Troubleshooting My Wurlitzer Electronic Piano’s Tube Amp: User Serviceable Edition

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Early Wurlitzer electronic keyboards dating before roughly 1962 all featured on board tube amplifiers. Tube amplifiers offer a unique warm & full sound - and what could be better to play your Wurly through, right?  To many, however, a vintage tube amp can appear intimidating, high-maintenance and something only audiophiles should own. In the world of vintage amplifiers, however, a tube amp is more likely to be easier to troubleshoot should anything go wrong with its functionality.  In general, vintage tube amps have a few large, easy-to-replace components, while vintage solid state amps often have smaller, more delicate components that cannot be replaced by the casual user.  This is true with vintage Wurlitzer amplifiers, so here are some common problems that might come up with your Wurlitzer’s tube amp and how to fix them. 

Totally dead / not turning on

The Wurlitzer’s 112 fuse is mounted on the narrow side of the amp, so you need to open up the lid to access it. In later tube models, the fuse is on the long side of the chassis and is accessible from the back of the keyboard.

A totally dead amp can either be the simplest fix of all or the hardest. The first place to look would be the fuse. To check the fuse, make sure the amp is switched off and unplugged.  After you are sure it is off and unplugged, unscrew the fuse cap by applying inward pressure and twisting.  As you examine the fuse, pay close attention to the inner fuse element, which is a fine wire within the glass tube. If there is an obvious burn-out or break in the fuse element, you’ve got a bad fuse which is causing your amp to not turn on.  Tube amps use slow-blow fuses, and replacements can be found at any hardware store. Make sure to replace your fuse with one of the same amperage rating to avoid damage to your amplifier.

The fuse is a safety feature of the amp, and it is designed to fail before another part of the amp can be harmed.  Typically, fuses fail because of a short, which could be a bad power cable, a bad wall outlet, or something within the amp. In addition to replacing a bad fuse, inspect your power connections.  If everything looks to be in solid shape, replace the fuse, switch the amp on and monitor your amp.  If the fuse promptly blows again, you should have the amp checked out and serviced. 

As a rule of thumb, unless you are absolutely sure the blown fuse was caused by an external problem such as a bad power cord or wall outlet, a blown fuse is an indicator that the amp should be serviced.  

If the fuse does not seem to be blown and the power cable is properly connected, the issue is likely an internal connection or major component failure and should be checked by an amp technician.

Fuzzy or distorted sound

Distorted sound can be caused by a failing or worn out tube.  Though vintage tubes from the 60s and 70s were very well built (especially compared to modern day tubes), over time they wear out, become weak and eventually fail.  As a tube comes to the end of its life, it will affect the sound of your amp, which can be illustrated by distorted or fuzzy sound.  

To remedy this, you could either A) replace the entire tube complement or B) replace the tubes section by section.  If you choose to replace the entire set, the old tubes can be saved and tested at a later date - there is no need to toss good vintage tubes.  If you prefer to replace only some tubes at a time, start by replacing the output tubes - in the 112 & 120, these are two 6V6 tubes, and in the 145 this is two 7868 tubes. If the sound s still distorted, you could try putting back in the original output tubes and swapping out the preamp tubes. The 112 uses two 12AU7, the 120 has one 12AX7 and the 145 uses a compactron 6K11. 12AU7s and 12AX7s are very common, but for the 6K11 you will need to source a vintage used or NOS tube.  Lastly, try replacing the rectifier tube. In the 112 and 120 this a 5Y3, and on the 145 is a 6CA4.  

 
The Wurlitzer 145 amplifier uses a  Compactron tube , which is no longer manufactured. You can purchase a NOS replacement, or install our Wurlitzer 145 replacement amplifier instead. We’ve replaced the Compactron with two 12AX7 tubes, which gives us an additional triode that can be wired as a gain stage or aux output.

The Wurlitzer 145 amplifier uses a Compactron tube, which is no longer manufactured. You can purchase a NOS replacement, or install our Wurlitzer 145 replacement amplifier instead. We’ve replaced the Compactron with two 12AX7 tubes, which gives us an additional triode that can be wired as a gain stage or aux output.

 

If replacing the tubes does not eliminate distortion, the cause is likely an internal amplifier component such as the electrolytic capacitors. Since this is the ‘user serviceable edition’ of amplifier troubleshooting we would recommend referring to an amplifier technician at this point.  But if you are comfortable replacing electrolytic capacitors and are able to do so safely, we offer a Wurlitzer 112 amp rebuild kit, a Wurlitzer 120 amplifier rebuild kit, and a 145 amplifier rebuild kit

It should be noted that a bad speaker will sound “fuzzy or distorted”. The best way to rule out the speaker as the source of the problem would be to plug a known good speaker cabinet (ideally 4Ω) into the speaker console output. For this, you would need an RCA to 1/4” cable.

Excessive Hum

Excessive hum can be caused by grounding problems, carbon composition resistors, old electrolytic capacitors or failing tubes. 

Tube-related hum

To check if tubes are the source of hum, try the tube replacement steps listed in the section on ‘fuzzy or distorted sound’. 

The Wurlitzer 200 filter capacitor on the right is physically leaking its electrolyte. This means that it should definitely be replaced.

The Wurlitzer 200 filter capacitor on the right is physically leaking its electrolyte. This means that it should definitely be replaced.

Carbon composition resistor noise

Carbon composition resistors, used in all Wurlitzer electronic piano tube amps are inherently noisy.  In fact, some tone-seekers enjoy carbon comp hum. If you have light hiss, crackling, or other occasional noise, the carbon comp resistors could be causing this (for further reading check out our carbon composition resistor article ).  Excessive hum also could be caused by failing resistors. If you are comfortable replacing these internal amp components, check out Wurlitzer parts shop, where we have rebuild kits containing resistors that we have specifically chosen for their suitability in point-to-point circuits.

Old electrolytic capacitor hum

Electrolytic capacitors slowly wear out over time and can cause hum or other issues, depending on their role in the circuit. For this reason, when repairing a noisy amp, electrolytic caps are a good place to start. Electrolytic capacitor sets are also available in our Wurlitzer parts shop.

Ground related hum and noise

There are many ground points in a Wurlitzer, both mechanical and electrical.  First, we suggest checking for loose screw connections on the harp and the amp chassis. The lid in tube Wurlitzers is also used as shielding, so a loose or ajar lid can be the cause of hum. The Wurlitzer 120 and 145 has a braided cable, which is wedged under the lid in order to connect the shielded lid to the electrostatic painted interior.  Finally, if you are comfortable checking grounding points within the amp chassis with a multimeter, you can rule out loose or missing grounds that way.  If you are unsure of your ability to safely check the inside of the amp, you may reach out to us for consultation and ideas.

Early tube amps were built a little noisy

There are some issues inherent to the design and layout of the amp cause noise. Wurlitzer was building tube amps while best practices for audio electronics were still being developed.  Today we know more about lead dressing and layout practices that reduce noise. Also, three-prong grounded power cables and low-noise components were unavailable at the time.

A normal level of hum in early tube amps could be described as a ‘noise-floor’. This is a noise level that has always been there since the time the amp was built.  At Tropical Fish, we build replacement amplifiers which fit into all of the 140, 145, 700 and 720 Wurlitzers.  If you are looking for a lower noise floor - or even just more tone options - you might consider one of our new Wurlitzer 145-style amplifiers.


Further Reading

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