140-series Wurlitzers
Pros:
Still have a very cool midcentury design
Both playability and serviceability is comparable to the 200-series
Extremely unique vintage amplifiers
720 console models arguably represent the pinnacle of electronic piano mechanical action
Cons:
140/145/720 (no "A" or "B" designation) have unobtainable reeds
Somewhat heavy and not as portable as 200-series models
Tube amplifier models have extremely idiosyncratic electronics
The earliest solid state models require careful repair and some modification for full functionality
The 140-series Wurlitzers represented a major redesign from the Wurlitzer 120. Their action is more sophisticated than earlier models, and their electronics is a lot more modern. Having a 140-series Wurlitzer is like having a little piece of history. But 140 Wurlitzers shouldn't be considered curiosities best-suited for display and dust-collecting. They also highly-functional, exceptional-sounding analog instruments that can still perform very well in professional environments.
A very 200-like mechanical action
140 series keyboards have a highly playable action that is similar to the Wurlitzer 200. If the instrument has been regulated, and worn-out felt replaced with new felt, it plays just as well as a 200. Some models, like the 720, arguably play even better.
An important note about the 140, 145, and 720: these models have a unique reed type which is not interchangeable with later 200-series models. The upside is that these early 140-series keyboards have an extremely unique sound. The downside is that reeds are scarce. If you are interested in having 140 reeds manufactured, let us know here.
The 140 amplifier and its germanium transistors
The Wurlitzer 140 is the first of Wurlitzer's electronic pianos to contain a solid state amplifier. This is a very early solid state circuit, and it has a lot of idiosyncrasies.
Selenium rectifier. Instead of silicon or germanium diodes, the 140 has a selenium rectifier. This is a type of semiconductor, but it works differently than a diode, and it is composed of a series of metal fins like a radiator. Selenium rectifiers fail with age, so they should be replaced with silicon diodes whenever encountered.
Unregulated power supply. The amplifier does not have a regulator circuit, probably because an regulated power supply adds cost and complexity to an amplifier. Instead, it uses cascaded filters to drop the DC bias voltage for the preamp transistors. In an unregulated circuit, supply voltages will fluctuate as the amplifier draws more and less current, changing the bias of transistor stages. The bias affects the sound of the amplifier, particularly its overdrive behavior. It is relatively unusual for a solid state amplifier to be entirely unregulated, and it contributes to the unique tone of the 140.
Germanium preamp and power transistors. Every transistor in the 140 is germanium. This contributes to the amplifier's unique sound. However, germanium transistors are unreliable and their characteristics can change depending on the ambient temperature. They are also somewhat fragile. Early 140 amplifiers require an additional resistor in series with the input to prevent incidental reed bar shorts from destroying the input transistor stage. (Later 140 amplifiers have this modification.)
Varistor tremolo. Tremolo in the 140 is powered by two varistors, a semiconductor device whose resistance changes with a changing voltage. The semiconductor material is very fragile and prone to crumbling. If the varistor acquires more than a small chip, it won't work anymore. New-production varistors have a different composition, and can't be used in the tremolo circuit.
Driver transformer. In the 140 amplifier, a driver transformer inside the chassis provides phase inversion and impedance matching. Transformers are not perfectly linear, and they affect the frequency response and harmonic distortion of the amplifier. However, in the 1960s, transformers were an economic way to provide both phase inversion and enough current gain to drive a power amplifier. Transistors were relatively weak, and designing a transistor driver that provided gain and phase inversion would have added significant cost and complexity to the amplifier. The driver transformer is certainly one of the reasons why the 140 sounds so cool.
The 145 amplifier
The 145 amplifier was the tube version of the 140. Like the 140, it integrated one of the technological advancements of the early 1960s—the compactron tube. As semiconductors miniaturized electronics, the compactron tube attempted to reduce bulk and heat in circuits in a different way: by stuffing more triodes into the same glass envelope. Although semiconductors obviously continued being developed, compactron tubes were only manufactured for a few years before being discontinued.
The 145 amplifier uses a 6K11 compactron tube, which has two triodes that are similar to high-gain 12AX7 triodes, and one triode that is similar to a 12AU7. This allows the 145 to have onboard tremolo without expanding the size of the chassis too much. Tremolo sounds excellent with 140-series Wurlitzers, so it's great that Wurlitzer added this feature. Unfortunately, the compactron topology allowed the Wurlitzer to fit all of these things in a very small amplifier. So, like the 120 amplifier, the 145 has limited modification potential.
A tube with three triodes in one envelope is a great idea, but its execution left a little to be desired. For instance, one of the heater pins is adjacent to a grid pin. For that reason, a center-tapped heater circuit cannot be used with the 6K11, because heater noise will couple into the grid circuit via the adjacent pin. The heater pin adjacent to the grid pin must be grounded. However, a single-sided grounded heater generally has more potential for noise than a center-tapped heater. A rebuilt 145 amplifier can be very quiet, but limitations such as the 6K11 tube will leave a bit of a noise floor even in a fully rebuilt circuit.
The 7868 power tubes provide another source of noise. The 7868 is a very cool-sounding tube that is capable of highly linear operation and a lot of headroom. However, many aging 7868s have an audible noise floor. Buying several sets of vintage 7868s will probably yield a pair that is pretty quiet; however, the cost of vintage 7868s is prohibitive. And although new 7868s are available, installing them in a vintage 145 tube socket will stretch the pin holes, and old 7868 tubes can never be used again. We have also found that new-production 7868s are not very reliable. The pins do not seem to be aligned properly, requiring a lot of force to insert them into the tube socket; this appears to compromise the glass-to-metal seal. Failure of the tube's vacuum typically appears within weeks; however, we do have new-production 7868s
The issue can be avoided entirely by replacing the 7868s with 7591s. The tubes are essentially the same, although the 7591 uses a different tube socket. In an amplifier rebuild, we recommend changing tube sockets as a matter of routine, in order to facilitate new heater wiring. Re-routing wires in general helps reduce the amplifier's noise floor and makes it more playable.
140-series "A" variants — mechanical modification
The 140A, 145A and 720A represent a revision of the mechanical action. From these models on, all reeds are interchangeable; this means that 200-style reeds can be used with both "A" and "B" type 140 keyboards.
The amplifiers, however, remained the same. So, the 140A will have a 140 amplifier, while the 145A and 720A will have a 145 amplifier.
Wurlitzer 140B
Like the Wurlitzer 140A, the 140B has a solid state amplifier. It is extremely similar to the 140A, containing driver transistors, germanium output transistors, and an unregulated power supply. The topology is very similar as well, and the physical layout is nearly identical. However, the 140B has a few crucial updates that make this amplifier highly usable in studios today.
Silicon rectifier. The 140B dispenses with the selenium rectifier in favor of silicon diodes.
Silicon preamp transistors. The 140B's silicon preamp transistors are very reliable—by the standards of 60+ year old electronics, at least. Sometimes, 140B small-signal transistors are noisy or quiet, and would benefit from being replaced. But it is easy to do so, and the circuit doesn't need to be re-biased to accommodate them (as a germanium transistor circuit would). The output transistors are still germanium—a cool detail that contributes to their sound.
Optocoupler tremolo. Instead of crumbling varistors, the 140B uses an optocoupler in the tremolo circuit. Whereas most optocouplers consist of a light-dependent resistor and a blinking light fused together with heat shrink, the 140B mounts its components inside of a little metal box on the PCB. It's a stylish little detail that we love.
140-series console models are extremely deluxe
Like the Wurlitzer 700, the Wurlitzer 720 is a console keyboard. However, the 720 is heavier and just generally has a more solid presence in a room. It has a similar action as its 140-series counterparts, so it feels excellent. Additionally, the keys are longer, and they have a sort of snap when you play them.
The 720 and 720A contain a 145 tube amplifier, while the 720B has a 140B solid state amplifier. In each case, the amplifier is mounted in the base of the console, as in a 700. Since all of these amplifiers are sized to also fit in a portable model, the 720 has plenty of space for a large customized amplifier.
The 140B amplifier sounds excellent in a 720B. The tube amplifier sounds great too, but the noise floor and lack of direct outputs are major obstacles to recording. Rebuilding the 145 amplifier provides a big improvement. However, since we replace all wiring and passive components when rebuilding the amplifier, building a custom amplifier yields all of the benefits of rebuilding the original, plus the potential for more features.
About the 720's oval speaker
Wurlitzer evidently realized that its customers were being spoiled with the 12" speaker, and switched to a 6x9" oval speaker for the 720 keyboards. It still sounds very cool, however, and is a fitting parallel to the portable models, which have their 6x9" speakers mounted in the back of the cabinet.
The only problem with recording the 720 is the fact mic'ing the 6x9" speaker will capture any mechanical noise originating in the sustain pedal. (This is also a problem with the 700.) The issue is easily solved by connecting an extension speaker, which can be positioned anywhere in the room. All Wurlitzer amplifiers have the capability for an extension speaker, and a great speaker provides both recording flexibility and an improved frequency response versus the onboard speaker.
See the 200-series for a discussion of the Wurlitzer 146
[1] Technically, you could store a pedal in the 920 tone cabinet, which was sold as an accessory to 112 and 120 electronic pianos. However, the 920 tone cabinets are also very rare; they seem to have gone to the same place that all the sustain pedals disappeared to.
[2] We are not aware of any surviving examples of the Wurlitzer 830 Monitor, but there is a manual with more information in the Archive.