Variable Amplified Thoughts

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.

From Rich:

This has been discussed multiple times in multiple forums. Usually, the discussion dies down quickly when someone with materials engineering experience provides some science. Bottom line is that cryogenic treatment will provide no benefit to the materials used in tubes (kinetics (changes) slow down exponentially as temperature decreases).

Cryogenic treatment makes sense for certain steels, when the desire is to induce more hardness (steels go through a martensitic transition at low temperatures).

Rich

(A materials engineer in a previous life, with a PhD in Nuclear materials from MIT)…

Then this: http://www.tubedepot.com/bsctfaq.html

Weigh each before deciding. I personally haven’t tried these treated tubes yet, so I won’t pass judgement.