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. Wally

    Wally Telefied Ad Free Member

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    I am glad that is settled...again...and until the next time. Lol...

    Matched loads allow the amp to operate most efficiently in the safest manner. Mismatching by 50% to the low side puts stress on the primary side of the OT...the power tubes. This is the arrangement that Fender allows for with their parallel speaker jacks. Mismatching to 100% over the OT’s design puts the stress on the OT secondary. Greater mismatches in either direction magnify the stresses.
    And....not all damage to an OT is sudden and total. They can keep on working although they are degraded in their performance. That degradation may or may not be noticed by the user. I had a fellow come around with a DR he thought sounded great. It was not worth listening to when compared to a healthy DR...or any other Fender reverb amp. He had already been told that the output was only 15 watts....and those 15 watts created a sound that was blurred, thin, raggedy. As noted above, there are no ways to visually see the moment when flyback voltage starts piercing the winding insulation and compromising the OT. Mismatching to the high side is what causes extreme flyback voltage spikes that do this damage.
    Can a mismatch be done ......sure, go ahead....it is not my amp. Any mismatch to the high side that I do will be very temporary and the amp will not be pushed hard. I am not too wild about mismatching to the low side, either. Ommv....and will until the end of it all.
     
  2. Dacious

    Dacious Poster Extraordinaire

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    They tell you not to exceed the redline of your car or motorcycle engine. Or drive a manual around at low speeds in hi gear.

    It's up to you, but the impedance reflected into the tubes is designed to keep those tubes, at that current and voltage, in their safe operation range.

    Fender recommended you can go 100% mismatch either way. So I'd say on such amps it's safe - you might wear out tubes faster, like towing heavy loads with your family car will wear out the driveline quicker. It might last 8 years instead of 10.

    An amp like a SF Bassman - might chew tubes quicker but otherwise probably immortal. They are really understressed. An AC30 Vox or some Marshalls, no.
     
  3. DannyStereo

    DannyStereo Poster Extraordinaire

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    Man that radio story is just so cool!
     
  4. DannyStereo

    DannyStereo Poster Extraordinaire

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    My wife tells me the same thing!
     
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  5. cousinpaul

    cousinpaul Friend of Leo's

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    Thanks Wally, I was unaware of the possibility of flyback voltage. Apologies to all for the misleading post.

    ~CP
     
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  6. jhundt

    jhundt Doctor of Teleocity

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    I think the effects of speaker impedance are different in solid state amps, compared to tube amps. I wonder, if you had not rewired it, would it still be working? It WAS working, and doing what you hoped and expected it to do. You changed it because someone suggested you do so. We'll never know, it might have worked just as happily all these years if you hadn't changed anything.
     
  7. vr6pilot

    vr6pilot TDPRI Member

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    I blew up a 65w Peavey tube amp running the built in speaker along with an extension cab. The onboard speaker, a Scorpion, was already at 4ohms. Killed the PT.
     
  8. gusfinley

    gusfinley Tele-Holic

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    There are a lot of variables to consider here.

    The Impedance itself changes with frequency! Most speaker are spec'd for nominal impedance at 1KHz.

    Looking at the response charts for a 4 ohm vs 8 ohm version of the same speaker shows this:

    Notice how each speaker has a impedance spike around 125Hz ( C on the Low E and A strings ) that is about 4 times the rated impedance of the speaker. The impedance is doubled starting 2 half steps down ( 112Hz ) to ( 150 Hz ) 3-4 half steps up )

    4 Ohm:

    https://www.jensentone.com/frequency-response-chart/44/4

    8 Ohm:

    https://www.jensentone.com/frequency-response-chart/44/8

    When you match the impedances of the output transformer and speaker ( of course the tube and OT impedances should also be matched) you get the most power transferred to the speaker - if not, where does the power go? You'll get signals bouncing around between the tubes output transformer and speaker.

    Think of it this way: You have three lengths of rope. Two are the same diameter and are spliced together - essentially one rope.

    Now the third length is twice the diameter of the first and second. What happens if you quickly flip the rope? You'll see a wave travel through the first and second length that will then hit the third length. Some of the energy from that wave will be transferred to the third rope, but you'll notice that the wave will also back bounce and come at you. If the beginning of the rope was attached to something massive then the wave would reflect back down the first and second rope until it hits the third rope again with the same effect as before. The wave will keep bounding around until the energy dissipates.

    If the third length is the half the diameter of the first and second, then you will see the wave travel down the first and second rope which will then hit the third rope and cause a bigger wave to travel down the third rope. If this rope were tied to something rigid on the end then it would hit the end of the rope and bounce back to the first and second rope. Some of the energy would be transferred into that rope, but a lot of it would bounce back and travel again down the third rope. The wave will keep bouncing around until the energy dissipates.
     
  9. redchapterjubilee

    redchapterjubilee TDPRI Member

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    I've run an 8 to 16 ohm mismatch on a Mesa Mark I reissue for most of this decade. The manual actually encourages that mismatch. I also run a mismatch from both of my Bassmen (2->4 ohm on my 59 LTD head and 8->16 ohm for the Bassman 20) without issue. I wouldn't say it's a one-size-fits-all solution for every amp but for these three amps I've had no issue.
     
  10. teletimetx

    teletimetx Doctor of Teleocity

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    So, about the typical response pattern as every other time this comes up, with maybe the same number of random variables, plus a new analogy using mismatched rope diameter wave theory entropy.
    Within given parameters, mismatch not likely to cause audibly discernible damage, but system will, over time degrade and in extreme cases, tranformer experiences total and therefore expensive failure.
    Expensive = bad.
    Bonus points for radio stories and days of old, when the new silver sparkle of the BF ruled the stage.
    My own radio story: once upon a time in a small mountain town, lived in a tiny box-like room that was added on top of the back porch of the family house. It was tiny, unheated, but it was my own space. My parents had just made the quantum leap of buying a new "stereo" which played not only records, but cassettes, too!
    I found an old speaker, the kind with the old brownish fabric grill and ran an odd assortment of wires, maybe 25 feet long or longer, to the extra extension terminal of the stereo and hey, music! Right kind of wire? Highly doubtful. Impedance match?
    Unknown.
    My mom still had that stereo when we moved her into assisted living in 2013.
     
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  11. Wally

    Wally Telefied Ad Free Member

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    Teletimetx, since there was a cassette player, that system was solid state. If there were extra contacts for speakers, it was built to handle it. I doubt if you were getting more than one side of the stereo, though, right?? Good story....
     
  12. teletimetx

    teletimetx Doctor of Teleocity

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    Not exactly high fidelity...but late 60's rock!
     
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  13. Asmith

    Asmith Friend of Leo's

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    Well dont forget the OT primary is in series with a pentode(s) and P=V^2/R, (where R is impedance) you can increase the impedance but then itll drop more voltage. My knowledge on pentodes is limited so I cant put any maths to it other than if by doubling the impedance the voltage drop across the transformer increase by more than a factor of 1.4 (sqrt(2)) then the OT would have to handle more power.

    FWIW I could be wrong.

    Regarding lower speaker ohms, lets just assume a scenario where the secondary is shorted. The impedance of the OT primary would be close to its DCR and wouldnt actually be able to drop a lot of voltage compared previously to the valve so Id imagine the OT wouldnt actually consume a lot of power.

    Again FWIW I could be wrong so no one quote me on this and if someone with more experience wants to correct me please do!
     
  14. stepvan

    stepvan Tele-Meister Silver Supporter

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    add fur if makes it an easier concept
     
  15. tele_savales

    tele_savales Tele-Meister

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    We used to wrap tin foil around our blown fuses and shove em back in the amp when I was a youngster. Worked like a charm. Mismatched impedances too, I'd wager.
     
  16. gusfinley

    gusfinley Tele-Holic

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    Na, the foil just helps you find the bad part by making sure it fails hard!
     
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  17. tele_savales

    tele_savales Tele-Meister

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    For the life of me, I can't remember anyone blowing an amp up back then. I never did, and everything I owned was on 10 at all times.
     
  18. tele_savales

    tele_savales Tele-Meister

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    Lovely.
     
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