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do I have the ohm thing backwards?

Discussion in 'Amp Central Station' started by Deeve, Feb 22, 2020.

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  1. Deeve

    Deeve Poster Extraordinaire Silver Supporter

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    okay, amp people, please be gentle, here, as the mistake is probably basic (but not obvious to me).

    I thought higher ohm count meant more resistance, so the plan was to take my tiny AC 108 Vintager ss power/6v6 pre combo into forced quiet distortion.

    The stock 8" spk says 8G20B4
    4ohms 20watts.

    I replaced it w/ JBL 2115
    16ohms unk watts - unlimited?

    Two low volume strums sounded fine, then nothing.

    What's my rookie mistake here?

    Peace - Deeve

    1582429034039-370080118.jpg 1582429065016-72298453.jpg 1582429116649-212758080.jpg
     
  2. DougM

    DougM Poster Extraordinaire

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    If you get no sound it's possible that you could have damaged the output transformer. Tube amps don't like a speaker impedance a lot higher than they're designed for, because it increases the voltage stored in the output transformer. The general rule is 1 step in either direction is ok, but two steps higher can be a problem. This is why you never use a tube amp without a speaker connected, because it wil fry the output transformer. They handle a lower impedance than they need better than a higher one. SS amps are just the opposite.They can run all day with no load, but a shorted output, or one that's lower impedance than they're designed for will fry the output transistors.
     
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  3. rolandson

    rolandson Tele-Holic Silver Supporter

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    No, you got it right...

    You killed it.

    A 16 ohm load in a system designed for 4...

    Dead.

    the mistake was in not asking ahead of powering up. But because it Berhringer, you'll live to fight another day.

    but never ever do that again.


    Nice speaker though.
     
    Last edited: Feb 22, 2020
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  4. AndyPanda

    AndyPanda Tele-Holic

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    Tube amps need to have the correct speaker impedance that the OT is designed for. If you want to reduce the volume at the speaker while driving the amp hard you can use a resistor network in combination with the speaker that gives the amplifier the expected load (4 ohms in your case) while the speaker is only getting a small fraction of the output power and the resistors are turning the rest into heat. But you need to know the basics of series/parallel wiring and Ohm's law and some common sense.
     
  5. Deeve

    Deeve Poster Extraordinaire Silver Supporter

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    The Amp ain't dead friends - it still runs when (re)hooked up to the stock spkr.
    The power section of this amp is ss; the pre-amp has a ceremonial tube - this is the opposite of my pv classic 2x12 (ss pre and a pair of 6L6 power)

    The "bulletproof" JBL, however, is quite silent.
    Or am I still not understanding.

    Yes @AndyPanda - Common Sense is what I came looking for -
    a) to see if I had the resistance thing backward and
    b) if that goes out the window when power section is solid state.
     
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  6. rolandson

    rolandson Tele-Holic Silver Supporter

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    Like I said...Behringer...it pulled a Lazarus on ya!!!
     
  7. gusfinley

    gusfinley Tele-Afflicted

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    With solid state amps more resistance = less current = less power.

    Still, the 16 ohms may be out of spec with the driver of the SS power amp and it could be going into shutdown.
     
  8. Deeve

    Deeve Poster Extraordinaire Silver Supporter

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    From how I'm reading some of these responses, it sounds like you believe the AMP has been killed or injured.
    I believe the Amp is yet alive.
    It makes sound w/ original speaker re-connected.
    The JBL, however, is now a lot like Silent Bob.
    :cry:
    new question
    If I can get the JBL re-coned, can its ohm-age be changed to 8 or 4 ohms?
    Peace - Deeve
     
  9. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Get a PP3 9V battery and connect some wires to the terminals of the speaker. Tap the other wires onto the battery terminals. Does the cone move in one direction?. If so, the speaker is OK. Don't worry about the direction.

    It may well be the output IC of the amp went into shutdown.

    As to changing the voice coil to an 8 or 4 Ohm unit, probably. Is it worth it though?
     
  10. AndyPanda

    AndyPanda Tele-Holic

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    I'm not familiar with your amp ... but in your OP you said 6v6 (which is a common output tube and not something I'm familiar with being used as a preamp tube). You also indicated that your reason for switching to a higher impedance speaker was so you could get "quiet distortion" So based on those two bits of info from your OP made it sound like you have a tube power section of your amp that you want to overdrive. Hence my reply about using a resistor network or an L-Pad to reduce speaker volume while driving your amp hard enough to get overdrive.

    But apparently you are saying that your amp uses a 6v6 tube as the preamp and has solid state power section ... which makes your amp something completely alien to me.

    Your JBL might just have a broken tinsel wire and not need a recone. Those tinsel wires wear out and are fairly inexpensive/easy to replace.
     
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  11. Deeve

    Deeve Poster Extraordinaire Silver Supporter

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    Andy - you're absolutely right!
    I've misidentified pre-amp tube- not 6v6 (of course)

    The Amp works.
    Attach to original spkr and - sound.

    The JBL spkr stopped working.
    Battery to leads test: nothing, nada, zilch, no movement
    until the battery leads were moved to original spkr - yes, movement, noise, signal success, and music.

    Will check JBL tinsel.

    The amp in original photo is a super cheap Berhinger "15 watt tube" combo w/ 8in spkr. On closer exam, it's a single tube in pre-amp (12ax7?) and a ss power section, the opposite of the pv dinosaur 2x12 classic (w/ ss pre and 6L6 duet for power)

    I just wanted a better speaker (JBL) and to push it into distortion sooner/quieter.
    I thought a higher ohm resistance spkr would help w/ that, and instead, have gotten helpful tips about output transformers and common sense I lack, cos I've been misunderstanding what's going on w/ a higher resistance spkr.
    And THAT was the idea behind the headline/title on the original post.
     
  12. AndyPanda

    AndyPanda Tele-Holic

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    being solid state I don't think you'll get the results you want by changing speakers ... solid state power amp doesn't give a pleasing distortion by making the speaker quieter to drive the amp harder .. but tube amps do. Good luck with it.
     
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  13. Dacious

    Dacious Poster Extraordinaire

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    If the JBL cone doesn't move with 9 volt DC connected it's blown, or seized. You can check with a DMM set to DCr. For 16ohms you should see a DC resistance of 10-15 ohms. And hear a slight scratchy static. If infinity reading voicecoil is open, if zero resistance voicecoil is burnt and shorted.

    A SS amp designed for 4 ohms through 16 will be very low output.
     
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  14. Guitarteach

    Guitarteach Doctor of Teleocity

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    The ‘efficiency’ of a speaker relates to the volume it produces for a given power. It is the sensitivity measure given in db in the specifications. The db scale is logarithmic so a 80-90db speaker may be half the volume output of a 100db speaker.

    You can crank the amp with a lower efficiency speaker more to get amp distortion at lower volumes.... Attenuaters soak up the power to enable the same.

    BUT... It may not sound good though as solid state (the power side of your amp) breakup is unpleasant.

    As others have said, a mismatch with the transformer will be a problem with using the wrong ohms.

    What it sounds like you really want is a clean boost or overdrive to distort the 12ax? vacuum tube preamp more though.
     
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  15. radiocaster

    radiocaster Poster Extraordinaire

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    It's common on the internet for people to say output transformers get easily fried, despite not having experience with that. I think it takes a lot to fry one, and although you can fry it with no speaker connected, even then you won't after 30 seconds.

    16 vs. 4 ohms, well, more resistance means less sound.
     
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  16. AndyPanda

    AndyPanda Tele-Holic

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    I think you have misinterpreted what I said --- or at least my intentions -- I didn't mean to imply that you or anyone else here lacks common sense. I was trying to say that armed with just a couple of simple tools (Ohms law, series/parallel wiring and some common sense) you could build yourself a series/parallel resistor network that would act like a power soak and lower the volume of the speaker while driving your amp hard enough to get pleasing distortion (this was when I believed your amp had a tube power section - now I know it is solid state, my suggestion no longer applies - and discussion about Output transformer no longer applies either).
     
    Last edited: Feb 24, 2020
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  17. Deeve

    Deeve Poster Extraordinaire Silver Supporter

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    we're still good, AP, still good

    what I SHOULD have done first, is ASK

    That's on me

    we're good
    Peace - Deeve
     
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