Vox AC30 Intermittent Buzzing Noise

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by MrKite89, Jun 24, 2019.

  1. MrKite89

    MrKite89 Member

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    Hi, I have a Vox AC30H2L, bought new 10 years ago, which lately is emitting a strange intermittent buzzing noise in the Top Boost channel ONLY. The Normal\EF86 chennel doesn't present this problem.

    The noise is affected by the volume control in the TB channel and the tone controls (treble, bass and cut). With the volume at zero the noise disappears. It is not VERY loud and seems to get worse once the tubes are warm.

    This makes me think that the problem is located in the preamp circuit, pre volume control, of the TB channel.

    Things I tried:

    - swapping 2-3 12AX7s in the first position with trusty ones: nothing.

    - cleaning all the tube pins and sockets: nothing.

    - cleaning jacks and pots: nothing.

    - gently tapping the components in that area looking for mechanical "reactions": nothing.

    - moving some cable around the TB preamp section: nothing.

    My next suspect would be the CC 220k plate resistor, but I don't know if the noise I'm hearing can be produced by a noisy resistor and I don't have on hand any 1/2W (or more) 220k resistor to try.

    Here's the schematic (I think the problem is located around V2) and a recording of the problem (SM57 + audio interface, recorded without guitar plugged in): you can hear a costant buzzing noise (along with the normal hiss) which becomes intermittent and then costant again. Around halfway and near the end you can hear me turning the controls to affect the noise: with the volume at zero the noise disappears. Also with the cut and\or bass at max it pretty much disappears (or gets masked), while increasing treble and volume definetely increases the buzzing.

    Thanks in advance for any idea! :)

    https://el34world.com/charts/Schematics/files/Vox/Manual_vox_ac30h2_ac30h2l.pdf

     
    Last edited: Jun 24, 2019
  2. pdf64

    pdf64 Member

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    No soundclip?
    Yes the input stage’s plate resistor r26 would be a likely suspect. But surely attempting a repair on a new amp would void the warranty?
     
  3. MrKite89

    MrKite89 Member

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    Can't you open it? It's at the bottom of the post, uploaded with Vocaroo.

    Read the first sentence of my post once again... ;)

    Thanks, but I'm not sure that it's that kind of noise: if you can have a listen then let me know what think...
     
  4. pdf64

    pdf64 Member

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    Oops sorry I ‘speed read’ that on iPhone as ‘new 10 months ago’ :rolleyes:
    There’s no link other than the schematic showing up. I’ll check on my win10 pc later.
    Buzzing type noise often indicates heater signal interference; are both 220ohm heater balancing resistors ok?
    I guess that the input stage plate Vdc is fairly low? That can cause otherwise good tubes to have problems, so it may be worth swapping in yet another known good one.
     
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  5. MrKite89

    MrKite89 Member

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    Mmh...it shows up on my screen... :dunno

    Maybe I'll try another uploader if nobody can see it...

    Are you referring to R102 and R103 on the schematic? I should check those, thanks for the tip...could also a bad rectifier tube cause this kind of problems in ONE channel only? I have a couple of GZ34s to try...

    Yes, the plate Vdc is low on V1 in the TB channel, ~130Vdc: lower gain tubes (measured with my tube tester) approach ~135Vdc, higher gain ~127Vdc, different brands too, new or NOS, no major changes. I have an EH 12AY7, maybe I can try it just to see if the problem disappears...

    I also tried different cathode followers with no success...

    UPDATE: it seems to disappear with a 12AY7 in V1, but the plate Vdc is ~75? :huh. I'm leaving it on to check it over time...And now? How to proceed? It doesn't sound bad, but I want the gain of a 12AX7 without buzzing! :D Maybe replacing the 220k plate resistor (10% CC right now) with a lower measured value, so to increase plate Vdc a little?
     
    Last edited: Jun 25, 2019
  6. ddog

    ddog Supporting Member

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    Its funny people always say power tubes...v1,v2 pre's are and lately have been culprits for me...I have small electricians spare parts box (many slotted shelves) for xtra tubes....youll always needem.
     
  7. pdf64

    pdf64 Member

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    OK, the adobe flash player shows up on the pc; is there definitely none of that buzzing from the EF86 channel?
    Has a power tube ever blown? eg causing fuse FS101 to pop.
    Yes, check R102 and 103. I don't think the GZ34 is a likely cause, but as you've got good spares, by all means try them. I can't see that the plate resistor R26 could cause this.

    Have you noticed if that channel may have lost a little bit of gain alongside this buzz developing? A possible cause could be V2 cathode bypass cap C26 going bad.
     
  8. MrKite89

    MrKite89 Member

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    Absolutely.

    Honestly in 10 years of playing I can't remember exactly...it can be, maybe once...I'm sure it happened with my AC15H1TV (which is quiet), not sure about my AC30H2L.

    I didn't notice less gain than usual, just this weird buzz.



    The amp has been on for all the afternoon with a 12AY7 in it and it seems fine. Would this indicate R102\3 as the main suspects at this point?

    Thanks for all the help!
     
  9. HotBluePlates

    HotBluePlates Member

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    Try grounding the grid of V2 with a jumper to ground.

    Input jacks and/or associated resistors/caps (and possible loose ground or switching contact) would be my primary suspects.
     
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  10. MrKite89

    MrKite89 Member

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    Thanks for the tip, I'll try!

    The jacks seem fine anyway: I tried to clean them accurately and the buzz remains the same with a cable plugged in (both inputs) and there's no reaction at all if I wiggle the cable or plug it in and out.

    So with the 12AY7 in V2 it's silent because the gain is too low to "pick up" the noise, in your opinion? I'm trying to understand what's going on...It is less gainy with the 12AY7, obviously, but it's not "weak" or "empty" sounding at all, there's still quite a lot of gain, weird that the buzz is gone: volume on ~6 with a 12AX7 equals to ~7.5 with the 12AY7...probably it's just driving "efficiently" the next stages and this amp has a lot of gain (for an AC30) anyway...
     
  11. pdf64

    pdf64 Member

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    The 12AY7 may not exhibit the buzz because it has less gain, or because it has good heater to cathode insulation, or a combination of the two.
    It’s worth trying another 12AX7 in there, one that’s quiet when used in the input stages of a high gain amp.
     
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  12. HotBluePlates

    HotBluePlates Member

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    That's why troubleshooting involves forming a hypothesis for where to look, then devising a test that systematically eliminates possibilities.

    This forum relies too much on blind guessing ("Try this. Try that. When I had that problem, I swapped ____.")

    What @pdf64 said.

    I don't have an opinion about "picking up noise." I do know a 12AY7 is lower-gain than a 12AX7, so it will amplify less anything that comes before it. Maybe the noise is related to the input jacks, and the 12AY7 doesn't amplify it as much as the 12AX7. Who knows?

    So we're going to do a simple test ("jumper the input tube grids to ground") to test the theory. If the noise remains, the jacks, components & connections are ruled out as problem sources. If the noise goes away, we then need to figure out what about the input jack circuit is malfunctioning (new theories, new tests).
     
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  13. MrKite89

    MrKite89 Member

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    I tried some tubes that are very quiet in the dirty channel of my JCM600 head and all of them make the same noise in the AC30: a couple of Sovteks WB\WC, 90's "square getter" 12AX7, China 7025 and so on...I'll try a JJ and an old Brimar ECC83 just to be sure. :)
     
  14. MrKite89

    MrKite89 Member

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    OK, I'm confused: put a JJ in, no noise. Put the Sovtek WC that was originally in when I recorded the noise: no more noise.

    I let warm up the amp and everything, it is still on and there's no noise...

    I don't know what to do: if it stays like this who cares, but I'd like to have a reliable amp...

    It is very hot these days, so with all the fridges, air conditioners etc, the voltages are quite low and not stable at all: do you think it could have something to do with the problem I'm experiencing? Although I remember the buzz also before the summer, so...I'm planning to buy a voltage regulator or something anyway...

    Any idea? Should I still try to jumper the grid to ground as HotBluePlates suggested?
     
  15. HotBluePlates

    HotBluePlates Member

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    No, wait until you start having noise issues again.

    Intermittent problems are the most difficult to troubleshoot.
     
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  16. MrKite89

    MrKite89 Member

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    S**t. :)

    Thanks for the help guys!
     
  17. MrKite89

    MrKite89 Member

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    The noise is back, and I have a new clue: the buzz, still present without the guitar plugged in, gets affected by the guitar controls: the tone knobs shape the "tone" of the buzz as you would expect (better near zero, worst on 10) and the volume reaches the maximum noise in its max out impedance position, ~75% with a Log pot.

    When the noise disappears the guitar is silent and doesn't buzz\introduce buzzing in any way.

    Should I proceed with the tube grid to ground or this new clue opens up to new possibilities?
     
  18. MrKite89

    MrKite89 Member

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    I jumpered the V2 input grid to ground and the noise seems to disappear: all the noise, buzz and hiss gets drastically attenuated, but I can't hear that buzzing when the input grid is grounded...Now, what would you do?
     
  19. HotBluePlates

    HotBluePlates Member

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    According to the service manual you linked, the input jacks have switched contacts and grounding washers. Additionally, there appears to be a fast-blow fusible link between circuit ground & "CGND" (which is a RF suppression cap running to the grounding washers).

    First, check resistance from each side of CGND to the ground side of R37 (the Top Boost channel input stage).

    - If you get open-circuit on both side of CGND, the link to circuit ground is broken & needs replacing.​

    If CGND has a side that is ~0Ω to circuit ground, then check the action of the switched contacts on the input jacks. Press on the contacts to see if they start to close & kill the noise. Of course, these contacts may need cleaning (with sandpaper or emory cloth) to restore a clean contact at the switched contacts.

    Last, The grounding washers themselves may have a coating of insulating corrosion. You will need to disassemble the input jacks to clean the washers (preferably with abrasion as well as a cleaning solution like DeOxit).

    One or more of these steps will likely solve the noise for good.
     
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  20. pdf64

    pdf64 Member

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    Also there's the possibility that the there's a problem in the circuit between the 'gnd_main' and the chassis itself, eg C101, D101 etc bad, or a bad connection between a solder tab and chassis. Or maybe the cab has a foil screen on the open side of the chassis which isn't getting a decent connection to the chassis metalwork.
     
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