VoxAC10 hum/buzz. Why?

Discussion in 'Fred's Barcalounge' started by Jelly Roll Horton, May 3, 2021.

  1. Jelly Roll Horton

    Jelly Roll Horton Country Gent

    Nov 10, 2017
    Portland, OR
    No. It seems to be related to the Gain knob.
     
  2. Jelly Roll Horton

    Jelly Roll Horton Country Gent

    Nov 10, 2017
    Portland, OR
    Today's efforts: I removed all the tubes (all four are JJs (EL84 and ECC83 S), cleaned the contacts, reinstalled making sure everything was snug. The hum/buzz/fuzz is gone, but there is still a slight hiss which increases if I increase the gain with volume up.With volume at zero there is no hiss, The hiss increases if I raise the volume. So the relationship seems to be between the gain and volume knobs. o_O Hiss is not eliminated by touching the strings as earlier described in my original post.
    Really, I could live with this, because the volume overruns the hiss when I play. But it is still annoying to know it ain't right. :mad:
     
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  3. Henry

    Henry I Bleed Orange

    Apr 9, 2014
    Petaluma
    Some amount of hiss is normal, as the amp is picking up some noise, and of course turning up the gain amplifies the hiss.

    How loud is the hiss? Noticeable while playing quietly?
     
    ZackyDog likes this.
  4. sgarnett

    sgarnett Synchromatic

    655
    Apr 14, 2020
    Kentucky
    @Jelly Roll Horton have you tried leaving your phone out of the room? Drove myself crazy one time chasing cable and tube socket problems until I realized my phone was usually either in my pocket or near the amp (both bad).
     
  5. Jelly Roll Horton

    Jelly Roll Horton Country Gent

    Nov 10, 2017
    Portland, OR
    I don’t have a cell phone, so that isn’t it. But thanks. :)
     
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  6. Jelly Roll Horton

    Jelly Roll Horton Country Gent

    Nov 10, 2017
    Portland, OR
    This hiss is pretty minimal. I’m thinking I can live with it.
     
  7. rockymtnguitar

    rockymtnguitar Gretschie

    128
    Jan 22, 2019
    Colorado
    I had the same amp, same issue, moved the amp all around, swapped out tubes, etc. Just sold it on CL.
     
  8. Really wild hair: What vintage is your house.
    Are your outlets actually grounded?

    You might have to change houses.
    * (only partially kidding).

    I blame the ghost of the last owner.
     
  9. stevo

    stevo Friend of Fred

    May 1, 2012
    Atlanta
    Not sure why it hasn't always had the hum and it's hard to say that what you have is normal without hearing it. But my initial take on this is that it's "normal" with caveat.

    I owned a Mesa Subway Blues for a while - bought it used. When it came to me, there was a significant hum after it warmed up. Mesa had a standard fix - replaced a board in it and it was fine. They said the hum showed up in the amp after about a year of use. I assume the heat cycles over time caused some physical change that created the hum. So maybe in your case, it's a similar issue. With that in mind, I wonder if Vox have acknowledged this and have a fix?
     
    Jelly Roll Horton likes this.
  10. Jelly Roll Horton

    Jelly Roll Horton Country Gent

    Nov 10, 2017
    Portland, OR
    House is 90 yrs old. Outlets for all musical gear are grounded.
     
  11. Ricochet

    Ricochet I Bleed Orange

    Nov 13, 2009
    Monkey Island
    Apartment complex, semi?
     
  12. Archtops

    Archtops Synchromatic

    525
    Mar 4, 2021
    SoCal
    I had a peavey solid state amp that when dialed to the clean settings I could hear a hiss. It would never go into a solid clean tone.
    I took out the chassis and inspected the board and one of the ceramic capacitors had started to erode and leave a white powdery residue around the capacitor.
    After cleaning the powder up the hiss was gone.
     
  13. Archtops

    Archtops Synchromatic

    525
    Mar 4, 2021
    SoCal
    If it’s happening in the gain circuit maybe check that out.
    Can you get an amp schematic so you can check the gain circuit on the board?
     
  14. Archtops

    Archtops Synchromatic

    525
    Mar 4, 2021
    SoCal
    PREAMP TUBE GAIN FACTOR


    CHOOSING PRE-AMP TUBES BY GAIN FACTOR

    One simple tweak that is favored by guitar amp users is to plug in a pre-amp tube with a slightly different gain factor. (Important: this only works for tubes that are pinout and plate voltage compatible.)

    The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 tube has a gain factor of 100, while a 5751 tube (which is often used in place of a 12AX7) has a gain factor of 70. This means that if you plug a 5751 into a socket that expects a 12AX7, the pre-amp will have about 30% less gain. Not only will this make the amp quieter, but it can also alter the sound by making the power section work harder when you turn it up. Many guitar amp users (particularly harmonica players) like to reduce the gain to get a different sound or calm the amp down to prevent feedback.

    In some cases, you can also go the other way and increase the gain factor. It is generally more risky to increase the gain factor rather than decrease it as the added gain may be too much for the rest of the amplifier circuit. An example of an acceptable up-swap can be found in Fender amps that use a 12AT7 (with a gain factor of 60) as a "reverb driver" tube. You can sub in a 5751 to increase the gain by just a touch.

    The tubes we list here also have several alternate names, which we list below. Note that the alternates do not have significantly different gain factors, they are simply a different name for the same thing.

    12AX7 - ECC83, 7025, ECC803, E83CC, 6681
    12AT7 - ECC81, 6201, 6679
    12AY7 - 6072
    12AV7 - 5965
    12AU7 - ECC82, 5963, 5814, 6189

    Before you discover the new possibilities in your amp, let us give a disclaimer. Your amp was originally designed for a certain type, and although tube amps are often forgiving, it may not have a tone that you like with a different tube type. For instance, changing the tube type doesn't only change the gain factor; there are other variables as well and your amp may have a circuit that is more particular than others. As a general rule, the substitutions we describe here should work well but there are exceptions and we don't warranty any problems that result from trying mismatched tube types. For the best results, find other people on the net who have experience swapping tubes with the amp you use.

    These substitutions can also work in home audio amps, but we don't recommend trying this unless you are having trouble finding the correct tube type for your amp.
     
    ZackyDog likes this.
  15. Archtops

    Archtops Synchromatic

    525
    Mar 4, 2021
    SoCal
  16. Jelly Roll Horton

    Jelly Roll Horton Country Gent

    Nov 10, 2017
    Portland, OR
    Freestanding house.
     
    Ricochet likes this.
  17. Its a very unlikely thing, but just because there is a 3 prong outlet,
    doesn’t mean its wired with a ground.

    Sounds like you know your music room.
     
    Ricochet likes this.
  18. Jelly Roll Horton

    Jelly Roll Horton Country Gent

    Nov 10, 2017
    Portland, OR
    Yes. I have a ground check device.
     
  19. G5422T

    G5422T Country Gent

    May 24, 2012
    usa
    Mine's been fine, but I changed the power tubes to some old organ take outs.

    Got a ton of various noises, including hums and hisses.

    Put in another set of old NOS tubes that I had stashed, and all better now.

    Also played around with various pre amp tubes. All old stash, and some sounded better than others, some more hiss, others not.

    IMO, at least on mine, the circuit seems to bring out the best or worst of all the tubes.

    Probably no help to you, but what I've seen with mine.

    Do really like this little bugger.

    20200118_231610.jpg
     
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