A Marshall JCM900 amp arrived.  The JCM900 combo is basically a 100 watt twin 12 inch speaker combo and is a true Marshall stack in a matchbox. Nice and easy to transport.   When new these amps  supplied in two variants, with one having 4x EL34’s in the output and the other variant having slightly lower power with 4x 5881’s.

This amp was dead. Zero output and a lovely hum.

Inside the amp proudly displayed a Marshall sticker stating that this amp should have 5881 output valves fitted.

This JCM900  was fitted with 4 odd Fender branded 6L6’s.  One was glowing a beautiful purple. It was now a neon light. This is a sign that it has a gas leak. It is now a non vacuum tube !

Put them on the tester to find one still Ok, two worn and one short circuited.  The clue was Fender branded valves as this implied they were a number of years old and this was a gigging amp.

So the task was simple.  Fit four Tung Sol 5881’s re-bias them. This amp has nicely accessible output fuses on the back panel. Both were blown.

You can see them in the picture below. One fuse per pair of output valves.


I PAT tested the amp and plugged in one of my Telecasters.  I made the fatal mistake of leaving the master volume at 10. I blew my ears off and rattled the windows.  Lovely sounding amp though !

The owner is happy to have this amp back, sounding wonderful again.

It needs a couple of new knobs on the front , otherwise this 10 year old has many years ahead.

Valves fitted were Tung Sol 5881 matched quad 

However the JJ Electronic 5881’s  ( as a matched quad) are good too and lower cost. Check them out here 

On an amp that is gigged regularly you can avoid this kind of hassle with your amp by replacing the output valves every 18-24 months . Click and read what Marshall say on their website

Marshall is good having the fuses to protect other components and the output transformer. Not all amps have this protection so beware !


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