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Thread: External Cab Circuit?

  1. #1
    Mentor fender3x's Avatar
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    External Cab Circuit?

    I will be getting a small tube combo amp with a single 12" speaker for Christmas. I am sure it will be plenty loud enough for my garage, and who knows if it will ever leave the garage? Still, it has an external speaker jack, and that has me thinking about an extension cab...

    When you plug in an extension cab, the jack disconnects the internal speaker. I want to have the option to run the amp with both the internal speaker and the extension cab. I thought I could do it more or less they way the guy in this video did it:

    https://www.youtube.com/watch?v=19azXJTeM-4

    Like the amp in the video my amp says it needs an 8-16 ohm load. HOWEVER, the dude in the video has 16 ohm speakers in his combo amp and extension cab so he can run simple parallel wiring. Since I have an 8 ohm speaker in my cab, I will need series wiring.

    Here is my idea:

    Click image for larger version. 

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    I clipped a bit out of the schematic for the graphic above. Hope the bits I added are readable.

    The only mod to the combo amp would be the addition of a "direct speaker in" jack. Could be just like the one that the video guy put in his. This would avoid opening the case and voiding the warranty.

    I would attach a lead from the "external speaker jack" to the extensions cab's "amp in" jack I would attach another lead to the "series out" jack which would go to the "direct speaker in" jack on the amp. So I would need two speaker leads rather than just one, but that should not be a problem. If I have done this right, this should be a series circuit with a 16 ohm load.

    I used a switching jack for "series out", That should allow the cab to be used as a conventional 8 ohm cab if nothing is plugged in to the "series out" jack.

    Does this look like it will work, or has Dyslexia Man struck again?
    Last edited by fender3x; 12-12-2020 at 01:37 AM.

  2. #2
    GAStronomist Simon Barden's Avatar
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    Is it going to be worth it? Why not upgrade the stock speaker for a more efficient one? Increasing the load impedance will drop the power output of the amp, so you loose about as much as you gain by having more speaker area. And if the ampís turned right up, then it wonít be doing the amp components much good to have the impedance mismatch. Running to stand still.

    If the external speaker is less efficient than the combo one, then youíll probably drop the overall volume. If itís greater, then it will be better off in the combo on its own.

    Otherwise make a 4x10Ē or 4x12Ē 8 ohm cab and just use that on its own, no internal speaker, and make the most of the extra volume you get from the much greater speaker area.

    But you might want to see how loud it gets on its own first before even thinking about doing anything.

  3. #3
    Overlord of Music dave.king1's Avatar
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    I've got a little Mustang 1 among the amp herd and put an isolation jack in the back of it for an extension speaker.

    Internal speaker is 8 ohm and so is the quad box so the load remains the same and it really does make the Mustang sound rather masculine, mind you 700 watts of speakers is a bit of overkill for a 20 watt amp

  4. #4
    GAStronomist Simon Barden's Avatar
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    The chap in the video doesn't seem to fully understand valve amp transformer matching, and has fitted a 16 ohm speaker as the combo speaker, so he'll be getting less output from it on its own. The V-type speaker is 1dB less efficient for 1W input than the 98dB Seventy 80, so he's lost a bit of output there as well. He gets back to 8 ohms when the second speaker is plugged in, so for him

    Looking at the Laney Cub 12 schematic, it's certainly an 8 ohm transformer, so if I was doing anything, I'd swap out the stock speaker for something with at least 100db sensitivity. The Eminence Redcoat The Governor puts out 102dB for 1W, and is a decent sounding speaker (I have one in a Laney Ironheart), but it is quite an expensive driver.

    The Seventy 80 is a pretty flat response speaker, so suits those amps which have a high level of mids to start with e.g. a typical Marshall or some of the tweed Fender circuits. It gets criticisms on amps lacking in mids, as it doesn't add any of its own and so can sound quite bright and lacking 'push'. I don't know what the basic sound type of the Stage Right is, but the video chap certainly fitted different speakers to his. So I'd listen to the amp as it is and decide whether it could do with less treble and more mids, or whether it needs something with a pretty flat response to keep the sound much as it is (assuming you so decide to change the speaker).

    But there's a good chance it will be more than loud enough as it is, even for possible gigs. A mate's 15W Fender Blues Jr was more than loud enough for a small club gig without needing to be turned up beyond halfway.

  5. #5
    Mentor fender3x's Avatar
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    I think what you are saying makes a lot of sense. I thought that the greater surface area of the 2x12 might make up for the lower power. But it would be a lot of effort to go to if it doesn't, and adding a higher sensitivity driver is closer to s sure bet. As you point out, I also don't yet know whether I like the sound of the seventy-80, nor am I sure about how loud it will be in real life conditions.

    I also like the suggestion about putting an Eminence driver in it if I don't like t he seventy-80. It may be that you pay more in England for Patriot and Redcoat speakers. They do cost more than the Seventy-80 which runs about $70, but not that much more, especially if you compare them to the better Celestions. Even the neos in the patriot/redcoat line only run around $110. All the ones I looked at have higher SPL than the seventy-80, and it's not at all bad at 98. So lots to choose from if I go that route. I have also had good luck with Eminence. All my bass cabs (including two that are waiting to be built) have Eminence drivers.

  6. #6
    Mentor JimC's Avatar
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    I'm always rather wary of things that are too clever, because if they go wrong they go horribly wrong. Series connections mean a cable issue has the amp running into open circuit, which is not good. Its not usually that great an idea to run odd speakers either, although you can get away with all sorts of misdemeanors with a guitar amp that would make a bass rig very sick. I'm with Simon though, I think if you need more volume than a 1x12 it would be better just to use an external cab, the saving by keeping the internal speaker in the loop isn't worth the complication.
    Last edited by JimC; 12-12-2020 at 06:47 PM.
    Build #1, failed solid body 6 string using neck from a scrapped acoustic (45+ odd years ago as a teenager!)
    Build #2, ugly parlour semi with scratch built body and ex Peavey neck
    Build #3, Appalachian Dulcimer from EMS kit
    Build #4, pre-owned PB ESB-4
    Build #5, Lockdown Mandolin
    Build #6, Sixty six body for Squier
    Build #7, Mini Midi Bass

  7. #7
    GAStronomist Simon Barden's Avatar
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    Or just buy a bigger amp for gigging if needed. Or mic it up (even better).

  8. #8
    Mentor fender3x's Avatar
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    Quote Originally Posted by JimC View Post
    I'm always rather wary of things that are too clever, because if they go wrong they go horribly wrong.
    I think that goes beyond advice, and is approaching wisdom. Also, I see your point. And Simon's too ;-)

  9. #9
    GAStronomist Simon Barden's Avatar
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    Probably hundreds of nearly-new amps being dropped off at at amp rescue centres a few days after Christmas that need new homes. ones that looked so cute in the store, but mom found just too much to cope with when little Jimmy let rip on it at home. And who could say no to a sad-looking amp staring up at you with its control panel in need of a good clean, with its valves glowing and its speaker giving out a slight "won't anybody love me" hum?

  10. #10
    Mentor JimC's Avatar
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    Yes, while building my midi bass I've been having a vision of a little battery powered midi bass rig for sessions, with the amp from some orphaned cheap practice amp, a small speaker, maybe a faital 8pr, and an old android phone to act as an FM synth. So then I have to go and tidy up the workshop until something in the vague region of common sense returns.
    Last edited by JimC; 13-12-2020 at 06:06 PM.
    Build #1, failed solid body 6 string using neck from a scrapped acoustic (45+ odd years ago as a teenager!)
    Build #2, ugly parlour semi with scratch built body and ex Peavey neck
    Build #3, Appalachian Dulcimer from EMS kit
    Build #4, pre-owned PB ESB-4
    Build #5, Lockdown Mandolin
    Build #6, Sixty six body for Squier
    Build #7, Mini Midi Bass

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