Marshall JCM 800 2203 Guitar Amp - Tube Swap and Re-Biasing

[jcrubin]

Chris from Cornerstone joins me again for this Marshall re-biasing video.  This is an all original JCM 800 - 2203 that is receiving new tubes all around.  Of course, a quick inspection is in order.  Measurements of the electrolytic caps show that most of the caps are good, one of the filter caps are about ready to go.  As I make incremental adjustments to the bias as the tubes cook, The question is, are the tubes really balanced as the box from mesa advertises..   I'd think not. As a matter of fact, the tube deviation is 6 milliamps from all 4.  Good thing I was biasing for the highest current flowing tube.

[Cyberdragon]

Really? That's some shenanigans! I thought matched tubes were supposed fo be...well...just that. Where did you your new tubes?

[Ray Gianelli]

This is a real issue, since Mesa amps have no adjustable bias.  They depend on you buying Mesa branded tubes, which are supposed to be matched.

[jcrubin]

When the packaging is shinier than the tube... I get suspicious...

[Kirkhaan]

For lifetime its better that they draw similar currents indeed.
But for sound quality, in such a (heavy metal) tube amplifier, I would not bother too much.
Guitar players want to hear harmonic distortion!

[Cyberdragon]

For lifetime its better that they draw similar currents indeed.
But for sound quality, in such a (heavy metal) tube amplifier, I would not bother too much.
Guitar players want to hear harmonic distortion!

Not just the tubes, the whole health of the amp depends on it (component heat). Some amps (as explained and shown by guitarologist) run the tubes so hard that any sudden change in load will blow them right up. Well, guitar players like pyrotechnics and smashing stuff.