased too. They make shielding paint but I'd be a bit leery using that inside a cab. It works well enough inside guitars but the heat from tubes and vibration might cause it to flake off and short circuits.

            For using the meter, I simple unscrew one of the speaker jack covers. As Onelife says, the speaker must be connected and the amp running. No input and volume turned off. You connect one lead to the chassis. Then you carefully touch the center lead from the jack with the meter set for low AC volts. You can start higher and work your way down on settings till you get a reading. Its normally a volt or less when you can hear a hum.

            From there you can turn the pot to find the lowest reading. It will be like a valley as you turn it, the voltage will reach bottom then start rising up. Tweak it back and forth till you find the lowest point. At the same time you'll hear it change with your ears too. You may hit a point where you see no change on the meter but you hear a small change. Just use your ears at that point to get the lowest level between the left and right.

            As I said, with good tubes, the hum should disappear below the amps normal hiss level. The hiss is normal and will vary depending on what tubes you use and the type of speakers you use. When the amps were designed the speakers weren't the best and they didn't produce allot of high frequencies so the hiss was masked. If you bough cabs with upgraded Altecs like mine had the hiss was more noticeable but added loudness made it well worth it.

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            • #36
              Thanks onelife and WRG. I have my work cut out for me.

              Comment


              • #37
                I had a hunch. It occurred to me last night.

                This new amp is basically a Princeton Reverb clone.
                Built as a head and speaker cabinet, just like my Princeton Reverb II
                after I separated it into the same form (from original combo form).

                My Princeton Reverb II was always dead quiet.
                Up until the time I separated it into head/cab form.

                Since then there has been a bit if a hum and hiss.
                And the hum/hiss is on par with the new amp build.

                So I thought, what is going on here?

                I was initially thinking about the grounding plate that I removed from
                my Princeton (it was rattling) and had replaced with aluminum tape
                in its new head housing. Maybe that was not sufficient grounding?

                And recently I discovered the new amp doesn't even have any grounding
                in its head housing.

                I thought beyond this. What else had changed?
                Well now there was separate head and speaker cab, instead of a combo.

                Then I thought, "What about the speaker cable that I built?"
                The 7.5' cable that I was using to connect the head and speaker?
                Maybe, just maybe it was acting like an antenna?

                The new amp was shipped with a 2' cable which I had straight away set aside
                and never used.

                I plugged her in today and the hum was reduced by more than 75%.
                Plugged her into my Princeton Reverb II and the same thing happened.

                Is it dead silent now? No.
                But I think I can live with the hum now.

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