Variable Amplified Thoughts

Lately I’ve been reminding myself to periodically write something here. Today I was thinking about all the things that can cause an amp to malfunction, and ways to prevent them from happening. One of the easiest things a gigging or touring guitarist can do, which doesn’t cost anything, is to make sure the transportation of their amp doesn’t not inflict excessive vibration. Vibration is the #1 enemy to tube amps. I know several guitarists that routinely haul their amps to their gigs in the back of their equipment trailer. For me this is a big NO-NO. Have you ever driven alongside a trailer as it’s being pulled at 70mph and observed it vibrating or even bouncing? Now picture your amp doing the same thing. One or more of the tubes in that $140 tube set is being slowly destroyed if not in one single jolt from a pothole. Along with solder joints, and possibly even the insulation on the transformer windings. Nuts and bolts can also become loose.

Be your amp’s best friend and always carry it with you in your personal vehicle if possible. Or at the bare minimum, pad your amp with pillows or similar items (a heavy duty flight case is optimal) if you have to put it in the trailer.

Of course if you choose not to heed this advice, that’s fine with me and other techs who also repair amps.

Hello all out there in tone nirvana land! I just finished up a brand new M50 Plexi build (Marshall-ish ‘72 era JMP50 w/ a master volume), fired it up on the current limiter (light bulb in series with AC mains), checked and recorded voltages, and biased at 36mA, both E34Ls within 1mA. Trouble-free build right? I start playing, and as I get progressively louder all is fine… then, a drop in volume, followed by a popped B+ fuse. Also notice V4’s plate is red as hell.

Remove the chassis, discharge caps, replace the fuse, replace V4 (and V5 so they’d be matched), and this time monitored cathode current on both tubes as I ran a 1KHz tone into the input and progressively got “louder” (with an 8 ohm dummy load attached). V5 steadily climbed to about 90mA, while V4 jumped to 180mA!

Removed the JJ ECC83S (12AX7) and popped in another new one. Problem solved.

So, for anyone that experiences red plating on one of the two output tubes on your 50 watt amp, try replacing the phase inverter tube, preferably with a matched one (matched triodes). A severely imbalanced PI tube can cause red plating, and its an easy cure before you spend half an evening chopsticking and poking around in your amp.