On the Wurlitzer 120/700 Amplifier
This vintage tube amp was removed from a Wurlitzer 700 electronic piano. It’s is the same amplifier as a Wurlitzer 120…unfortunately. The 120 had very limited cabinet space, so that’s why the physical design of the amp is so cramped. On the other hand, the 700 had a cavernous cabinet that could definitely have handled a larger amp chassis. Oh well: that ship has obviously sailed.
We’ll probably re-cap this amp before we re-install it in the keyboard. But since the cabinet on this keyboard is enormous, we might opt to replace it with a new amp instead. Maybe with spring reverb? Unlike most Wurlitzer models, the 700 is not limited by cabinet space.
About the 700. The 700 is the console version of the Wurlitzer 120. It resembles a very small spinet piano, although it's much lighter since the cabinet is mostly empty. We’ve seen it in light oak and dark mahogany. Are there other finishes as well? We wouldn't rule it out.
This amplifier is all-tube and, like the 112, contains no onboard vibrato. Unlike the 112, it has just one preamp tube and a very small chassis, which makes it less suitable for mods. It definitely has plenty of tube warmth, as well as that natural Wurlitzer shimmer. Don't forget that the Wurlitzer 120 and 700 have a specific style of reed, which contributes to their unique tone.
Features of this amplifier
For a more technical discussion of the circuit, check out our article here.
Late 1950s or early 1960s manufacture. This is a later 700 amplifier. You can tell because of the presence of a cathode bypass capacitor on the input triode. Also, the input circuit is absent from this chassis, because it is mounted on the reed bar. (Supposedly this reduced noise and “frying” sounds.) The cathode bypass capacitor is intended to reduce the amount of noise that can leak into the cathode via the heater filament.
Point-to-point wiring. Like most amps of the period, the 700 amp has point-to-point wiring. Point-to-point wiring can sometimes look messy, and it can be difficult to achieve a layout that avoids hum and oscillation. However, when done correctly, it is possible to achieve very short and direct leads, which contributes to a clean signal.
This Wurlitzer amp is a really nice example of point-to-point wiring. There is definitely a logical arrangement to the components. In the preamp, components from each half of the 12AX7 are physically separate, which avoids noise and cross-talk. Each pin of the 12AX7 has its component mounted directly to it with short and more or less straight leads. Some details could be improved: heater and transformer wires aren't twisted, for instance, but they never are in Wurlitzer amps. The heater is also a somewhat more noisy non-center-tapped design. If this is a concern, it is relatively easy to re-wire the heaters and add a center-tap, since the chassis is partially open.
Partially open chassis. Which brings us to our next point: the shape of the chassis. In order to fit in a Wurlitzer 120, the amp is long and narrow. However, it is not made of one continuous piece, like later Wurlitzer amps. Instead, it is composed of two L-shaped pieces, each with one deep wall opposite a shallow lip. Both pieces fit together to form a completely enclosed box. When screwed together, the chassis is completely enclosed, which is not only good for shielding but prevents dust and debris from getting inside. When apart, the amp has one long, open side, which is convenient for making repairs.
Octal socket connecting the wiring harness. This is a nice vintage detail. Fancy octal plugs like this were eventually superseded by plastic Molex connectors, so you won't find any plugs like this in later gear (unless it's completely custom). Molex connectors take up less space, but, since the amp is point-to-point wired, there is an advantage to the octal socket. It has large pins, so it's possible to connect components directly to them. You can see in the photos that some components are mounted from the octal socket directly to the tube socket, eliminating the need for additional wiring. You can't mount components directly to a Molex connector, so you'd have to run wires from each Molex pin to some intermediate point, like a terminal strip.
The first Wurlitzers to use Molex connectors were 140-series solid state keyboards, which used space-saving printed circuit boards and had plenty of room to run wires to and fro. In contrast, every square inch of space in the Wurlitzer 120 is spoken for. The ability to mount components to the wiring harness socket is very important to its clean layout.
Vintage components. All low-wattage resistors in this amp are carbon composition: unsurprising, since this was the only style of vintage resistor available at the time. (Note the high-wattage one connected across the power tube sockets. Why do we have a sudden craving for tootsie rolls?) Carbon composition resistors can produce crackling or fizzing sounds as they deteriorate. Even when working correctly, they produce hiss. This is actually the sound of the carbon particles becoming agitated and colliding into one another.
If we needed to replace these carbon composition resistors, we have two options. The first is to install new carbon composition resistors, which are still being made despite (or because of?) their poor and very distinctive noise characteristics. Alternatively, we could install low-noise metal film resistors. Or, we could install a combination: low-noise in sensitive areas, and carbon composition to provide subtle vintage character.
Moving on to the capacitors. The Astron Minimite capacitor is actually relatively desirable, because it are often found in vintage Fender amps. Electrolytic capacitors tend to go bad whether they are used or NOS, so even people that are attempting to restore an amp to perfect period-correct condition won’t re-cap it using vintage electrolytics. Instead, they will remove the cardboard tube from a vintage capacitor and use it to disguise the replacement new-production capacitor.
Note how long the leads are in both electrolytics. The Minimite spans half the length of the chassis. The positive lead on the smaller cathode capacitor is long enough to wrap around the input and arc back towards the tube pin. These capacitors are part of the reason why Wurlitzer could get away with such a cramped chassis in the first place: the capacitors could be mounted in any convenient spot, because their leads are long enough to reach the appropriate pin. Today, it is all but impossible to find capacitors with leads like these, because components are built for circuit board mounting, not point-to-point wiring.
Also, have you noticed the candy apple red ceramic capacitors? We’re not going to pretend that glossy red coatings do anything for tone. (Or do they?!) But it certainly looks great.
The ground tab for the power transformer center tap. The power transformer’s center tap must be grounded. There are an infinite number of things that could be mounted in order to create a secure ground. Or, you could make a convenient little cutaway in the chassis, bend it up, and wrap the wire around the resulting tab. This is definitely one of the best details in the amplifier.
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