Music Man HD150 Retube Cap Job, Tube Amplifier Repair, Capacitors

I made a video while recapping a client’s Music Man HD150. The video can be found here:

Also worth noting is a question I get often about capacitors in these old amplifiers.

First and foremost, be sure you know FOR SURE how to discharge remanent voltages from the amplifier. There are lethal levels of DC voltage stored in the power supply caps even after the unit is turned off and unplugged.

If the original polar (aluminum electrolytic) capacitors are still in there, it’s definitely overdue for a cap job. Depending on the electrolyte used in a capacitor, they degrade overtime whether used or not. Typical shelf life of capacitors in these old amps is about 20 years (very roughly). Seeing as the amp is 40+ years old, always a good idea to get some new cans in there.

You can explicitly see a capacitor failure if the electrolyte bursts or oozes out of the package. It’ll typically look like little crusty paste on the faces of the cylinder. These MUST be replaced to ensure proper power filtering and bypassing, depending on the location within the circuit.

You can hear a capacitor losing its capacity to hold charge if the amplifier starts to sound flat, loose, or unresponsive during play. Alternatively, if the capacity is too high (in the case of a replacement of higher capacitance in the power supply section), the amp will sound very stiff and overly ballsy. This is sometimes a cool thing to experiment with, but be sure to always match or exceed the voltage ratings.

I’ve had great success using F&T capacitors for the high voltage and bias power supply sections. The Sprague Atoms are well known to provide boujee filtering as well… Nichicon also makes nice options for bypass and power applications. I tend towards 105ºC and large voltage ripple ratings.

Good luck and let me know if you have any questions.

Casey

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