Tubes & Biasing

Category: Main -> Loud Speakers & Impedance



Q: Can I safely use a speaker attenuator between my amp and cabinet?

If you are a live player and wish to bring the volume down by about 6db this can be quite effective. Just hook up the attenuator between your amp and cab. As you continue to reduce the volume to the speaker the damping effect of the speaker on the amplifier is reduced. If you want 30db or wish to completely eliminate the speaker cabinet it's a little more complicated.

A loud-speaker is not a fixed impedance device, an 8 ohm speaker is only 8 ohms at it's resonant frequency, 75 hertz in the case of a Celestion G12M. At higher frequencies the inductance of the voice coil causes the impedance of the speaker to rise to 35 ohms and to attenuate the highs. The output stage of a tube amp interacts with this impedance as well. A loud-speaker is easier on the amp then a purely resistive 8 ohm load.

High-strung 100 watt Marshalls can blow very easily if you drive them hard with only a load box. Smaller amps are recommended. Feed the output of your load device to your fx rack then into a power amp and speaker cab. A solid state amp will do but a tube amp is better.

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Q: Can I plug my 16 ohm Marshall cabinet into my Fender Twin Reverb instead of using the Twin's 2x12's?

For short term use it is generally not a problem, but, if this is to be your permanent set up I would advise to rewire the cabinet to 4 ohms. To do this wire all the speakers in parallel (all positive terminals together to the jack tip and all negative terminals to ground).

When using mismatched impedances between the amp and speakers there is a greater strain on the output stage. Total output power to the speakers is reduced, the tonal balance changes, and part of the output signal is reflected back to the amp. This reflected signal can exceed the voltage ratings of your output tubes and cause them to arc or short out - even worse the output transformer can be damaged. Try to keep the output below maximum, this will prevent possible problems.

See my ImpedanceMatching Section for more info

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Q: Can I plug my 8 or 16 ohm Marshall as an extension cabinet into my Fender Twin Reverb? (Don B.)

Most Fender Twin Reverb amps, including the Blackface reissue, operate only on a 4 ohm load and have no means of selecting any other impedance. Exceptions to this are the last original silverface issues that are rated at 135 watts, and later channel-switching models. Operating both together brings the speaker load to 2.7 ohms which is not a problem at all for the Twin Reverb to handle. If, however, you use only the 8 ohm Marshall cab and unplug the Fender speakers, then the amp could be at risk if you play at loud volume.

Whenever you mismatch a tube amp with a much higher load (speaker) impedance, a higher signal voltage is produced across the primary of the output transformer. One loud popping note on the high E string and you could arc-over the output tube sockets, fry the tubes, or zap the output transformer. This is why you cannot run a tube amp without a speaker connected to it. If you must mismatch the load impedance use a lower than rated one, as you are doing now by connecting both cabinets.

Having said this, and making it very clear that I do not advise anyone to connect a higher than rated impendence to any tube amp, generally there is no problem when operating the amp at 8 ohms if your amp is well designed and in good working condition. Connecting a 16 ohm load to a 4 ohm amp is, of course, more risky.

See pages on ImpedanceMatching and LoudSpeakerImpedance

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Q: What is the correct wiring for a 4x12 Marshall cab? (Mario P)

This is the basic circuit. 16 ohm speakers wired this way will result in a 16 ohm cabinet impedance. If you are using 8 ohm speakers the cabinet's impedance will be 8 ohms

See LoudSpeakerImpedance for more info

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Q: What's the correct impedance setting for an amp with an extension cabinet connected? (Dean T)

As with all tube amps the correct impedance setting for the amp is the one that is the same as the total impedance of all the speakers that are connected to it. Set the impedance selector in the back of your amp to match the load (speaker) impedance that you are using.

You will get the same performance (but not necessarily the same tone) with a 16 ohm cabinet by setting the selector to 16 ohms as you will if you use a 4 ohm cabinet and set the selector to 4 ohms. If the amp has no impedance selector (Fender Twin etc.) then choose a combination of speakers that comes close to the rated impedance... preferably lower. The speaker output jacks on a Marshall JCM 900 are connected together in parallel so the total impedance of the internal speakers and the extension cab together will be a parallel combination.

See the following questions to determine what the total load impedance will be. A 100 watt Marshall will produce up to 175 watts of output power when running full blast. You will eventually damage the speakers in your combo if they are rated at less than 100 watts each. The total power rating of the speakers (RMS or continuous power, not peak power) must always exceed the maximum output power of the amp. There is no danger in using higher powered speakers, they do not draw more power from the amp and they will last much longer.

The tone of higher powered speakers may not be to your liking, however, and it may be preferable to use more speakers than to install higher rated ones. If the impedance of the speakers that you connect together are not the same then the lower impedance units will receive more power than the others, so take this into account when calculating the operating power of the speakers

See ImpedanceMatching for more info

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Q: How do I know the impedance of my cabinet?

Use an ohmmeter or multimeter to measure the coil resistance directly. If you are using an analog meter (one with a needle) then set the range to ohms x 1. Short the probe tips together and adjust the zero offset potentiometer to read zero ohms. Measure across the speaker jack cable from tip to sleeve.

If you are using a digital meter then no zero adjustment is necessary. In either case the coil resistance is approximately equal to the rated impedance of your cabinet. (Your measurement will be slightly less than the actual impedance). To figure out the impedance of the individual speakers in a multi-speaker cabinet you must know how they are wired together.

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Q: How do I know the total impedance of a multi-speaker setup?

If they are all in parallel, just divide by the nubmer of speakers. Four 16 ohm speakers in parallel will give you 4 ohms.

If they are in series then add up all the impedances. Four 4 ohm speakers in series add up to 16 ohms.

With 4 x 12 cabs the impedance stays the same. Four 16 ohm speakers will result in 16 ohms total impedance when connected in series/parallel.

See LoudSpeakerImpedance for more info

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Q: How do I rewire my four 16 ohm speakers to be 8 ohms (or my four 8 ohm speakers to be 16 ohms)?

Unfortunately, this is impossible!. Four 16 ohm speakers can be wired to be 4 ohms (parallel) or 16 ohms (series+ parallel). Four 8 ohm speakers can result in 2, 8, or 32 ohms but not 16 ohms. This chart shows all the possibilities

Number of Drivers Series Parallel Series+Parallel
2 x 4 ohm
2 x 8 ohm
2 x 16 ohm
3 x 4 ohm
6 or 2.667
3 x 8 ohm
12 or 5.337
3 x 16 ohm
24 or 10.667
4 x 4 ohm
4 x 8 ohm
4 x 16 ohm

See LoudSpeakerImpedance for more info

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Q: I have a 2 speaker setup, a 15 inch woofer and a horn. If I add a high-pass capacitor in series with horn does this change the overall impedance?? (Lorrie G)

Yes, the series capacitor changes the impedance of the circuit. At very low (bass) frequencies the impedance of the capacitor will be very high (kilohms or megohms), in effect cutting off the sound to the speaker. The overall impedance will be the nominal impedance of your woofer, as if there were no tweeter connected at all. (eg 8 ohms)

At high frequencies the impedance of the cap will be negligible and it will be equivalent to not having any cap at all, In this case the total impedance will be the nominal impedance of the woofer in parallel with the nominal impedance of the tweeter.(eg 8 ohms // 8 ohms = 4 ohms). This change of impedance is what causes the bass frequencies from damaging your tweeter.

In practice however the rated impedance of a speaker is measured at it's resonant frequency and it rises sharply as the frequency increases. A speaker that measures 8 ohms at it's resonant frequency of 75 HZ can measure as high as 35 - 40 ohms at higher frequencies.

(I have greatly simplified things for simplicity's sake. The true impedance is the result of the vector sum of the DC resistance of the voice coil of the speaker, the capacitive reactance of the capacitor and the inductive reactance of the voice coil.)

See LoudSpeakerImpedance for more info

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Q: Why are 32 ohm speakers used in Ampeg SVT cabs (Ron T.)

Higher impedance speakers sound more precise than lower impedance designs. A 16 ohm voice coil has more turns and a greater coil length than a 4 ohm model. Since there is less current flowing through the speaker a smaller gauge of wire can be used. This means that the magnetic field can be distributed over a wider area, the windings are closer together, and that the movement of the voice coil is more controlled. Given the choice, I would always buy a 16 ohm model over a 4 ohm one. (I am not including tonal changes due to increased inductance of the coil nor inter-winding capacitance etc - there are always other considerations, design is a matter of compromises)

4 ohm designs only became popular with solid-state amps. These amps do not have an output transformer and consequently require a low speaker impedance to supply their maximum power (often rated at two ohms). With a tube amp you can simply match the speaker impedance by using a different tap on the output transformer.

There is always some power loss in the wiring and solder connections of the speaker cabinet. As an exaggerated example, assume that there exists a 1 ohm resistive loss due to wiring and jack connections and that you are feeding a 2 ohm load. You will have lost 33% of your power by the time the signal reaches the speaker. In the case of a 32 ohm load the loss is reduced to 3%.

I do not know the specific reason that 32 ohm speakers were selected, other than to avoid using a series/parallel connection. The SVT is in a class of it's own and was designed for performance and tonal quality rather than economy. One would assume, therefore, that the choice of speakers was based to some degree on tonal characteristics rather than simply cost effectiveness.

Parallel speaker connection offers an advantage due to the fact that the same signal is applied to all speakers. In a series connection, any dissimilarities between speakers in their frequency related impedance curves, inductance, etc., will cause one speaker in the chain to act as a filter of sorts and alter the signal to the other series-connected speaker(s). As speakers age and the voice coil is repeatedly heated, the coil can become deformed, the lacquer insulation (used in older speakers) can soften, and the electrical characteristics can change, thereby increasing the filtering effect.

In some series/parallel wiring combinations (eg. Marshall), if one of the speakers should become open, then the other series-connected one(s) will cease to function. In a parallel combination all the remaining speakers will still be active.

See LoudSpeakerImpedance for more info

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Q: Can I connect an 8 ohm speaker cabinet and a 16 ohm speaker cabinet together and at what impedance do I set my amp?

When you connect speakers or cabinets of different impedance together there is always a mismatch. If they are connected in parallel (eg. Marshall head) then the 8 ohm cab will be operating at twice the power of the 16 ohm cab and will be louder. Of course, your cabs were probably quite different to begin with so this may turn out to be OK. The overall impedance will be 5.3 ohms, so set the amp impedance selector for 4 ohms. (Using 4 ohm and 8 ohm cabs will result in 2.6 ohms when connected in parallel)

If the speakers are connected in series (eg 135 watt silverface Twin Reverb ) then the opposite occurs.The higher impedance 16 ohm cab will be operating at twice the power of the 8 ohm cab. The overall impedance will be 24 ohms. (Using 4 ohm and 8 ohm cabs will result in 12 ohms when connected in series)

The greater the mismatch in impedance, the greater the power mismatch as well. Combining 4 ohms and 16 ohms gives a power ratio of 4 to 1.

See LoudSpeakerImpedance and ImpedanceMatching for more info

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Q: The impedance switch on my Marshall JCM900 reads "(4)16 "on the left side and "8" on the right side. How do I set this switch? (Nicholas D.)

The "normal" wiring according to the Marshall schematic is 16 ohms on the left and 8 ohms on the right. The 4 ohm setting is not available. The alternate wiring scheme is 4 ohms on the left and 8 ohms on the right making the 16 ohm setting unavailable.

As it comes from the company you do not have access to both 4 ohms and 16 ohms at the same time. It is possible to rewire the amplifier internally to conform to either wiring scheme provided that the output transformer has the necessary impedance taps.

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Q: My 4x12 cabinet is currently loaded with 8 ohm speakers, and I would like to add a 2x12 cabinet (at this point, impedance unknown). How can I wire these together to create a virtual 6x12? (Dave S.)

To distribute the power evenly among all 6 speakers you must use 8 ohm speakers. Wire the 2x12 speakers in series to obtain 16 ohms.

Connect the 2x12 cab in parallel with the 4x12 cab. The total impedance is now 5.3 ohms (3 pairs of series connected speakers = 16 /3 ). Set the amplifier output impedance to 4 ohms.

t is not possible to obtain 4 8 or 16 ohm impedance when using 6 speakers of equal impedance, so you must compromise.

For a "perfect" solution you must use another 8 ohm 4x12 cab instead of a 2x12. Then you would obtain 4 ohms when using the two cabs in parallel.

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Q: Do the speakers in my 4x12 cab have to be all the same power wattage? (Serge G.)

No they don't, as long as they are of sufficient wattage to handle the amplifier output. Try to use loud-speakers with the same sensitivity (rated in db for 1 watt at 1 meter) so that all the loud-speakers operate at the same volume.

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Q: I bought a new Fender Champ 600 tube amp to power a Talk-Box that I built, and want to A/B between the Champ's speaker and the Talk-Box. I've been told that this could kill the amp as there may be a split second of no load, so I was curious of your opinion on the matter. (Jason A.)

Suddenly disconnecting the load to an amplifier while playing at full volume is absolutely the worst thing to do. Your camp is in mortal danger whenever you do this.

In order to use a talk box safely you must stop playing for a second (just mute the strings or be sure that no sound is coming from the speaker) whenever you switch the talk box on or off. That way there won't be any danger to your amplifier.

Look at it like driving a standard transmission automobile. If you use the clutch whenever you change from first to second gear you'll have no problem. Forget to use the clutch and you can blow the transmission.

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Can I connect my Marshall JCm900 4x12 cab together with my 8 ohm 2x12 cab? (Andy T.)

Yes, since both cabs have 16 ohm speakers you can do this without any unbalance of power amongst the speakers but you will have an impedance mismatch.

Set the impedance switch on the cab to 16 ohms, and connect both cabinets together in parallel. All 6 speakers will now be in parallel and the resulting impedance will be 16/6=2.67 ohms. Set the amp impedance to 4 ohms.

For a better impedance match use only half the JCM900 cab by plugging into the other speaker cabinet jack instead (Stereo mode). This will disconnect 2 of the speakers and the JCM900 cab will now be 8 ohms. Using both cabs together (in parallel) you will obtain a load of 4 ohms, a perfect match.

Another way still is to rewire the 2x12 cab to a series combinaton of 32 ohms and to set the JCM900 cab to 16 ohms. Using both cabs together (in parallel) you will obtain a load of 10.67 ohms. Set the amp to 8 ohms. This will result in a closer match than with the original 2x12 cab wiring but is only useful if you don't plan on using this cab alone (since it will be 32 ohms)

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