How is a Wurlitzer tube amp different from a guitar amp?
A Wurlitzer 112A tube amplifier (which, incidentally, was found in a Wurlitzer 112).
Wurlitzer tube amps[1] are in fact very similar to guitar amps.
But before we talk about that, let’s back up a little. The Wurlitzer itself is very analogous to an electric guitar: the heart of both instruments is a pickup that converts vibration into an electrical signal that is ultimately sent to an amplifier. In a guitar, the amplifier is almost always external, but a Wurlitzer’s amplifier is tucked into the body of the instrument.
You can clearly see the pickup in this 200-series Wurlitzer, since the dampers have been removed. Note that this Wurlitzer has a reed bar shield installed over the pickup, which reduces noise.
Why? First of all, Wurlitzer was a piano company, and the on-board amp contributed to the illusion that the Wurlitzer is a piano. (The Hammond organ, invented in 1935, provided precedent for this concept.) Like any acoustic instrument, the traditional pianos that Wurlitzer built were self-contained. Every component required to make sound is (of course) already present within the case, no accessories required. And, unlike a guitar, a piano isn’t meant to be handheld, giving Wurlitzer more real estate inside the body to hide electronics. So Wurlitzer’s electronic piano was also designed to be ready-to-play upon delivery, its amplifier present within so that the sound came from the keyboard and not some external unit.
Another point: Wurlitzer didn’t make amplifiers.[2] And because the Wurlitzer’s pickup requires voltage to function, third-party amps would not be compatible with the electronic pianos. There’s no indication that Wurlitzer wanted to make cabinets or otherwise get into the amp-making business. Plus, the a major selling point of the Wurlitzer was its portability. Requiring an extra 20-lb amplifier would undermine any ads touting the keyboard’s convenience.
So, a Wurlitzer is different from an electric guitar in two ways. First, thanks to the physical nature of a piano, Wurlitzer had the opportunity and the motivation to conceal the amplifier inside the instrument. Second, the Wurlitzer’s electrostatic pickup made it incompatible with contemporary instrument amplifiers, which assumed that any input signal originated in a magnetic pickup.
So, although the Wurlitzer amplifier is very similar to a guitar amp, its design was informed by a completely different perspective on the musical instrument market. With minor modification, Wurlitzer amps and guitar amps are pretty interchangeable. The differences are mostly a matter of style and philosophy rather than function.
Differences between a wurlitzer amp and a guitar amp
Wurlitzer amplifiers send voltage to the input. A Wurlitzer’s electrostatic pickup requires voltage to function. The pickup itself is a capacitor composed of a vibrating metal plate (the reed) which is separated from a stationary piece of metal (the reed bar) via an air gap. When a voltage is applied to the plates, the capacitance of the pickup varies based on the reed’s vibrations, inducing a signal current.
The Wurlitzer’s relatively high reed bar voltage (180v) is meant to mitigate distortion in the keyboard’s lower register. That distortion would be caused by the increased size of the lower reeds, which must vibrate over a larger area.
In contrast, a guitar amp does not send voltage to the input, because guitars use magnetic pickups. Magnetic pickups create a signal based on the vibrations in the pickup’s magnetic field which are induced by the vibrations of the guitar strings - no voltage required.
Wurlitzer amplifiers have a minimal, low-gain, cost-saving circuit. Note that “cost-saving” doesn’t necessarily equal “cheap” here. The 50s and 60s produced some truly awful electronics, but Wurlitzer amps are well-made and have decent components. This isn’t one of those noisy single-ended amps that came in a fiberboard box and might literally electrocute you when you plug it in. Wurlitzer accomplished what was necessary in the circuit - and not a single thing more. That’s fine: it’s cutting costs without cutting corners.
It was understandable that Wurlitzer wanted to cut costs in the amplifier. Producing the mechanical portion of the keyboard itself was expensive, and Wurlitzer wanted to market it to budget-conscious musicians: beginners, amateurs, people who played dive bars or didn’t have the funds to lug a piano around on tour. Going all-out on the amplifier would price those people out. Remember that, at the time, the electronic piano was a gimmick. If you were a serious player and had the resources, you’d probably skip the Wurlitzer and buy a nice acoustic piano instead. (Unless you were a trend-setter like Ray Charles. Also, gear acquisition syndrome was definitely a thing in the 60s too.)
To this end, Wurlitzer used the least amount of tubes possible for their circuits. To have a functional push-pull amp, you need two things: at least one gain stage to amplify the circuit and a phase inverter to split that signal into two versions, one for each of the two power tubes. A Fender amp, for instance, would accomplish this with a minimum of two preamp tubes: one wires for two gain-stages, and one for a relatively high-gain, two-triode, long-tailed pair phase inverter.
However, tubes add to the size and cost of the amp. Plus, Wurlitzer wasn’t interested in gain: they prioritized linearity, and they were only driving a single 6x9” oval speaker anyway. The amp also had to fit into a small cavity in the back of the keyboard.
We built a new 145 amplifier for this Wurlitzer. We build the amp bigger than the original, taking up all of the available space, but the amp cavity is so small that we really only get a few extra inches.
So both the Wurlitzer 120 and 145 use just one preamp tube,[3] which is wired to provide just one gain stage and a one-triode phase inverter.[4] This is an exceptionally minimal tube preamp circuit. A guitar amp designer that wanted to cut down on tubes might have gone the single-ended route instead, eliminating one expensive power tube as well as the need for a phase inverter. But Wurlitzer wanted the clean linearity of a push-pull amp, so they could only cut back on preamp tubes.
The Wurlitzer 112 is an exception. This is probably just because the 112 is so early that its designers were more interested in simply getting the keyboard into the market. Bringing costs down would come later.
The 112 has two preamp tubes, which are wired to provide a total of two gain stages, a phase inverter, and an alternative phono input. (Think two-channel guitar amp.) However, both preamp tubes are 12AU7s, which helps keep the circuit’s overall gain low. Because of the extra tube, the 112 can be modded in multiple ways. (We like to install an fx loop so that the 112 can be played with reverb and tremolo.)
The tube rectifier from a Wurlitzer 112 amplifier.
Wurlitzer amplifiers are built for clean tone. A lot of guitar amps at the time were built for clean tone, too, but Wurlitzer took the concept further by borrowing elements from hi-fi amps. Wurlitzers don’t sound exactly like acoustic pianos - how could they, with no strings? - but contemporary marketing materials never mentioned a tube Wurlitzer’s unique tone. Instead, Wurlitzers were basically marketed as portable pianos.[5] True to that characterization, Wurlitzer designed the electronic piano’s tube amp to have a very clean, transparent tone.
To achieve as much linearity as possible, Wurlitzer used several strategies. We already discussed the minimal, low-gain circuit that all tube Wurlitzers are outfitted with. However, because their circuits were so minimal, there were limits as to how linear they could truly be. With the exception of the 112, Wurlitzer often biased their preamp stages aggressively, in order to achieve a reasonable amount of gain from very few tube stages.
The output tubes are another characteristic clean-tone strategy that Wurlitzer used. In all amplifiers, they are wired in a push-pull (class AB) configuration. This is less noisy and more efficient than a single-ended design. Designed well, a push-pull amp can provide ample output power with a minimum of distortion. The 112 and 120 use 6V6 power tubes, which have a relatively low wattage output but distort easily. For the 145, Wurlitzer switched to a pair of 7868s, which are less prone to clipping and capable of greater output.
This obviously contrasts with a guitar amp, where some amount of distortion is desirable. However, it is sometimes possible to modify a Wurlitzer in order to achieve some distortion. For instance, the preamp in a 112 can easily be biased hotter, and the 12AU7 tubes can be swapped for 12AX7s. Or, you could install a replacement tube amplifier containing a master volume and an extra triode for ease-of-overdrive.
Summary. On the whole, a Wurlitzer is pretty similar to a guitar amp. A Wurlitzer tube amp has the same topology as a typical guitar amp, but in many cases Wurlitzer amps are biased for low-output, clean operation. The most significant difference between a Wurlitzer amp and a guitar amp is that a Wurlitzer’s electrostatic pickup requires the amp to send 180v to the input, while a guitar amp does not.
How do I turn a Wurlitzer amplifier into a guitar amp?
Sometimes, for whatever reason, Wurlitzers amps come up for sale without their accompanying keyboard. The amp is definitely more valuable in the keyboard than as a guitar amp, so we wouldn’t suggest removing one from a working Wurlitzer. But, as an illustration, here’s what you’d have to do to make it a functional guitar amp:
The input circuit of a Wurlitzer 700 tube amp.
Note that all tube amplifiers carry high voltages that may persist even when the amplifier is switched off and unplugged. Do not proceed with these steps without following the appropriate safety protocols.
Remove the voltage from the input. This is very important. Wurlitzers use 180v of polarizing voltage at the input. There is a current limiting resistor but you still don’t want that voltage anywhere near your guitar. Cut that resistor completely out of circuit in order to remove the power supply’s connection to the input jack. You should also use a multimeter to confirm that there is no voltage at the input jack when the amplifier is on.
Revise the input circuit so that it is more appropriate for a guitar signal. There is always a coupling capacitor at the input jack. It is is intended to prevent the high-voltage DC from reaching the grid of the input triode. We removed the DC voltage in the last step, so we don’t need the coupling cap anymore. The other thing about the input circuit is that it may have some oddball resistors. For instance, the Wurlitzer 145 amp uses a 1M grid stopper resistor. This is unnecessarily high for a guitar input. Resistors generate noise, and, the higher their resistance, the more noise they generate. Grid stoppers are usually between 22k and 68k ohms, so pick one of those and mount it as close to the socket pin as possible
Mount the amplifier in a cabinet. Now you have a guitar amp! And since we barely modified the amp at all, you can always reverse your mods and put it back in a Wurlitzer later.
One final complication: the input jack of a Wurlitzer is an RCA. You can of course connect your guitar using a 1/4”-to-RCA cable. Or, if your Wurlitzer amp originally had a 1/4” phono jack, you can wire that as your guitar input instead. Make sure you cut everything off the RCA input anyway, or it could introduce noise (not to mention that 180v we mentioned earlier) into the circuit.
Isn’t a Wurlitzer amp too clean to perform adequately as a guitar amp? Not necessarily. We recently plugged a guitar into a Wurlitzer 120’s phono input (after confirming that there is no leakage voltage across it), and it sounded great, even though the 120 has half the gain stages as most comparable guitar amps. You won’t hear overdrive through the amp, but for clean tones a Wurlitzer amp is definitely an interesting sound.
Don’t have a Wurlitzer amp to mod? Consider these guitar amps, all of which have similar circuits to Wurlitzers.
Fender 5E3 Deluxe. Some people describe the Wurlitzer 112 as 5E3-like, but where the 112 installed a phono input, the 5E3 opted for its famous Normal channel instead.
Fender 5E9 Tremolux. Ever play a 5E9 and think, “Two whole preamp tubes? Does Fender think I’m made of tone??” If so, you’d love the Wurlitzer 120, which was basically the 5E9’s phase inverter and nothing else.
Vesper Tremolo. We love the 145 so much that we were inspired to make a guitar amp using 7868s. It has the same clarity as a Wurlitzer amp, but with more gain. Check it out here.
[1] We are focusing on Wurlitzer 112/120/145/700/720 tube amps here, just because it’s more common to think about modding a tube amp for a guitar input. Although a lot of the discussion above applies to the solid state models of Wurlitzer as well, we’ll focus on tube amps since that’s the kind most guitarists ask about.
[2] The briefly-produced Model 920 amplifier is an exception to this rule. It was intended as an auxiliary amp for the Wurlitzer 120.
[3] The 145 also has a one-triode tremolo circuit, further underscoring the amp’s tube minimalism. Some guitar amps use a two-triode tremolo, but clearly the one-triode version fit better in the 145’s cabinet. Despite the addition of tremolo, the Wurlitzer still uses the same number of preamp tubes as the 120 (that would be one). This is thanks to the release of Compactron tubes in the 1960s. Compactron tubes were GE’s attempt at creating a tube that was small and convenient enough to compete with transistors. They had up to three triodes in one envelope, and never exactly took off.
[4] The 120 has a paraphase inverter, so the gain stage is technically part of the phase inverter. The 145 has a gain stage followed by a cathodyne phase inverter.
[5] Consider one advertisement from 1971: “Plays like any piano, but because it is electronic the volume is variable. All you have to do for quiet operation is turn down the volume…or for brilliant high power performance, turn the volume up.” That’s a lot of words for “has a volume knob.”
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