Running 16 Ohm Speaker Into 8 Ohm Amp

Discussion in 'Marshall Amps' started by Peter Karakondis, Aug 12, 2019.

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  1. Peter Karakondis

    Peter Karakondis New Member

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    I just noticed that my DSL401 was at 8 ohm setting instead of the 16 ohm setting, which is what the internal speaker is. I switched it back to 16 ohms and the amp still works. Is it possible that any damage may have occurred?
     

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  2. Emtbreid

    Emtbreid Well-Known Member

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    How long was it set like that for?
     
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  3. MarshallDog

    MarshallDog Well-Known Member Premium Member

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    I have always been told it is OK to run the amp at a lower setting than what the speaker(s) is but dont run the amp at a higher setting than the speaker(s)!
     
  4. Emtbreid

    Emtbreid Well-Known Member

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    There’s your answer dude. You’d know if something was messed up, be it tubes or an output transformer. Now had you had it set to 8 ohm and tried to run a 4 ohm speaker? Could be different. Keep that switch on 16 and all’s well.
     
  5. Peter Karakondis

    Peter Karakondis New Member

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    I don't how long it's been set like that, probably a long time, but I haven't used the amp much, and have hardly ever used it for more than an hour in one day.
     
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  6. Peter Karakondis

    Peter Karakondis New Member

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    Thanks for your quick reply! I think I'm OK then. The tubes still light up and the amp works. If the transformer was affected, what would the symptoms be?
     
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  7. Peter Karakondis

    Peter Karakondis New Member

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    So if it was the opposite situation, with a an 8 ohm speaker plugged into a 16 ohm amp, that would be a reason to panic I guess.
     
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  8. Emtbreid

    Emtbreid Well-Known Member

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    Loss of volume, decrease in “good” tone, hums, strange noises, etc.
     
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  9. Emtbreid

    Emtbreid Well-Known Member

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    Yeah. Anytime you mismatch a load it’s potentially bad for the amp, but there’s “bad” and “really bad” I suppose. Sounds like you’re a ok!
     
  10. john hammond

    john hammond Well-Known Member

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    If you look at d3 and d4 on this schematic centre section, up the top

    https://www.thetubestore.com/lib/th...all/Marshall-JCM2000-40W-DSL401-Schematic.pdf

    you can see the bridge rectifier that rectifies the wall power to high voltage dc.

    what this means is that you can use any speaker out , 4, 8, 16ohms into either 4, 8 or 16hms speaker.

    Just pick one that sounds the best and go for it.

    The reason people are edgy about this type of thing is because techs have taught people that using the wrong speaker load in an old marshall will kill it, and it will if it's an old 100 watter, because they use voltage doubling, not bridge rectification.( 50 watters dont use voltage doubling, so again, pick the load that sounds best).

    These killer spikes can be called 'reverse voltage spikes'.
    If you have an old 100 watt marshall, you can get these diodes , 3 in series, put in from plates to ground on two inner output tubes which stops the spikes from damaging your amp.

    The reason voltage doubling kills amps is because large transients ( think SRV fat e string hits) force current back all the way to the rectification diodes, and then back forward again ( like breathing ) and after the current goes through those rectification diodes, the voltage doubles AGAIN, so, your copper insulated wire in the primaries of your output transformer see around 900 to 1000 volts ( even more in a 1966 or 1967 marshall 100 ).
    That wouldn't be a problem in some transformers, but those drake and dagnalls just dont like that much voltage, and the insulation breaks down and causes short circuits between one winding and another = dead output transformer.

    Having said all this, pls remember to have SOME speaker attached to your dsl401, 50 watt marshall or any fender amp, or you'll kill the output transformer eventually anyway because the voltage fed into it cant go anywhere and it will 'divine wind' itself.
     
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  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Whaaaaaat???
     
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  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    If you never maximized output (gain and volume) then you are more than ok. When you force the amplifier out of its normal paramaters then it will have problems.
    It is always best to have all settings be appropriate especially the output impedance of a tube amplifier.
     
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  13. john hammond

    john hammond Well-Known Member

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    edit..66 and 67 marshalls dont use voltage doubling, i think, so thats an error right there.
    I should add too that if you use the 4 ohm out into an 8 or 16 ohm speaker, which almost always sounds better than 4 into 4...you'll reduce output tube life as they have to work harder, for me its a no brainer because youll hear more ' tubes' and that's the point in the first place, don't worry they'll still last for ages.
     
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  14. john hammond

    john hammond Well-Known Member

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    wanna hear how to make your amp sound better than it ever has?

    1) find some output tubes that have been test run and pre-hammered, like mesa tubes.
    2) bias it hot, real hot..don't worry about bias readings etc..set the amp chassis up in a black room, no light whatsoever..use a torch for this it will help..turn the bias pot up slowly and watch the plates on the output tubes, watch the fins and watch the plates..anyway the plates and or fins attached to the plates will start to glow red ..back off until they don't glow and stop right there. presto, youve just biased your amp with no tools except a screwdriver and your eyes.
    3)run the amp from the 4ohm out into a 16 ohm load and enjoy.
    turn the amp backwards and watch those plates while you crank it up in a black room as you may have to back off some more on the bias pot.

    4) don't try this with a 100 watt marshall or any amp that utilises voltage doubling, check before you do this.

    don't forget step 1)..use virgin untried tubes and sure as god made little green apples, one will pop.
     
  15. john hammond

    john hammond Well-Known Member

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    uh...run your 4 ohm out into your 16 ohm speaker BEFORE you bias, sorry about that guys i'm doing three things at once here
     
  16. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

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    It wasn't full up and you used a 16 ohm in 8 ohm output. It works, you're fkne. I agree about the paranoia out there about mismatching.
    See Santiagos many posts about this. The guy designed some fine Marshall amps.
     
  17. MarshallDog

    MarshallDog Well-Known Member Premium Member

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    Luckily, that’s a question I can’t answer as I have never had an OT blow. Hopefully someone else will chime in!
     
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  18. Emtbreid

    Emtbreid Well-Known Member

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    There’s no 4 ohm out on a DSL401... only 8 and 16.


    Let me rephrase, there’s two outs, but the selector switch only does 8 and 16ohm.
     
  19. john hammond

    john hammond Well-Known Member

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    " I agree about the paranoia out there about mismatching. "
    - that's another thing entirely.
    you WANT a mismatch.
    So, if they're mismatched , you're getting a considerable amount more harmonic activity..the more mismatched, the more harmonic activity but the more hum.
    When 4 matched tubes are given a certain current, they produce harmonics within a certain range, the same range.
    When you vary the current, you vary the harmonic output of the tube.
    If there is say two tubes, I like to mismatch them on purpose to within say 15-20 percent. So if one reads 40, i like the other one to read 32-35. The hum is barely noticeable, certainly not while playing and it just sounds better.
    When setting up mismatched tubes, just make the hottest one is running under the threshold you want...if following the above instructions on biasing without a meter or anything else, just bias until one starts to cherry up, then back off.
    People simply make money from convincing other people how important it is to use matched tubes, what an utter load of bollock.
     
    Last edited: Aug 12, 2019
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  20. john hammond

    john hammond Well-Known Member

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    " There’s no 4 ohm out on a DSL401... only 8 and 16."

    - marshall cheapout alert.
    Just run from the 8 ohm tap to 16 ohm speaker you'll love it.
     
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