Tube amp Ohm Mismatch - Settle this for me once and for all

Discussion in 'Glowing Bottle Tube Amp Forum' started by DannyStereo, Aug 17, 2019.

  1. DannyStereo

    DannyStereo Poster Extraordinaire

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    Simple scenario - you have an amp with an 8ohm tap and want to run a 16ohm cab. Safe? Not safe? Good? Bad? Why?

    I see this question gets a MILLION different answers. Dr. Z once answered an email of mine on his AMA video series and he seems to think it’s totally fine to do so. Other folks I’ve talked to say otherwise.


    Discuss.
     
  2. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    I have definitely run a mismatch before by adding an extension cab to a YCV40 Traynor combo. This was before I know anything about it. It wasn't quite half. The amp was fine.

    As far as I've read, it might depend on the amp and the robustness of the transformer.
     
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  3. radiocaster

    radiocaster Poster Extraordinaire

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    :lol:
     
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  4. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    Generally a 100% mismatch is safe.
    100% being double or half the impedance the amp is spec'd for.

    But generally is always open for debate.
    One problem is that we can run amps for years or even decades and keep doing minor damage to the OT without knowing it.
    I have not seen any study where an OT is fitted with a tiny camera to record any arcing in the windings or whatever may happen when "flyback" voltages nibble on wire insulation.

    Now we have more cheaply made amps with transformers that may not be built to as high specs as "the old days".
    Of course some old amps also fell below spec like the Princeton power transformer borrowed from the Champ and not spec'd for running that many tubes. They failed eventually but it took many years and those old poorly engineered amps are among the most valuable cheaply built amps today.
     
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  5. Tonetele

    Tonetele Poster Extraordinaire

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    Rule of thumb. Stick to the specified Ohms and double, at least, the wattage of the output.
    See google for diags.
     
  6. schmee

    schmee Poster Extraordinaire Silver Supporter

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    There is no consistent answer because each amp is different. Yes it's generally safe to go double or half the factory match. But if you have an amp designed with a marginal sized transformer etc, then maybe not. Certain amps, like the Princeton for instance, are known for blowing transformers more than others, even with the factory matched speaker ohms.
     
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  7. DannyStereo

    DannyStereo Poster Extraordinaire

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    Good stuff y’all. For the sake of argument, let’s call the amp in question a Silverface Bassman. Those go for a song here locally and I see them fairly regularly ran into 8ohm cabs.
     
  8. jhundt

    jhundt Doctor of Teleocity

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    I spent 90% of my life playing into mismatched cabinets. I had a beautiful blackface Super Reverb; the speakers were blown when I got it, so I cut a new board and put in a single 12" 8 ohm speaker. Never had a problem. Actually, we didn't understand that ohms stuff when we were kids. We just plugged anything into everything. I remember doing an hours-long recording session on bass, using a direct box from the speaker output of my Bassman 100, with no speakers connected at all.
     
  9. DannyStereo

    DannyStereo Poster Extraordinaire

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    Lol that’s exactly it! When I was younger I didn’t understand impedance at all and just ran my JCM900 SL-X 8ohm out into a 16ohm 4x12 and played the crap out of it.
     
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  10. jhundt

    jhundt Doctor of Teleocity

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    I do sometimes worry that we - with all of our vast internet knowledge - are taking a lot of the fun and spontaneity out of electric guitar music. I was just trying to remember when was the last time I heard a bunch of kids in a garage, trying to play music. It seems like ages ago...
     
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  11. DannyStereo

    DannyStereo Poster Extraordinaire

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    Agreed man. Ten years ago i could not have cared less about boutique, handwired this, true bypass that... just a Les Paul and a Marshall and my dudes.
     
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  12. nojazzhere

    nojazzhere Poster Extraordinaire

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    Didn't you see Toto'sDad's post on a garage band experience?.....it's not as much fun as it used to be! ;)
    And yes.....we didn't know anything about speaker ohmage, amp biasing, or tube matching.....and everything sounded great. If an amp had an extra speaker output jack, we'd plug whatever we could get our hands on into it. ;)
     
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  13. 1300 E Valencia

    1300 E Valencia Friend of Leo's

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    "Simple"? No.
    Which amp? They are all different. Call/write/email the builder, ask them.
    Dr. Z engineers his amps to work that way. Others, maybe not so much.
    Fenders are generally regarded as being "robust", which means they can take abuse and keep on working. Other amps, maybe not so much.

    What often gets lost in the discussion is that the sound of the amp changes. Once again, which amp? You may like the change, you may not. Generally, you lose bottom end, and the amp sounds like it's farther away. The sound gets smaller, thinner, and there's either less or more distortion depending on how loud you play. The well-worn phrase "blanket over my amp" applies.

    Any mismatch is probably going to wear out your power tubes faster, but how much is the question, once again different for every amp, and how often and how hard it's played. There's no one answer. You pays your money, you takes your chances.
     
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  14. jhundt

    jhundt Doctor of Teleocity

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    ha ha! I would say the same, but for me it's 45 years ago, a '68 Strat and an old BF Bassman.
     
  15. cousinpaul

    cousinpaul Friend of Leo's

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    Output is tied to the amount of load. Your bassman will lose a little output power with an 8 ohm load but should run safely.

    Cutting the load severely , or running the amp with no load, can cause the amp to run beyond the limits of it's components and create problems.
     
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  16. jvin248

    jvin248 Poster Extraordinaire

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    .

    In theory the 16ohm would lower or "resist" the current draw from the 8ohm-designed amp and should thus be easier on it.

    I don't quite get the comment "double it or halve it" because setting up for 4ohms would draw more current from the amp and stress it.


    When I was a kid I had one of those Sears digital clock radio affairs with a two inch speaker in it and I had collected a handful of speakers out of broken televisions, radios, and cars that I put in shoe boxes and wired into the radio so I had noise all around my room in middle school and into high school. First electronics class I had I mentioned it to the teacher and he asked if I wired those speakers in parallel or series.... uh, parallel. So he suggested I do some rewiring before the radio can't get me up for class. So I did. When I left for college I unhooked the extra speakers and used that alarm clock radio through college, first job, combining houses in marriage, and sometime into the second job when I replaced it. I hadn't stressed it too much after all but it was a lesson.

    .
     
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  17. johnDH

    johnDH Tele-Meister

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    it must depend on how hard you push the amp? Ie, if you arnt maxing it, a mismatch would be safer, even if not safe when fully cranked.

    On tone, theory says that on a given amp, an 8ohm tap has a lower real output impedance than a 16 ohm tap (which is not directly related to the value 8 or 16, it depends on tube type and NFB, plus how hard it runs). This affects the tendency to damp the speaker. Id expect that for small signals, a 16 cab in an 8 tap would have less upper treble, and maybe less bass.
     
  18. RottenTheCat

    RottenTheCat Tele-Holic

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    Truly no simple answer. I know that solid state drivers at the output are generally not happy with lower impedance. I know that you can dead short the output driven by tubes, and nothing bad happens at all.

    Some of the mystery can be explained by understanding where the "excess" energy (watts) go. The answer is they turn into heat. Also need to consider the nature of transformers, how the impedance ratio really doesn't tell us much more than that. Consider the wire gauge, total number of turns regardless of ratio, proximity to core, insulation used, interleave.... Think in terms of how much "current" the primary and secondary can withstand. For single ended transformers... you probably have to consider the air spacing as well. Just a heap of variables.

    Old Marshall heads really don't like higher impedance being presented to them. They tended to lose the output transformers. On the other hand, I've run a pair of 2 ohm loads in parallel off a Super Reverb. I also know an "open" speaker load on a classic Fender amp will tend to want to blow the OT.

    And when you think you got it all boiled down to simple rocket science... consider that tube impedance is not always what we think it is, due to various factors. Early 6L6 tubes from Sovtek were not 6L6 tubes, but some Russian "fairly close" valve for instance. Lot to lot variation, tube variation, design variation... you get the picture. As a side note, we see many variables in transmitting tubes from China vs Russian (or ex-Russian) manufacturers - the 811a and 572b are two that come easily to mind.

    So, transformers vary, tubes vary... designs vary... I think the safe bet is to never use too low impedance for a solid state driver, nor to high impedance with a valve driven output.

    We know that Leo overbuilt his amps, and Fender continued to overbuild them after Leo's departure, which probably contributes to the abuse they can withstand, while others cant.
     
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  19. JD0x0

    JD0x0 Poster Extraordinaire

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    It changes the loadline of the tubes, but if the Xformer is decent, it should be able to handle a 1:2 mismatch.
     
  20. scottser

    scottser Tele-Afflicted

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    Plug the right thing into the right hole unless you like doing unnecessary maths.
     
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