What speaker impedances can I safely connect to my tube Wurlitzer electronic piano?
Every tube amplifier has a preferred speaker impedance: usually, this is something in the neighborhood of 4 Ω or 8 Ω. The preferred speaker impedance of tube Wurlitzers (according to their schematics) are as follows:
Wurlitzer 112: 3.2 Ω
Wurlitzer 120: 3.2 Ω
Wurlitzer 700: 3.8 Ω
Wurlitzer 145: 4 Ω
Wurlitzer 720: 4 Ω
With the exception of the 145 and 720, these are all pretty oddball numbers. We’ll explain why later. In general, though, you can consider all of these impedances basically equivalent to 4 Ω. Because impedance varies with frequency, the impedance of any particular speaker is necessarily an estimate. (For instance, 3.8 Ω and 4 Ω should be considered basically equivalent.) For this reason, you should always keep some mental flexibility when thinking about speaker impedance.
So, the onboard speaker of tube Wurlitzers should be 4 Ω. However, in a pinch, you can use a speaker of a different impedance, as long as that impedance is only slightly higher or slightly lower. For instance, you can usually use an 8 Ω speaker with a 4 Ω amplifier, or vice versa. When you use a higher impedance speaker, you are reducing current flow through the amplifier’s output circuit. This reduces the amp’s output power, but small mismatches in this direction should be safe for the amplifier. However, as you increase the speaker impedance, you also change the characteristics on the primary side of the transformer, including the voltage and primary impedance. If you connect a speaker with too large of an impedance — for instance, over 16 Ω — it may appear to the power tubes as no load at all. This will result in damage to the tubes or the output transformer.
Detail from the Wurlitzer 120 schematic.
On the flip side, using a lower impedance speaker increases the current flow. If the speaker impedance is low enough, this will potentially increase current to a level that the amplifier cannot safely handle. This may result in unwanted distortion, blown fuses, or the eventual failure of the output transformer.
Speakers with a lower impedance than 4 Ω are difficult to find outside of car audio. However, if you connect multiple speakers to the Wurlitzer using the extension speaker jack, the amplifier sees that speaker in parallel to the onboard speaker. When connecting multiple speakers in parallel, it is very easy to end up with a total impedance well under 4 Ω. Although a lower impedance in the neighborhood of 3 Ω would be fine, connecting 2 Ω or less can be risky to the amp. For this reason, you should be very careful about which speakers and devices you connect to the headphone and extension speaker outputs of a tube Wurlitzer amplifier.
Whenever possible, you should use the correct speaker impedance for your amplifier. If you must mismatch impedances, stay as close to the amplifier’s desired impedance as possible. The further you stray from this impedance, the more stress you put on your amplifier circuit. If your amp is particularly deteriorated, this stress might be the thing that finally put it over the edge. However, using a slightly mismatched speaker should be fine under most circumstances, particularly if those circumstances are temporary. Some people like to baby their output transformer; some people just want to finish their recording sessions; these are both valid positions and we’re not going to judge you for improvising a speaker solution (or holding out for your new speaker shipment).
A little background: understanding speakers in parallel vs. speakers in series
Connecting speakers in parallel. There are two ways to connect multiple speakers: parallel or series. When you wire speakers in series, the total impedance is the sum of the impedances of all speakers in the series circuit. So, two 4 Ω speakers in series equals 8 Ω. One 4 Ω speaker and one 8 Ω speaker equals 12 Ω.
However, when you wire speakers in parallel, the total impedance is less than the impedance of the lowest-impedance speaker in the parallel circuit. You can calculate this with the formula z = 1 / [ (1/z₁) + (1\z₂) + (1/z₃) + … ], where z₁ is the impedance of the first speaker, z₂ is the impedance of the second speaker, etc., continuing for all the speakers in your parallel connection. So, two 4 Ω speakers in parallel equals 2 Ω. One 4 Ω speaker and one 8 Ω speaker in parallel equals 2.67 Ω.
An amplifier that has a lot of speakers — like this Fender Super Six — usually combines parallel and series wiring in order to achieve a reasonable impedance (here, 8 Ω speakers totaling 5.3 Ω impedance).
There is a shortcut for calculating speakers in parallel when every speaker has the same impedance: divide impedance in question by the number of speakers. Two 8 Ω speakers in parallel equals 4 Ω. Two 4 Ω speakers in parallel equals 2 Ω. Four 8 Ω speakers equals 2 Ω; four 4 Ω speakers equals 1 Ω. However, for speakers of difference impedances, you have to use the formula instead.
Identifying series and parallel circuits. In most amplifiers, extension speakers are wired in parallel. The tips of the jacks are both connected to the secondary of the output transformer, and the sleeves are both connected to ground. If the sleeve of the first jack was instead connected to the tip of the next jack, the speakers would be wired in series.
You can also see the difference in a schematic. In a series circuit, the current has no path to the next component without passing through the one before. If you remove any component in a series circuit, the circuit will be broken. (This is why one burnt-out Christmas light makes the rest of the string go out.) In a parallel circuit, current flow can reach both components at the same time. Removing a component does not interrupt the circuit.
In every Wurlitzer amplifier, the extension speaker is wired in parallel with the on-board speaker. If you want, you can even disconnect the onboard speaker and only use the extension speaker. However, you must make sure that there is always at least one speaker plugged in at all times. Operating the amp without a speaker load will damage the output transformer.
How the Speaker & Headphone Jacks Work in Wurlitzer Tube Amps
The extension speaker jacks and headphone jacks follow the same general wiring scheme in every Wurlitzer electronic piano tube amplifier. The specific wiring might vary, and it may travel through different pins in the wiring harness, but the principle is the same in every amplifier.
The onboard speaker jack. The onboard speaker is mounted at the back of the keyboard. Connections vary depending on the model. In the 145, it connects to the amplifier with an RCA plug. In the 700, it connects via the wiring harness. In the 112, it connects with a very old, obsolete rectangular plug. It looks hardwired, but you can actually plug and unplug it.
The Wurlitzer 112’s layout is unique in that all controls are mounted to the amp chassis, accessible from the side of the amplifier. Note that the aux output is tapped at the preamp circuit, so plugging into that jack does not affect the speaker impedance.
The extension speaker jack. The onboard speaker jack is wired in parallel with the extension speaker jack. This means that:
You can use both jacks simultaneously, or either jack on its own
If you plug speakers into both jacks, you will hear sound out of both
If you plug a speaker into both jacks, the total impedance will be less than the impedance of the lowest-impedance speaker
The headphone jack. The headphone jack on each model is a switching jack. When you plug into this jack, the switch cuts the onboard speaker. However, the extension speaker, which is wired in parallel with the headphone jack, is not cut and will still pass signal.
In addition to cutting the speaker, plugging into the headphone jack also engages a high-wattage 3 Ω or 4 Ω resistor (depending on the model). This resistor is the load resistor, and it ensures that the amplifier always has a reasonably acceptable load regardless of what device is connected to it. This is because tube amplifiers cannot run properly without a load, and doing so for any period of time can damage the power tubes and output transformer. The resistive load on the headphone jack doesn’t exactly model the impedance of the speaker, because it is missing the reactive element. However, it is close enough for the purposes of a headphone jack.
Connecting headphones to the headphone jack
Headphones have an impedance of at least 32 Ω. This impedance will appear in parallel with the load resistor. Because the load resistor is much smaller than the device resistance, the total impedance will be something close to the load resistor’s resistance. Headphones with a 32 Ω impedance, in parallel with a 3 Ω load resistor, result in a total impedance of 2.7 Ω. This is close enough to the 3.2 Ω output transformer primary. If you are risk-adverse (which is understandable here, since the headphones will be going on your head), you can bump up the load resistor to 4 Ω. In that case, 32 Ω headphones will give you a total resistance of 3.6 Ω, which is still a little off the mark, but in this case current flow is inhibited rather than increased.
You can also use the headphone jack to connect to another device, such as a mixer or a recording interface. If you do this, you should use a DI box between the headphone output and the device. Note that the headphone out will give you a very hot signal compared to an aux output (which, among tube Wurlitzers, only the 112 has), so you may need to attenuate the signal before it reaches your final device.
A DI box isolates the two circuits, minimizing the potential for ground loops and diminishing the possibility that unwanted signals could cross from one device to the other. It will also have a more predictable input impedance: usually 600 Ω, but as much as 1800 Ω. These impedances are so high that you don’t need to calculate the parallel impedance, because you can safely estimate that it will be equal to the load resistor. For instance, 600 Ω in parallel with 3 Ω is 2.985 Ω and 1800 Ω in parallel with 3 Ω is 2.995 Ω.
Connecting multiple headphones or devices to the Wurlitzer
The 112 service manual, on page 13, suggests that “a second set of earphones may be plugged into the jack marked ‘speaker.’” This advice is also found in the Wurlitzer 120 and 700 manuals. Note that connecting multiple devices in this way will show the amplifier three things in parallel: the 3 Ω load resistor, the impedance of whatever is in the headphone jack, and the impedance of whatever is in the extension speaker jack. (The onboard speaker jack is out of the equation because plugging into the headphones brings it out of circuit.) If you plug a pair of 32 Ω headphones into both jacks, the total impedance will be 2.6 Ω. Now we’re flying a little closer to the sun. Remember that lower impedances stress the amplifier and in this scenario the headphones will be attached to your head. I would personally skip this exercise.
You should also watch out if you have headphones plugged in at the same time as an extension speaker. If your headphones are 32 Ω and your extension speaker is 16 Ω, the total impedance will be 2.3 Ω. if your extension speaker is 4 Ω, the impedance is 1.6 Ω.
We often use this Gallien Krueger 4x10 with our 1960s Wurlitzers.
The Wurlitzer manual does not give good advice on connecting an external speaker. Page 13 of the Wurlitzer 112 service manual also advises that “any external low impedance speaker (3 to 8 ohms) may be plugged into the jack marked ‘speaker.’” The Wurlitzer 120 and 700 manuals have similar language. Recall that both the 112 and the 120 manuals have a 3.2 Ω speaker tap. Assuming that they are using a 4 Ω speaker, connecting a 3 Ω speaker results in 1.7 Ω total impedance. We do not recommend running the amplifier with a speaker impedance this low. Connecting an 8 Ω speaker gives you 2.67 Ω, which is better but still relatively stressful for the amplifier.
However, a 16 Ω external speaker combined with a 4 Ω onboard speaker results in a total impedance of 3.2 Ω. This is exactly what the output transformer wants to see, according to the schematic. There are alternative, equally suitable speaker configurations — for instance, an 8 Ω extension speaker and an 8 Ω onboard speaker results in 4 Ω, which is basically equivalent. However, the fact that 16 Ω and 4 Ω in parallel equals 3.2 Ω begins to explain the strangely specific output impedance Wurlitzer listed int he schematic.
There is evidence that Wurlitzer did not love mounting the speaker in the back of the amplifier. One patent depicts a keyboard that appears to be a precursor to the Wurlitzer 120. Here, the speaker is mounted in front of the action assembly, facing up. This is definitely better for both convenience and sound quality, since here the audio is first of all closer to your ears, and you can also position the keyboard close to a wall with a clean conscience.
However, a speaker in this position would have made it more complicated to remove the lid, and, as we know, Wurlitzer continued to mount the speaker at the back of the keyboard. Was the 3.2 Ω transformer impedance intended to encourage users to connect a 16 Ω speaker to the amplifier? Was it because Wurlitzer bought a truck full of 3.2 Ω output transformers at a cut-rate price? We will probably never know.
Still, it is a fact that Wurlitzers sound better with a larger, round speaker. We usually use a 12” speaker, and the fact that the 700 and 720 already have one built-in makes these models criminally underrated in our book. If you want to connect an extension speaker to your keyboard most of the time, any combination that brings you close to 3.2 Ω will make your amplifier happy.
Further Reading
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