What components should I replace in my vintage Wurlitzer amp, and why?

The main circuit board of a Wurlitzer 200a electronic piano.

Tropical Fish now offers several kits containing replacement components for all Wurlitzer amplifiers. If you already have experience working with electronics, these kits offer a convenient DIY option for restoring your amp. Each kit contains the same high-quality components that we use in our own restorations.

For all amplifiers, we offer the Basic kit as well as several add-on components packages. The Basic kit contains all electrolytic capacitors, power supply resistors, and a few input components for the preamp. For many amplifiers, replacing these components is sufficient for improved reliability and performance.

However, you may want to replace components other than these basic components. There are a few reasons why:

  • The amplifier has noise or distortion that persists even after the electrolytics and high-wattage resistors were replaced[1]

  • The amplifier contains obsolete components that can be replaced with better-quality new components

  • The amplifier contains components that are shorted, open, or physically damaged

In this guide, we’ll talk about the different categories of components in an amplifier, and some signs that they should be replaced.


Note that Wurlitzer amplifiers contain high voltages that can persist even when the amp is turned off and unplugged. Do not attempt to service your amplifier without following the appropriate safety protocol. if you are unsure how to repair an amplifier or otherwise install the components in these kits, contact us and we can install them for you.


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Types of Components in Vintage Amplifiers

ELECTROLYTIC CAPACITORS

Note the Astron Minimite electrolytic capacitor in this original Wurlitzer 700 amplifier. The leads are very long, which is convenient for mounting in a point-to-point circuit like this one. On the right, there is a 7 watt wirewound resistor as well as a high-wattage (probably 2 watts) carbon composition resistor.

What are electrolytic capacitors? In general, a capacitor is composed of two conductive plates separated by an insulator. The insulator is called the dielectric. Electrolytic capacitors are special because they have polarity: that is, a positive plate (the anode) and a negative plate (the cathode). The positive plate is typically made from anodized aluminum. Anodizing beefs up the natural oxides on the surface of the aluminum until it is covered with a continuous layer of aluminum oxide. Because this oxide layer is an insulator, it can function as the capacitor’s dielectric. The oxide layer is then covered with an electrolyte, which is often made of paper soaked in conductive gel.[2] This forms the capacitor’s cathode.

Electrolytic capacitors can have large capacitance values because the dielectric is very thin, and the anode has a comparatively large surface area. Unfortunately, the unique composition of electrolytics also has its downsides. The electrolyte can evaporate over time, decreasing the capacitor’s capacitance. Depending on the quality of manufacture, the electrolyte may even start physically leaking out of the capacitor. And, if the capacitor is dormant for long periods of time, the oxide layer can corrode, creating leakage currents.

Because of their high capacitance values, electrolytic capacitors are typically used as filter capacitors in the amp’s power supply. They are also commonly used as cathode bypass capacitors in tube amps, or emitter bypass caps in solid state amps. Electrolytics are also used as coupling capacitors where relatively high capacitance values are required.

 

These electrolytic capacitors, original to this Wurlitzer 200 amplifier, are physically bulging and leaking.

 

When should electrolytic capacitors be replaced? Because electrolytic capacitors are so prone to aging, it is a safe bet that all electrolytics in a vintage amp are ready to be replaced. Depending on their location in a circuit, deteriorated electrolytics can cause hum, poor frequency response, or otherwise reduced performance. Physically leaking electrolytics can furthermore damage other components in a circuit.

Note that, although new electrolytics can improve the noise floor of your amplifier, they are not a magic bullet. There are many factors that can introduce hum into a vintage circuit. But even if the electrolytics are not the primary cause of your Wurlitzer’s hum, it is always worthwhile to replace them for the overall health and longevity of your amplifier.

HIGH-WATTAGE RESISTORS

What are high-wattage resistors? Resistor wattage - also known as the resistor’s power rating - is a measure of how much energy a resistor can dissipate in the form of heat. The power dissipated in a resistor can be calculated by multiplying the voltage across the resistor by the current through it.

This 5-watt, 0.33 ohm resistor, found in a Wurlitzer 140a amplifier, is part of the output transistor’s biasing circuit.

High-wattage resistors are typically found in the output and power supply sections of the amplifier. Power supply resistors have a role in filtering the amplifier’s B+ voltage, and also help decouple one preamp stage from the next. Bias resistors set the operating point of the output tubes or transistors, and typically require higher power ratings in order to handle the large currents that pass through them.

When should high-wattage resistors be replaced? Most amplifiers use 1/2 watt resistors by default, even in areas where the resistor won't be dissipating anywhere near that much power. This is particularly beneficial in the preamp, because oversizing the power rating of the resistor helps reduce noise. Therefore, resistors in the preamp may be 1/2 watt for a variety of reasons that have nothing to do with actual heat dissipation, such as noise, convenience, or voltage.

On the other hand, if the circuit designer went through the trouble and expense of installing a large high-wattage resistor, you can pretty safely conclude that there was a reason behind that decision. And that reason is almost certainly because that resistor will be dissipating a lot of heat. Basically, a high wattage rating is a red flag that says, “This resistor is under a lot of stress.”

Over time, hot, high-current conditions can cause the values of a resistor to drift, which means that they’re not functioning as designed. Deteriorated resistors can also introduce noise into circuits, even if their value has not drifted. This is particularly true for carbon composition resistors. We recommend replacing power supply resistors at the same time as the electrolytic capacitors.

INPUT CIRCUIT COMPONENTS

What are the input circuit components? The input circuit varies from amp to amp. Wurlitzer electronic pianos always have a coupling capacitor between the input and the grid of the input triode (or the base of T1). This prevents DC voltage from the reed bar power supply from reaching the triode or the transistor. Depending on the Wurlitzer model, there may also be a grid leak resistor, a grid stopper resistor, or both.

 

The input components in the Wurlitzer 200a amplifier are on a separate circuit board that is mounted above the reed bar. Note that, unlike previous models of resistors, the 200a has carbon film resistors. These are less noisy than carbon composition resistors, but not as low-noise as metal film.

 

When should input components be replaced? Components at an amplifier’s input circuit aren’t necessarily more likely to deteriorate than components elsewhere in the circuit. However, because the input is so sensitive, deteriorated input components will have the biggest negative impact on the amplifier. Any noise or poor frequency characteristics at the input will be amplified by the entire circuit.

For that reason, we prefer to replace the input components in many amplifiers that we repair. We have included them in the Basic kit so that you have that option as well.

COUPLING CAPACITORS

What are coupling capacitors? In this case, “coupling” describes the capacitor’s function. Coupling capacitors are placed anywhere two parts of an amp must be connected in a way that AC signals can pass, but DC signals cannot. (Basically, capacitors behave like an open circuit where DC signals are concerned. However, AC signals can pass right though.)

Any style of capacitor can be a coupling capacitor. In solid state Wurlitzers, the majority of coupling capacitors are electrolytics. In early tube Wurlitzers, coupling caps are typically ceramic. Ceramic capacitors aren’t ideal in audio applications, because their characteristics can drift depending on the circuit conditions. However, ceramics were the only readily-available small-value capacitor when Wurlitzers were manufactured.

This Wurlitzer 140b mostly has electrolytic coupling capacitors. Tube Wurlitzers are more likely to have ceramic coupling caps.

When should coupling capacitors be replaced? Coupling capacitors aren’t indestructible: they can fail like any other component. However, because they age a lot better than electrolytics, they are often perfectly functional and can last indefinitely. The question of whether to replace working coupling caps is up to your personal preference.

Because coupling capacitors are in the signal path, they have a huge influence on the amplifier’s tone. New-production capacitors are less noisy and more reliable than even the best vintage capacitors. However, some people appreciate the lo-fi randomness that vintage coupling caps offer.

This is particularly true when the original coupling caps are ceramic, which are not ideal for audio due to their non-linear frequency response. Ceramic capacitors are also prone to microphonics. However, they were the only real option during the manufacture of Wurlitzer electronic pianos.

Replace your coupling caps if you prefer the amp to have a more predictable frequency response with less vintage coloration. The high-quality polypropylene coupling caps that we include in our kits are robust and well-suited for audio. However, if you like the tone of the amp as-is, leave the original coupling caps.

Note that you must always replace shorted or otherwise dead coupling capacitors.

TREMOLO COMPONENTS

What are tremolo components? The heart of Wurlitzer tremolo circuits is an oscillator. For a circuit to oscillate, it requires both gain and a phase shift. In the Wurlitzer 145, gain is provided by an aggressively-biased triode. In all other tremolo models, the tremolo circuit is transistor-based. The phase shift is provided by a series of resistor-capacitor networks. For this reason, it is easy to visually locate the tremolo circuit in a Wurlitzer amp: look for three identical capacitors in a row.

These three capacitors are part of the tremolo circuit in this Wurlitzer 200 amplifier.

If the circuit’s active component (the tube or the transistor) fails, the tremolo will stop working. If a biasing component drifts or fails, the tube or transistor may not achieve enough gain, which will also result in no tremolo. If a component in the phase shift network drifts, the circuit may not achieve enough phase shift to oscillate. This, too, results in no tremolo.

When should I replace tremolo components? If the tremolo section contains electrolytic capacitors, these are probably deteriorated and should be replaced. (They are included in the Basic kit.) Otherwise, if your tremolo is working, you can leave everything original.

On the other hand, if your tremolo is not working, you should first check that the appropriate connections to the tremolo potentiometer via the wiring harness are in place. If the potentiometer is connected correctly, there is probably a bad component in the tremolo section. Likely culprits include the phase shift capacitors, the tube or transistor, and (if applicable) the optocoupler.

RESISTORS

The Basic kit includes all high-wattage and a selection of input resistors. However, we also offer an add-on kit that contains the remaining resistors in the amplifier. We offer this kit for convenience. It is very common that a few resistors in the amplifier will need to be replaced, but it’s pretty impossible to predict which ones (outside of the usual suspects, the high-wattage resistors, which we already include in the Basic kit). But, if you have the all-resistors add-on, you’ll be covered if, during the course of your restoration, you discover a bad resistor in some unexpected part of the amplifier.

For these resistors, we have sourced high-quality, low-noise metal film resistors that are well-suited for audio applications. For the tube Wurlitzer kits, we include resistors with extra-long leads whenever possible. This is very convenient when working with a point-to-point amplifier.

Carbon composition and carbon film resistors coexisting in a Wurlitzer 200a amplifier. This is the power supply section. It’s possible that carbon composition resistors were used here because they tolerate high transient currents better than other styles.

When should I replace resistors? The original resistors in the Wurlitzer 120 amplifier are carbon composition. Carbon composition resistors are inherently noisy and, as they age, they can introduce crackling sounds and other unusual forms of noise into the signal. However, some people appreciate the lo-fi hiss that carbon composition brings to a vintage circuit.

If you are happy with the overall tone of your amplifier, you can leave the original resistors in place. However, if you are hearing unusual noise (and you are positive that it isn’t coming from the reed bar), you may want to replace some of the original resistors, paying close attention to any resistors in the signal path. This kit includes all the resistors in the amplifier, but you can replace as many or as few as you’d like.

[1] If your amplifier has a noise problem, you should always try to narrow down the problem area first, before replacing any components (other than electrolytic capacitors and high-wattage resistors). Otherwise, you might end up replacing more components than you originally intended to. See our guide on hum for details.

[2] This describes a “dry” electrolytic capacitor. Despite their name, dry electrolytics can in fact contain liquid electrolyte. They’re called “dry” in contrast to early “wet” electrolytic capacitors. These were basically made of thin aluminum plates that were folded like instant ramen noodles and submerged in a box filled with water.


Further Reading

Browse all of our articles on restoring vintage gear. Or, click on an image below.