Please help me avoid electrocution in a few days

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by RandyFackler, Dec 4, 2017.


  1. RandyFackler

    RandyFackler Member

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    Greetings,

    I have a Vox AC4tv amp. This is the one that everybody warns about opening up and working on because of something to do with the chassis design (I think) and how it can hold dangerously high voltages even when unplugged.

    So I just ordered a pair of new tubes and this will be my first ever tube swap. They should arrive in the next few days. Yesterday morning I unplugged my amp and it is now just sitting on its own, no cables or anything plugged into it. If I just let it sit like that for a few days will that create a safe situation for me to take it apart and get in there? Or is there something more to it that I am unaware of?
     
  2. HaroldBrooks

    HaroldBrooks Member

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    Wear rubber gloves and shoes with rubber soles when you change the tubes. Don't touch anything that is grounded.
     
  3. xtian

    xtian Member

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    Get a multimeter and measure the amp's DC voltage. Learn how to discharge the caps. It's not hard. And then you don't have to WONDER, you can KNOW. Far safer.
     
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  4. stahlhart

    stahlhart Supporting Member

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    You should be fine as long as you leave it unplugged for the tube swap. I doubt that there will be enough of a residual charge in the filters by then for it to be a safety issue.

    Since there's nothing more dangerous than a wounded mosquito, you could take an alligator clip lead and ground the (+) side of the power supply filter capacitor to the chassis.
     
  5. jthomas666

    jthomas666 Member

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    I think that HaroldBrooks is pulling your chain a bit. (Probably you knew that). That said, rubber gloves and rubber soles on your shoes and not touching anything that is grounded would help keep you safe.

    EDIT- Ok... I found a picture of this amp and what I wrote earlier today was incorrect.

    http://www.voxshowroom.com/uk/amp/ac4tv_hood.html

    When you open this amp to replace the tubes, you will be exposed to the guts where the dangerous electricity resides. Keep the amp unplugged, but realize that if there is no bleed resistor in the power section of the amp to drain the filter caps, then touching the guts of the amp could be lethal, even if it is still unplugged.

    When you take the back panel off you will see the tubes sitting in their sockets. The tubes in most amps can be pulled out and replaced without touching anything more that the tubes. That will not expose you to dangerous levels of electricity, as long as you touch only the glass of the tubes.

    It looks like you will be replacing the one power tube and in the ideal world you should set the bias. The bias controls the flow of current within the tube. Too hot and the guts of the tube can be fried. Too cold and you don't get the kind of grind and harmonic distortion that makes tube amps sound so good.

    Sometimes you can get away without biasing (that is what would have happened back in the day), but biasing would be best. Given your question here, it's likely that you do not yet know how to do that. At the very least, keep an eye on the power tubes and make sure that when you power up, the plates do not begin to glow bright red (i.e., red plate). A tube that is red-plating is burning itself up by passing way too much current. Look at this video starting at about 24 minutes in.

    A Google search using the phrase "red plating" will bring up a lot more on this. Again, you might do fine just replacing the tubes without re-biasing. That is what guitarists did for years and years, but re-biasing might help you get the kind of tone out of the amp that you want.

    Please be careful and take no risks.
     
    Last edited: Dec 4, 2017
  6. RandyFackler

    RandyFackler Member

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    I think I like your idea the best. I ordered a multimeter, only problem is I don't know how to use one. I'll watch some YouTube videos and figure it out - but if you have any quick insight for me I would appreciate it!
     
  7. ylo

    ylo Member

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    But . . . doesn't the Vox AC4tv use cathode bias on all the tubes?
     
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  8. kbgear

    kbgear Silver Supporting Member

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    Yes on the EL84 and (of course) the 12AX7. No biasing should be necessary unless the tube is way outside normal spec.

    Also for reference, here's the service manual (for the head).

    I recently replaced the OT on an AC4TV, and it was draining the filter caps okay. Also, you'd need to unmount the PCB to have access to directly drain the filter caps, which pretty much defeats the purpose. As long as you're careful as mentioned above (amp unplugged and left to sit for a few minutes, rubber-soled shoes, use only one hand) you should be fine.

    But be gentle when removing the tubes: use a slow round rocking motion with moderate upward pull, don't just pull straight up. The sockets are mounted directly to the PCB, and it's not a particularly thick PCB.

    Also, the wire tube retainers are only held in with tension and tiny little bent hooks, and can easily pop out of the PCB when released. Keep upward pressure on them while you angle them to the side. Not impossible to get back in, but poking metal spring wires around in an amp is a very Beavis and/or Butthead thing to do. ;)

    HIH!
     
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  9. skytrench

    skytrench Member

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    Keep your left hand behind your back while poking around in the amp with your right! Both hands in the amp guts can lead to electrocution through the heart passing through both arms....especially with rubber shoes!
     
  10. pdf64

    pdf64 Member

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    I don't know for sure, but it's a pretty sure thing that the EL84 is in cathode bias.
    With an intended 4 watts output, I really doubt that the tube is being pushed near its limiting values.
    Cathode bias will sort itself out in these circumstances, I doubt that any functional EL84 will run too hot.
    So I think it's fine to just swap the new tubes in and play.

    Safety wise, as the amp is unplugged from the wall and nothing else is connected to it, it's effectively isolated from ground (unless you're working in a swamp and the plug is trailing in a pool).
    So no benefit in rubber boots or underwear (well, no safety benefit).
    A rubber glove and one hand behind your back / in pocket may mitigate the worst case scenario, ie HT caps still have dangerous charge remaining and the EL84 glass envelope breaks as you try to get it in/out, allowing you to touch the tube plate (which has a low resistance path to the HT); with both hands bare, one may be likely to be holding the chassis while the other touched the tube plate, facilitating a shock via your chest.
    As the amp is fairly light, it may be necessary to hold it steady while pulling the tubes out, in which case have a rubber glove on both hands.
     
  11. kbgear

    kbgear Silver Supporting Member

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    The schematic is a pretty good confirmation of this.

    You need to remove the entire back panel and chassis to get at the tubes, but yeah, using a glove or rag when bracing the PCB itself is a good idea. The PCB isn't mounted to the chassis near the sockets, so that's a very good idea to prevent snapping the PCB.
     
  12. pdf64

    pdf64 Member

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    Sorry, not sure from that whether you are positively confirming that you've seen the schematic and it is cathode bias, or whether you meant that IF the schematic was available, THEN reference to it could identify the bias arrangement?
    You give the impression that you're familiar with changing tubes on them, so I assume the former.
    If so, do you know what the power tube operating conditions (a, g2, g1, k Vdc WRT 0V) and the cathode resistor value are?
     
  13. kbgear

    kbgear Silver Supporting Member

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    I posted the schematic which is in the service manual above.

    I measured plate voltage and current to verify the Ruby EL84 after I replaced the OT, but I don't remember them off-hand. Dissipation was in a pretty safe range for Class A. The schematic lists the cathode at 10.2V with a 220Ω resistor and 220µF bypass cap, screen at 315V, and plate at 317V before the primary winding.

    If it becomes useful to the OP (or others) to know, I'll pull the back of the amp off and measure these directly at idle. I'm only hesitant because the cabinet is particle board (like, cardboard particles, not even true particle board much less MDF) and the screws strip the holes very easily. :p
     
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  14. Muttlyboy

    Muttlyboy Member

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    I'm sorry to go off topic, but would you happen to know if there is a Vox service manual for the Vox TB18?
    I've been looking for the schematic for that amp.
     
  15. kbgear

    kbgear Silver Supporting Member

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    Curiosity peaked, I found it, assuming you're talking TB18C1.
     
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  16. Muttlyboy

    Muttlyboy Member

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    Wow, thank you very much.

    For some reason I can't open it on my phone, but when I get home, I think I'll be able to get it on my laptop.
     
  17. Chris Scott

    Chris Scott Member

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    Changing tubes out's a simple task as long as you only touch the glass part of the tube(s) with one hand, and nothing metallic with the other...you don't NEED to touch ANYTHING metallic when swapping tubes out (hold the amp steady on the cabinet), so unless you're just not paying attention to what you touch, you'll be fine.

    The really bad $hit happens when you touch a live connection with one hand, while simultaneously holding the chassis with the other, as you now have a closed circuit in which the current can flow, which is right through the ol' ticker.

    Pay attention to what you touch

    Hold tube with only glass part in hand

    Line the pins of the tube up with the corresponding holes in the socket, and carefully push it home...job done. If you're having difficulty, get someone more qualified to help.
     
  18. Jon C

    Jon C Silver Supporting Member

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    Wiggling by the glass can, in some cases, stress the tube at the joint with the base.

    I try to grab it by the base, not the glass, to the extent I can.

    In any case, IMO OP's level of experience counsels proceed with caution here, I'd consider having someone more experienced show OP in person what's up and what to do and not do. Just my $.02. :cool:
     
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  19. Chris Scott

    Chris Scott Member

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    I thought he had EL84's ...maybe I need to re-read the post...
     
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  20. Jon C

    Jon C Silver Supporting Member

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    You may be right, I maybe missed that. :bonk
     

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