Tube Amp - Cab/Ohm Query

Discussion in 'Amps and Cabs [BG]' started by andy_p6, Apr 27, 2004.

  1. andy_p6


    Apr 5, 2004
    London, UK
    I own a sunn 300t and am in the market for a new cab. I am leaning towards the 900w Lab Systems 6x10 (i live in Australia) as this falls within my budget, is competitively priced and is an aussie product with a good reputation. My question is regarding the resistance, as on the website it says that the nominal impedance of the cab is 5.3ohms. My sunn has a switch between 2,4,or8 ohms. I'm assuming that i would set the head at 4 ohms but I wanted to make sure that this would in no way cause the tubes or transformers any stress that could damage it. I know that ohmage is not a static thing but I just want some affirmation from those who are a bit more tech-savvy than myself.

    I also would like some advice if I decide to go with a 4x10 & 1x15 or similar setup. Would I be best to choose 2 X 4ohm cabs and run the sunn @ 2ohms, or choose 2 X 8 ohm cabs and run the sunn @ 4ohm load? My concern is more associated running the head at its most efficient and reliable (rather than sound quality and volume levels, as I assume there would be little difference here). For example - will the tubes/transformers work harder @ 2ohms than 4ohms. etc?

    PS. This is my first post - its good to be a member!
  2. MrBonex


    Jan 2, 2004
    New Hampshire
    My understanding is that the stated impedence of any speaker system is an average of sorts. So it is perfectly possible for a 5.3-ohm bin to dip below that -- especially at lower frequencies.

    The other thing is that the speaker rating should not be below the amp impedence. In other words, it's ok to run an 8-ohm bottom with a 4-ohm amp selection -- it just reduces the power of the amp because it "sees" more resistance. Somewhere around half-power. It's NOT ok to run a 4-ohm bottom on an 8-ohm tap. I makes your amp run hotter that it was designed to -- and KABOOM!

    So, the best choice would be to run the amp at 4 ohms.

    The above is simplified science, so please forgive me for not blathering on about technicalities that I don't understand anyway.
  3. Actually from my understanding of how tube amps react to impedance mis matches, you've got it backwards. It is better to go from a higher tap to a lower impedance ( 8 ohm tap out of the amp to a 4 ohm load) than to go from a lower tap to a higher impedance (4 ohm tap to a 8 ohm load) Psycho! help me out here......
  4. Greybeard, you nailed it. I wouldn't run a 5.3 ohm cab off a 4 ohm tap at high volumes. You're better off to get a cab with eight speakers and a 4 ohm impedance. 5.3 ohms isn't going to do any immediate damage to your amp, but you'll lose some power tube life.
  5. andy_p6


    Apr 5, 2004
    London, UK
    cheers for that... unfortunately a good 8x10 is out of budget at the moment so may look into a high powered 4 and upgrade from there.
  6. I think you can run a 5.3 ohm cab safely from the 8 ohm tap.
  7. MrBonex


    Jan 2, 2004
    New Hampshire
    Hold On! Do I have it wrong???

    Psycho, forgive me, but I don't know who you are, so please explain in more detail! I am certainly willing to learn!!!

    This is how I understand it (and I believe how it was explained to me):

    Impedance is resistance. Therefore, the greater the impedance, the more resistance the amplifier "sees." Therefore any amp (tube or solid state) will react to that resistance by having more or less power available -- depending upon that resistance. If any amp sees less resistance, it will put out more power (because there is less "stuff" holding the current back). Therefore: If your 100 watt amp sees less resistance than it is designed for, the amp will poop. More resistance, and it will put out some fraction less.

    How does a tube amp differ from solid state in that regard? I regularly gig with an 8-ohm box on my tube amp that has only a 2 and 4 ohm tap (I set it at 4 ohms) and have ever had a problem. And I can't see how it would. So, some education here would be helpful.

  8. Saetia


    Mar 27, 2003
    What about those guys who run 3 8ohm cabs to get a 2.6 ohm load, if they have an amp that goes to 2 ohms they run it off of the 2 ohm tap. My understanding of how it works is if the impedence on your cab is lower then what your amp will put out the amp will end up failing on you. My understanding would be if your cab is 5.3 ohms and your amp is capable of putting out a 4 ohm load your cab would see just a smidge less then the 4 ohm rated output? Usually if an amp puts out say 400 watts @ 4 ohms it will put out 260 some odd watts (usually 2/3rds) at 8 ohms. I wouldn't see any reason why you wouldn't be able to run a 5.3 ohm cab with an amp that is stable at 4 ohms. Correct me if I'm wrong. But I don't see why you wouldn't be able too.

  9. That's a solid state amp you're talking about. Tube amps use an output transformer to match the speaker's impedance to the tubes' impedance, so full power can be delivered into any of the acceptable loads.

    They're not that picky, like putting 5 ohms on a 4 ohm tap probably won't kill anything, and neither will putting 3 ohms on a 4 ohm tap, but put 16 ohms on a 2 ohm tap and you may just fry your output transformer due to high voltage flyback action.

    Solid state - doesn't care what the load impedance is as long as it is above the minimum, but can only deliver full power to the minimum rated load, like 4 ohms in most cases.

    Tubes - care what the load impedance is, but can deliver full power into any load it has a tap for, like 2, 4, 8 or 16 ohms.

  10. See the above post by me. The tube amp output transformer matches the speaker impedance to the tubes desired impedance. This is why there are different taps and such, and why tube amps can put out full rated power into different impedance loads (as long as they have a tap corresponding to that load).

    Tube amps are not as picky as some people think, and that's why your 8 ohm cab runs fine on the 4 ohm tap. You may be getting a little less Wattage by runnning that, but not enough to worry about.

    If you plot output power versus impedance for a tube amp, it looks like a hill with maximum power delivered to the optimum tap impedance. If you get 100 Watts from a tap into 8 ohms, then you'll probably get 60 Watts from that same 8-ohm tap when run into a 4 ohm cab, or 50 Watts from the 8-ohm tap run into a 16 ohm cab. It's not a simple P=V^2/R equation like it is with solid state.

  11. I looked for a link that would sum this up since it seems I've explained this about 100 times. I couldn't find one that that adequately covered things in one post, so here goes. To understand WHY tube amps don't like higher impedances and why they are MUCH more dangerous to tube amps than lower ones, you have to understand what it is in both types of amps that actually does damage.

    In solid state amps, the output transistors are connected directly to the speaker load. Since solid state amps put out a constant voltage, the varible that determines power/current is the impedance of the load. Ohms law: P (power)= i (current)/r (resistance). The lower the load (resistance), the greater the amount of current is passed by the output transistors. Current also generates tremendous amounts of heat, which is why if the load is too low, the heat stress on a solid state's output transistors can damage them.

    The same rules apply to tube amps, but there is a deciding factor that makes a huge difference. Tubes are extremely high impedance devices, and in order to be connected to a speaker, must be coupled through an output transformer which takes the high voltage/low current output of the tubes and makes a high current/ low voltage output that can drive a speaker. Instead of being constant voltage amplifiers, they are constant current amplifiers.

    A transformer is an inductive coil that works by amplifying current and/or voltage. If the transfromer is not connected through a circuit on both its primary and secondary taps, it will feed back on itself and generate a large voltage spike at its connected end. What happens with too high a load connected to the transformer's secondary is that it acts like an open connection and causes feedback within the transformer. Since the primary of the transformer is connected to the plates of the output tubes, this voltage spike hits them first. As long as it doesn't exceed their limit, you're OK, but if it does, you can short them AND your output transformer.

    The reason a lower impedance is not so dangerous has to do with the nature of the tubes themselves. If the transformer secondary is connected to a lower than expected impedance, the transformer obeys Ohms law and tries to maintain a constant current, which causes a drop in the voltage of the plates of the tubes and they emit less, so power actually decreases. Some tube amps short themselves to ground when no load is present as a means to protect themslves from inductive feedback. That's as low an impedance as you can get!

    That's the short, quick, and dirty, and I can get a lot more detailed if necessary. So if you have any questions, just post them.
  12. Saetia


    Mar 27, 2003
    Thanks for clearing that up throbbinnut,

    I have very limited knowledge of tube amps, mostly just tube pres for me.

  13. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    PBG has pretty much explained it right as far as what to do.

    I don't think I would be quite as fussy about the exact numbers. There is a range of usable impedances for any tap, just because of the way a speaker acts.

    A speaker labeled 8 ohms is probably 8 ohms only at about 400 Hz. At resonance, it may "look like" as much as 50 ohms. At 2 kHz, it may easily be a net impedance of 14 or 15 ohms. These changes are due to resionance and due to the inherent inductance of the speaker voice coil.

    So exact impedance numbers are not even really known.

    That makes it difficult to "match" exactly, and that is taken into account in design.

    But, it is true that if an 8 ohm speaker is 50 ohms at resonance, and 16 ohms at 2kHz, a 4 ohm speaker will likely be about 25 ohms at resonance, and only maybe 9 or 10 ohms at 2kHz. So the 4 ohm will surely vary over a smaller range and stay closer to correct for a 4 ohm tap than an 8 or 16 ohm speaker.

    A lot has to do with the exact amplifier and tubes. Minor mismatches are not as catastrophic. In fact on many (not all) amplifiers, 5.3 ohms is perfectly OK on the 4 ohm tap.

    It also matters a lot if you are clipping the amp or staying "linear". Clipping causes way more problems.

    Technically, a transformer has inductance. Inductances store energy as current. So once you get current flowing in an inductance, you cannot instantly reverse it. The faster you reverse it, the more voltage it takes to do that.

    Conversely, if you try to stop current flow by opening the circuit, the stored energy causes an increase in voltage until current flow occurs. This is very much like the ignition in your car, which uses this principle to make a high voltage spark.

    Typical "linear" operation of a tube amp, even without a load, tends to charge and discharge the inductance slowly, so the voltages are low.

    But clipping involves much more rapid changes of voltage, and as a result can generate large "spike" voltages. These may cause voltage breakdowns in the tube or the transformer.

    If a proper load is connected, the energy has somewhere to go without inducing the high voltages.

    If a load of too high impedance is connected, the voltages may peak above the safe range even though some energy is dissipated at lower voltages.

    So there is a range of "safe" and a range above that that is marginal or unsafe. With any amp design, the ranges can be approximately known, but for sure we are not guaranteeing it outside of "normal" operation.

    A lower than optimum load does affect tube emission, and can draw extra current, potentially wearing the tube's emissive material out early. If you really mis-match, they may wear out VERY early. But this is usually both slower and less catastrophic than voltage breakdown.

    Typically you can go about 1 1/2 times up or down with most amplifiers. So 4 ohm tap is usually OK with 6 ohms, maybe even with 8. The 8 ohm tap may be OK with 6 or possibly even 4 .

    But there are no guarantees..............sorry.

    Some amps really push the voltage ratings, and almost no increase is safe with them.

    BTW, the other problem with high loads is oscillation. Most tube amps have some negative feedback to flatten response and create a lower impedance source. All is generally well at normal or below-normal loads.

    But with no load, that "negative" feedback may change phase and become "positive". Just as with a PA, this leads to oscillation, an uncontrolled self-generated signal, which may be at so high a frequency that you can't hear it. But you may see the tubes glow red due to the dissipation from this signal. Usually one instance of this wears out or otherwise destroys the power tubes, possibly as fast as a few seconds.

    Most amps that short the output with no speaker, do so for anti-oscillation reasons as much as anything else.
  14. softcover


    Jul 19, 2005
  15. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    Moderator, can we have a sticky, please? :D
  16. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    I'm not sure if it fits the budget or not, but Jim Bergantino's NV610 is a 4 ohm cab. He uses 24 ohm drivers.

  17. Yeah, either that or an echo efffect on PBG's posts, i feel sorry for the amount of times he's had to say that stuff!