Mysterious Resistor and the Red-Plating Saga: 1990 Marshall JCM900 4501 Combo

Partial restoration of a 1990 Marshall JCM900 4501 combo with a twist. Loss of negative DC bias voltage during testing. On the fly diagnosis and repair. Electrolytic capacitor replacement, potentiometer replacement and tube replacement.
MENDITTO AMPLIFICATION
[e-mail]
mendittoamps.com
MAGGIO MUSIC CENTER
8403 18th Ave, Brooklyn, NY 11214
Phone: [phone]
maggiomusicschool.com

[@mendittoamplification]

I point to the wrong resistor in one of my graphics. This is why you shouldn't edit videos at 4:30AM!

[@brooklynsoundgarage]

Wow that amp actually looks pretty good!

[@mendittoamplification]

It's a really nice survivor. I am not a big JCM 900 fan, but this one sounded great. Thanks for watching my video!

[@riffs66]

Good stuff!

[@mendittoamplification]

Thank you!

[@waynegram8907]

Why is C15 using a BRICK X-Class Capacitor? I never heard of an X-Class capacitor

[@mendittoamplification]

Thank you for the question. An X class capacitor will most likely fail closed or short. In this circumstance, I think that the intention of the electrical engineers at Marshall was in the event of capacitor failure, the X class capacitor would prevent the loss of negative DC bias voltage to the grid, and prevent the tube from drawing too much plate current, aka "red-plating". The problem is that I have only experienced the opposite when these capacitors fail in Marshall amps. Every one of these capacitors that I have changed has failed open. Thank you for watching our video!
-Mike Menditto

[@waynegram8907]

@@mendittoamplification I wonder why the X Class Capacitors often fail OPEN? because mostly the capacitors dielectric material insulator will get shorted out by the excess of the DC direct current. Maybe the 250v rated dielectric material didn't like the bias voltage

[@mendittoamplification]

In this bias circuit, the capacitor is acting as an AC voltage dropper. Rectification occurs after this capacitor. The capacitor that Marshall chose is just not up to the task. A 250 volt rated capacitor was a poor choice by the engineers. The common consensus among other repair technicians that I have spoken to about this is to just replace this capacitor with a much higher voltage film capacitor, and forget about the "X Class" rating.

[@waynegram8907]

@@mendittoamplification So they are using this X Class capacitor as a Voltage Divider to step down the High Tension voltage down to a Bias Voltage? Using Resistors has Voltage Dropper to create a Bias Voltage but using a Capacitor has a Voltage dropper to create a biasing voltage is Poor Engineering

[@mendittoamplification]

I recall measuring about 190 volts AC being fed to this capacitor.