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Need help with simple AC30 mod

Discussion in 'Recording' started by CombatWombat, Dec 31, 2005.

  1. CombatWombat

    CombatWombat Active Member

    Well, at least I THINK it should be simple.

    I have one of those new chinese AC30s and it's a 212 combo. I want to be able to link an attenuator between the amp and the speakers. Anyone know the best/easiest way to do this?

    I believe that if I cut the wires going from the amp to the "extension" I can then go out of the "external" jack, into the attenuator, and then out of the attenuator and back into the "extension" jack.

    Will this work?

    Thanks a ton,

  2. moonbaby

    moonbaby Mmmmmm Well-Known Member

    Anything that you do to that amp is grounds for VOX to void your warranty. And that may be a BIG problem for you down the road, as those "made in China" amps have no track record as to reliability.
    There is also the potential that the output section of that amp will not "like" having a power attenuator put on it. I have had varied results with power attenuators over the years on Marshalls and Fenders.You should check with Vox tech support regarding this. They may have some recommendations. They may also tell you not to do this.
    Whatever the case, if you go through with it and get an attenuator, instead of going INSIDE the amp to do surgery on that "ext" jack, why not simply disconnect the leads going to the internal speakers AT THE SPEAKERS. Place a 1/4" female jack ( like a Switchcraft jack-in-a-metal-tube) at the end of the leads, wire a 1/4" male (ie Switchcraft 280) plug to some leads connected at the internal speakers. Then you can patch the attenuator between them when you want it, using the appropriate speaker cables. This will also afford you with the added flexibility of using different cabs with the Vox, or a different amp with the internal speakers. And you are free to simply patch the Vox to the speakers if you decide to do that.
    Also, don't be tempted to use the cheesey connectors from Radio Shack. They don't hold up well, and that Class 'A' ouput section may not appreciate any arcing or intermittency from bad connections.My 2 cents...
  3. CombatWombat

    CombatWombat Active Member

    Hey, thanks for the reply. That is definitely a better idea than what I was thinking. I don't necessarily want to do anything to the amp, but I got a hot plate for christmas, so I thought if I could make it work without racking my brain or spending much money, I may as well keep it.
  4. moonbaby

    moonbaby Mmmmmm Well-Known Member

    The THD Hotplate is probably the best of that type of device. Just be careful that you are using the right one for the output of that Vox. Hotplates are very "load-specific". They are also great to have in an environment where you are not able to "get loud"...Lucky you..Enjoy!
  5. CombatWombat

    CombatWombat Active Member

    Moonbaby, need your expertise once more!

    So I took your idea to a friend and he made another suggestion that seems like it might work quite easily and for a bit cheaper.

    I bought a 5' speaker cable with 1/4" connectors and a 1'4" cable coupler. There are 4 wires running between the amp and the speakers.

    I plan to cut the speaker cable in half and cut the 4 wires between the amp and the speakers in half. Then, attach the 4 wires coming from the amp to the cut end of the speaker cable and attach the 4 wires coming from the speakers to the cut end of the other half of the speaker cable. Then, I can patch the hot plate in and just use the coupler when I don't want to use the hot plate.

    Do you think this will work? I really have almost no idea what I'm doing when it comes to electronics, but this makes sense to me. :)

    Just want a second opinion before I have at it.
  6. CombatWombat

    CombatWombat Active Member

    Is there any reason why I can't connect both positives and both negatives in the amp to the one positive and negative in the speaker cable?
  7. CombatWombat

    CombatWombat Active Member

    Finally, and to prove just how dense I am in the electronics field...

    How do I determine which wire is the positive and which one is the negative?

    Going to one speaker, there is a red and a yellow wire. Going to the other speaker there is a white and a black wire. I would assume that the red and the black are positive and the yellow and the white are negative. But, I'd rather not ASSUME anything. :)


  8. moonbaby

    moonbaby Mmmmmm Well-Known Member

    OK. So Vox is making this a whacko task. I haven't seen that particular amp. Typically, there are 2 wires (a + and a -) coming out of a combo amps speaker output, and the (2) leads simply jump from 1 speaker to the next. As far as color codes are concerned, look at the actual loudspeaker to determine which is "+", "-". Celestion usually will mark the "positive" with a red or silver dot, nothing on the "negative" side. But now that they are Chinese, haven't seen one of those yet, either.
    Now we have to determine what those Voxese bastards did to wire the speakers up. You will need to look at the speakers and see what the "ohms" rating is on each one. They are probably 8 or 16 ohms..."ohms" may be shown as an "upside-down'U'", or "horseshoe" after the #. Does your amp list the "ohms" rating at the speaker jack? What do they say in the owners' manual? My concern is that they have done some whacko wiring inside the amp, from the output transformer.
    Simply cutting the leads will not do. Say, if the speakers are each 8 ohms, and the actual load the amp needs to "see" is 16 ohms (from that pair), this would indicate that they have wired them in series inside the amp. Not cool for what you've described. I don't want you to do anything that will harm that amp or void the warranty!!!!! I will go to the Vox website and see what I can glean. Later.
  9. CombatWombat

    CombatWombat Active Member

    Moonbaby, I really appreciate all the help.

    Each speaker is 8 ohms and my manual says that the output select switch should be set at 16 ohms when using the internal speakers. Sounds like this is a bad thing? Ha. I suppose I will go snooping around the Vox site some more, but I couldn't find anything the last time.

  10. CombatWombat

    CombatWombat Active Member

    Alright! Vox hasn't gotten back to my email to them, but I was able to talk to a guy on the phone at THD and he basically assured me that patching their attenuator into the path in the way I described above would not damage anything and that the amp would still see 16 ohms.

    Shall I take his word for it? :lol:
  11. moonbaby

    moonbaby Mmmmmm Well-Known Member

    Ok, so it sounds like Vox "seriesed" (2) 8-ohm speakers together to yield a 16-ohm load. This means that at some point in the amp, the connections go from "+" to "-" on 1 speaker, and on to the "+" and "-" on the other, in a big circle-like circuit (as opposed to parallel, which is "+" to "+", "-" to "-", which is more typical. The way the've done it, you can't tell which is which. If someone at Vox or THD can draw you a schematic, fine...doubt either will. I am not sure about the color-coding. Different colors mean different things in Europe, Asia, and the US. You need to get the lowdown from Vox on what's what. You might look at the actual speakers to see if there are any markings to indicate polarity. Sorry.
  12. CombatWombat

    CombatWombat Active Member

    I got a flashlight out last night and actually discovered that the polarities were marked at the leads. Whoops. :oops:

    My old roomate who is an electrical engineering major is coming over tomorrow with his voltage meter to get this all sorted out. Thank god for friends huh?
  13. CombatWombat

    CombatWombat Active Member

    Alright, unless someone tells me that I'm gonna blow something up, I think I got this figured out. Please tell me if this is no good. Actually, the reassurance that it's ok would be nice too. :)

    So, the amp is most definitely wired in series. I checked it with an ohm meter this morning. Based on that knowledge, what I decided to do was simply cut the end off of one speaker cable and wire the (+) cable to the (+) lead of the first speaker. I then wired the (-) part of the speaker cable to the (-) lead on the second speaker.

    So, to complete the loop, I have a speaker cable going from the EXTERNAL jack on the output...which mutes the speakers...into the hotplate, and then one coming out of the output of the hotplate and connecting to the (+) of the first speaker and the (-) of the second.

    I fired it up and it worked fine. All the resistance readings worked out just fine as well. (Although they all came out to 6 and 12 rather than 8 and 16...maybe some extra resisitor somewhere in the amp?)

    Ok, so any cause for concern or am I good to go? Thanks for all the help so far, I just need that last bit of reassurance to know that I'm not going to totally *^&% something up. :)
  14. moonbaby

    moonbaby Mmmmmm Well-Known Member

    If I read you correctly, this sounds like it will work.As long as that EXT spkr jack DOES mute the internal speakers! Ya learn something new every day,eh?

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