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Pro Junior with 3 x 10 Vibro King?

Discussion in 'Amp Central Station' started by jj69, Jul 10, 2018.

  1. jj69

    jj69 Tele-Meister

    151
    Aug 27, 2010
    rhyl
    I've got a Fender Vibro king and when I plug my little pro junior into the VK's speaker jack, the sound coming through the 3x10 cab is phenomenal.
    The problem is the vibro king speaker outlet is 4 ohms and the Pro Junior wants to see 8 ohms so.....
    being a technophobe on these things, am I gonna cause damage to the pro junior doing this and if so, is there a safe way to configure 3x10 speakers to 8 ohms ?
     

  2. ruger9

    ruger9 Poster Extraordinaire

    Dec 31, 2004
    Hackettstown, NJ
    Looking forward to reading the answer to this one...

    To be clear: you are plugging the PJ's SPEAKER into the VK, right? You aren't coming out of the VK speaker jack into the INPUT of the PJ I hope?

    I can tell you this much: the 3 speakers in the VK make for an ohms mis-match to begin with; not a dangerous one, and it is part of the VK (as well as Bandmaster 3x10) tone.
     

  3. jj69

    jj69 Tele-Meister

    151
    Aug 27, 2010
    rhyl
    Sorry to be clear... yes i'm using the VK's speakers into the speaker jack off the pro junior so the PJ is basically used like an amp head.
    I take it from your response ruger8 that I'm heading for trouble?! I have only done this on a couple of occasions
     

  4. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    It's fine. Your Pro Jr. will see half the impedance, which will half the resistance load to the EL84. Normally, the EL84 will see about 7k, so now they see 3.5k.

    Checkout the datsheet: http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/EL84/8BQ5.pdf

    The tubes will operate fine at 3.5k, the tone may be noticeably different. But that might be why you love it so much.

    [​IMG]

    Look at the graph above, you'll see that the tubes will run with slightly more harmonic distortion at 3.5k (Load Resistance).
     
    Last edited: Jul 10, 2018

  5. ruger9

    ruger9 Poster Extraordinaire

    Dec 31, 2004
    Hackettstown, NJ
    Ah... I was reading it backwards, I thought were using the PJ speaker CAB as an extension speaker for the VK (in conjunction with the 3x10s in the VK)
     

  6. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Your Pro Jr. is designed for an 8 ohm load. I would not be afraid of a mismatch of 50%...4 ohms. I get cautious when hinking that the Pro Jr. can operate a load of 2.67ohms over a long period of time without some unnecessary and possibly unsafe stress put on the tubes.
    You could simply disconnect one of those 8 ohm speaker’s in the VK to get to a 4 ohm load. Or..better yet...play through the VK. It is a superior amp in any assessment, ime and imho.

    I just have to observe that the first sentence of your thread starter post does not in any way describe what you are doing. Reading that first sentence, one would think that you plugged the Pro Jr’s Speaker cable into the VK’s speaker out jack. One would think that because that is what you wrote. (;^) I was alarmed when I read that....and I am thinking that Ruger was affected the same way.

     
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  7. jj69

    jj69 Tele-Meister

    151
    Aug 27, 2010
    rhyl
     

  8. jj69

    jj69 Tele-Meister

    151
    Aug 27, 2010
    rhyl
    Description now made a bit more clear! Thanks for the replies so far and but seems like I'm not doing the PJ any damage
     

  9. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Sometimes damage does not become total at any one moment. I am going to take a WAG that your tubes are going to live a shorter life....especially if the amp has not been biased. Those Pro Jr’s come biased very hot from the factory. A mismatch tothe low side is less of a problem for the OT than it is the tubes. A mismatch to the high side puts the stress on the OT....and that damage can be gradual and ‘incomplete’. That is, the OT can still pass signal although that signal will be weaker and at some point degraded. Some people don’t notice it...and keep playing until the preforations in the secondary windings due to flyback voltage cause a complete short...and the amp goes silent.
    Know that a matched load is preferable, more efficient and better sounding...imho....than any mismatch.
     

  10. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    His plate current is going to be about 8mA higher at 3.5k than it is at 7k (if I've interpolated the graph properly). So that could be a difference in 2-3W plate dissipation.

    1. Easiest solution: Check for red plating. If no red plating, play on. It's a cheap disposable amp.
    2. Best solution: Check the bias, and change resistor if necessary (install adjustable bias pot).

    The OT doesn't care one way or the other. Except into really, really high resistance.
     
    Last edited: Jul 10, 2018

  11. schmee

    schmee Tele-Afflicted

    Jun 2, 2003
    northwest
    I'd be a little cautious about half load on that little amp. But most amps it's ok. If you could use the ProJ speaker also and wired correctly, you could have an 8 ohm load, but then you'd have to change back to use the Vibroking!
     

  12. Syrinx

    Syrinx Tele-Meister

    Age:
    54
    340
    Aug 7, 2016
    Champaign illinois
    I have the 93 pj and the best cab I found for it was the tweed peavy 4x10e stereo cab. I used it in stereo with two pjs or a pj and a classic 20-which was a even ohm situation. I also used it sometimes as a 4x10, which was a 50% mis match, but I had no issues playing it full out for a few hours at a time. Of course this is not a go ahead-but an experience.
     
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  13. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    I will observe again that we are not talking about a 4 ohm load on an OT designed for optimum operation with an 8 ohm load but rather we are talking about a load of 2.7 ohms on that 8 ohm OT. I have never seen good come from such a mismatch. Ommv.

    That said, I vote for playing the superior amp..the VK..through its own speakers. If I owned those two amps, the Pro Jr. would be sold while it is still working. Fwiw, the basic premise of the two circuits is very similar. But....one of them is a pro unit and the other is an inexpensively built, make-do amp, imho and ime.
     
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  14. jj69

    jj69 Tele-Meister

    151
    Aug 27, 2010
    rhyl
    Im now looking into possibly getting a hot rod deville 410 and taking out the amp chassis as a cheap way of using my PJ into a 410 cab ......does anybody know if this is viable?
     

  15. robt57

    robt57 Telefied Ad Free Member

    Feb 29, 2004
    Portland, OR
    You could wire the 3x10, 2 in series, and that pair in parallel with the third 10. One 10 will get 10 watt and the other two 2.5 watts, and you will hear the one 10 louder than the other two in series. But if my amp, I'd run it that way @ 5.33 load before a 2.66 load. Unless I had some series OT transformer more than the Original in it, I'd not go to 2.66. Even @ 4 ohm load I'd not gig it or play it for long periods unless playing low and clean personally.


    upload_2018-7-13_15-32-23.png
     
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  16. robt57

    robt57 Telefied Ad Free Member

    Feb 29, 2004
    Portland, OR
    It is all about the load. So that is viable especially if wired for 8 OHMS. Perhaps even 4 ohms.
     
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