Mojotone 1974x Cab Build. Worthwhile Upgrades?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Last, May 5, 2016.

  1. Last

    Last Member

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    L@@king at possible 'meaningful ' upgrades for a Mojotone 1974x Cab build.

    Anything worth serious consideration?

    This would be a completly Mojotone supplied kit. So she comes with everything needed.

    PRODUCT DETAILS
    • 2 JJ Electronic EL84 Tubes
    • 1 EH Ez81/6CA4 Tube
    • 3 JJ Electronic 12AX7 Preamp Tubes
    • 1 BV 30V 60 Watts Nominal (Combo Version)
    • Carbon Composition Resistors, Sprague, Tad, Mallory 150 Series and Mojo Dijon
    • Mojotone Custom black board with forked turrets
    • Mojotone Custom Reproduction Transformers

    The two components that leap out (or maybe not?), are the OT and speaker.

    Any and all suggestions very much appreciated.

    Thanks in advance.
     
  2. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Build it bone stock and get it running well. Then play and listen.


    I built a Weber 18 watt TMB years ago. Over time, I swapped the Chinese Weber OT for "Good" one's, a Heyboer and a Classictone. Both times I put the cheap Weber supplied OT back. It sounds best to ME.

    I've made a few circuit changes over the years, and went through a dozen+ speakers
     
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  3. Last

    Last Member

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    Good advice. Thank you. I'll try, though it can be difficult to keep any potential upgrades (during the actual build out), at bay.




     
  4. RussB

    RussB low rent hobbyist Silver Supporting Member

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    "Upgrade" is often justification for a different part
     
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  5. Last

    Last Member

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    Absolutely agree. Certainly not averse to any potential changes that would be cost/performance beneficial.

    Id like to get the most bang for buck obviously
     
  6. RussB

    RussB low rent hobbyist Silver Supporting Member

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    The 1974 is a great circuit. You would get the most tonal variations with different speakers, but that's another rabbit hole, one I jumped into a decade ago and still can't find my way out!
     
  7. EFK

    EFK Supporting Member

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    Mojo uses Heyboer transformers for many of their products, so good luck 'upgrading' those. You will hear most dramatic differences with speaker swaps and - in this case - preamp tube swaps as JJ 12AX7 tend to be on the dark side. However, in this case 'different' doesn't necessarily mean better, just different.
     
  8. Last

    Last Member

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    I was initially looking at the Guytron Output transformer as the OT can influence the tone quite a bit.

    However. Just trying to figure out how much of what I've read is really snake oil and how much better/different it would be as compared to Mojotone's OT by Heyboer.
     
  9. RussB

    RussB low rent hobbyist Silver Supporting Member

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    You're overthinking the transformers
     
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  10. mikebat

    mikebat Member

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    Transformers, more than meets the eye.

    (Sorry...the urge was too strong)
     
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  11. Last

    Last Member

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    I think you may be right.

    Thank you for being the voice of reason.

    Now. About those speakers and tubes. :confused:
     
  12. pdf64

    pdf64 Member

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    http://www.mojotone.com/Amp Kit Sch...P&vid=AmEx0YIlAit9Mye4&cktime=140674&gc=clear
    That standby arrangement is unfortunate, as it acts to 'hot switch' a capacitive load on to a hot rectifier; the surge from this will likely exceed the rectifier's peak plate current rating.
    Standby offers no technical benefit, and in cases such as this, its use presents a significant drawback, as it may promote early failure of the rectifier.
    It would be best not to use it.
    I suggest that the standby switch is eliminated from the circuit; the front panel space would be much better used for a HT fuse.
     
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  13. Last

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    Wow. Interesting. I'm not going to act like I understand exactly what you're seeing but I'm certainly going to look into it. Thank you.

    Can't believe they must send out hundreds of these kits and most are likely unaware.

    Really appreciate you pointing it out.

    edit: found this with a quick search.

    http://www.valvewizard.co.uk/standby.html
     
    Last edited: May 6, 2016
  14. Last

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    Ok I'm going with an HT Fuse and eliminate the standby switch altogether.

    After skimming through the above link I couldn't agree more with you and now understand not only why a standby shouldn't be used but also why they are.

    I've built several amps and to be honest never even gave it a second thought. I do like having another fuse in the circuit, that if I'm understanding correctly, saves your OT if something goes south further down the line from the other fuse.

    Question I have is what value would you recommend here, and I'm assuming it's placed in the circuit just like the standby though it's grounded and placed in the always active state.

    My apologies for what may be newbie questions. Though I've done a few amps I'm by no means proficient as most reading this are by any means.

    I am learning a lot for sure. Thanks again.
     
  15. pdf64

    pdf64 Member

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    Yes, it seems bizarre, but with regard to standby, the tube guitar amp industry seems to have been labouring under a misapprehension for 5 decades:crazy

    To such an extent that when vintage designs (which gained their status without standby) get re-issued, a standby switch often gets added, eg AC30.

    Regarding the HT fuse, Merlin has more helpful info http://www.valvewizard.co.uk/fuses.html
    With a single HT fuse, I suggest to put it in the 0V return of the power transformer HT winding; that will protect the amp from more failure modes than one on the dc side of the rectifier, eg in place of the standby switch.
    However, as Merlin points out, this won't protect the winding from a plate to plate or plate to cathode short in the rectifier tube; an additional silicon diode (eg 1N4007) in series with each tube rectifier plate will act to cover that eventuality.
    As the tube rectifier will provide a smooth ramp up of HT voltage, a quick blow HT fuse can be used; I think that a F150mA should cope with extreme cranked operation whilst giving good protection.

    Bear in mind that a light bulb limiter is extremely useful when powering up a new build, even for an experienced builder, never mind a first timer. So it would be a great idea to build yourself one of those alongside with the amp.
     
    Last edited: May 7, 2016
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  16. Last

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    Excellent! Just what I needed! Thank you.

    As for the light bulb limiter, already built one a while ago. Won't say I did my first build with one but definetly use one now.

    Thanks again. You've been very helpful.
     
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