5E3 Multi Jack Speaker Impedance

Discussion in 'Amp Tech Center' started by funandfunny20, May 18, 2019 at 3:19 AM.

  1. funandfunny20

    funandfunny20 TDPRI Member

    Age:
    34
    Posts:
    21
    Joined:
    Dec 1, 2018
    Location:
    Southern CA
    Hello,

    I'm currently building a 5E3. Kinda confused about what jacks I should use if I want to have separate 4ohm & 8ohm outputs. Do I use shorting jacks for both so that one of the secondaries is shorted when not in use? I have the Classictone OT that has 16/8/4 secondaries. I'm also wiring in a NFB switch...

    I'm not going to use the 16ohm tap for now, and I'm assuming I just tape it off, correct?

    Thanks! Hopefully my question makes sense.
     
    Andy44 likes this.
  2. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

    Posts:
    1,041
    Joined:
    Feb 2, 2012
    Location:
    Silver City, New Mexico
    I set up my DR build with both 4&8 ohm possibilities. Use a shorting jack for the "main" speaker, and a switching jack for the "aux". The "main" is 8 ohm until something is plugged into the "aux", then the 4 ohm tap is switched in.
     
  3. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,430
    Joined:
    Dec 29, 2012
    Location:
    United States
    You don't want to use a shorting jack in either speaker jack unless you will always use either the shorting jack or both jacks. If you try to use the aux jack alone the shorting jack will short part of the output transformer secondary.

    You must be careful to always have a speaker connected when you fire up the amp because you won't have the shorting jack protection.
     
    Paul G. and Wally like this.
  4. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

    Posts:
    1,041
    Joined:
    Feb 2, 2012
    Location:
    Silver City, New Mexico
    I like having the protection there. If you need to run a 4 ohm speaker only, install a plug in the "aux", and the shorting jack still protects against an open circuit.
     
  5. D'tar

    D'tar Tele-Afflicted

    Posts:
    1,876
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Impedance switch....


    upload_2019-5-20_9-26-1.png
     
    robrob and Wyatt like this.
  6. philosofriend

    philosofriend Tele-Holic

    Posts:
    970
    Joined:
    Oct 28, 2015
    Location:
    Kalamazoo
    None of the wires from the transformer should be shorted to any other wire from the transformer ever. Yes, you just tape off the extra wire, and the unused 4 or 8 ohm wire should not short out to anything, it should be electrically disconnected, the same as the taped off wire is.

    You just have to have one pair of the transformer wires hooked to a speaker, while the other wires go to nothing.

    Some amps with a built in speaker use a switching jack to disconnect the internal speaker when you plug into the "external speaker" jack.

    A picture of the circuit you are thinking of would help... I think you want one jack for 8 ohms and one for 4. You will have enough sense to always have a speaker plugged in (duh). You don't need a circuit to protect you from plugging nothing in or using both jacks at once because you know not to do that. If so, all you need is two plain jacks. Hook both grounds to the common winding and each tip to its own winding. Sorry if I don't understand the question.
     
  7. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

    Posts:
    1,041
    Joined:
    Feb 2, 2012
    Location:
    Silver City, New Mexico
    Notice that Fender uses the system that I described on their Hot Rod amps, the "main" is shorted to ground unless something is plugged in.

    https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Hot-Rod-Deville-Schematic.pdf

    D'tar's switch idea works well also, and you still have the protection from open circuit! My Twin build I used the OT from a "94 Twin" and a Marshall switch for 4-8-16 ohm output through either series, or parallel "aux" jacks. The "main" of course is a shorting jack.

    https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-94-Twin-Schematic.pdf

    (I did install an XLR for a line out, but have never used it!)
     
  8. funandfunny20

    funandfunny20 TDPRI Member

    Age:
    34
    Posts:
    21
    Joined:
    Dec 1, 2018
    Location:
    Southern CA
    Thanks everybody for the info!

    I finished assembly last night and we have an issue—amp won't amplify my guitar.

    The amp does have a very low hum with my guitar plugged into input 1. However, the amp oscillates when I unplug the guitar. When I measure my voltages, the amp just do the normal like crackle, pop, like we're all used to when we check our B+'s, etc.

    I'm using a single point ground, using the sleeve tab on input 1.

    Connected to my lightbulb current limiter, my voltages on the output tubes are within "normal" range taking into account the voltage drop from the lightbulb. Lightbulb glows very softly, like it should. No bright lights.

    It's P2P, and I've quadruple checked my wiring and re-flowed every joint—it feels like the issue is right in front me. I suspect my jacks are connected incorrectly and/or my grounding scheme is messed up.

    FYI, the red wire off to the side, waiting to be installed when I wire in the NFB,

    Here are some photos.

    IMG_4498.JPG IMG_4488.JPG IMG_4489.JPG IMG_4490.JPG IMG_4491.JPG IMG_4492.JPG IMG_4493.JPG IMG_4494.JPG IMG_4495.JPG IMG_4496.JPG
     
  9. philosofriend

    philosofriend Tele-Holic

    Posts:
    970
    Joined:
    Oct 28, 2015
    Location:
    Kalamazoo
    Bill is right. I was a bit confused (bad experiences from early transistor amps). I looked in Jack Darr's amp book and saw my mistake.

    The shorting function of the jack on the Deluxe is to protect the output transformer in case no speaker is plugged in. A tube amp run with no load can burn out the power transformer. When a winding is shorted the tubes can't deliver hardly any power to the transformer because the almost zero ohms speaker load asks so much from the transformer that the input impedance rises. So you would not want any windings shorted at the same time that you were trying to drive a speaker.

    In the hot rod circuit you can't get the low impedance into just one speaker jack. It only delivers the low impedance if both jacks have a plug in. You could plug a dummy switch into what they call the main to turn off the short (and get the low impedance out from the other jack). This system assumes that you want to use either one or two speakers both of the high impedance.

    The impedance switch allows you to get either impedance at will. If you put that into the stock 5e3 output jack setup you would have the shorting protection and the choices that two jacks gives you. For example, two eight ohm speakers, one into each jack, with the switch in the 4 ohm position, would be theoretically correct. Two 16s with the switch at 8 would be correct also.

    The only weak point in this setup is that you are trusting the switch to not fail open which could lead to no sound and losing the protection of the shorting jack. I would use a high quality dpdt switch with a high amp rating and bridge the two sides together (an old trick to increase reliability). I wouldn't change the switch unless the amp were turned off.
     
    Bill Moore and robrob like this.
  10. funandfunny20

    funandfunny20 TDPRI Member

    Age:
    34
    Posts:
    21
    Joined:
    Dec 1, 2018
    Location:
    Southern CA
    Thanks Philo!

    I think I'll get the impedance switch when the time comes.

    In the meantime, any thoughts on my problem?

    Thanks....
     
  11. Wally

    Wally Telefied Ad Free Member

    Posts:
    31,322
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    It appears to me that you do not have the inputs wired correctly. The shorting tab needs to be connected to the ground lug tab, which needs to be grounded.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.