I recently picked up a Marshall JCM900 SL-X Model 2100. I switched the amp over to triode for half the power and then back over to pentode for comparison. In doing so, I noticed one of the Fail LEDS was lit for Valve Fuse OPV2-OPV3. With either fail LED lit, the amp runs at a lower power to reduce the chances of damaging any other components.

Here's the manual along with schematics for the SL-X.

I know I can do some troubleshooting to determine where the problem may be, such as pulling tubes, but I've never had to do this on any other amps since I've never blown a fuse. I'm assuming the problem will be with one of the preamp tubes, but I'm not exactly sure how to go about testing them.

The previous and original owner replaced and rebiased the amp back in June. He had the receipt for the tube change and rebias. I know there's a somewhat high failure rate for preamp tubes, so that's why I'd guess it's one of the preamp tubes over the power section. There are no fluctuations in volume.

I'm planning on picking up a pack of fuses this weekend. Before then, any help on troubleshooting would be greatly appreciated.
I have a somewhat similar amp so here are my thoughts.

I didn't read the manual but do you have to cut your ohms in half when cutting power in half - like I do? Have you tried running amp with only the inner 2 or outer 2 power tubes? I would number the power tubes and then try them in different sockets and keep track.

Obviously you may not be able to do some of this until you get some new fuses. I take it the amp still turns on. Remember too that fuses blow for a reason so there IS a good chance that something is seriously wrong and continually turning the amp on and off COULD cause more problems.

I'm not thinking preamp tubes at all when I first read this. I'm not an amp tech though either. Here is a vid I made of one way to test for a bad preamp tube.


I also have a biasing blog that may or may not help.

Again - not a tech here
Last edited by 311ZOSOVHJH at Sep 9, 2010,
The cool thing about tube amps is that each tube serves one, sometimes two, functions - at least in the preamp. Using that knowledge, plus the diagram/manual that shows what each tube does, makes troubleshooting tube amps pretty easy. Use this info to methodically test the amp.

Does channel 1 work?
Does the reverb work?
Does the gain circuit work?
Does the effects loop work?

If all of these questions can be answered "yes", then we know all of the tubes, or tube halves, associated with channel 1 are good. Let's apply the same test to channel. Run through the same four questions, but substitute "2" instead of "1". Does everything in channel 2 work? If we find something that's not working, then we need to figure out which preamp tubes are possibly at fault.

This is where we can start swapping same type preamp tubes around. Something dead in channel 2, but channel 1 works? We can eliminate the phase inverter. Both channels dead? Suspect the phase inverter. Does the reverb work? We can assume that tube is good and can be used to swap out with other possibly suspect tubes. Do all the functions work? It may not be a tube at all. It could be a passive component, such as a capacitor or resistor.

Lastly, you probably already know it, but it's a good idea to place your amp in standby, prior to switching functions such as pentode/triode, different power levels (if your amp supports that) or from silicone to tube rectifier.
311ZOSOVHJH I'm not sure about cutting the ohms in half when switching the power, I didn't even look into that. That could have been the culprit, because I don't remember the fail LED being lit before switching between power modes. I'll try switching the power tubes around this weekend once I get new fuses. The amp turns on and works perfectly fine, but the fail LED keeps me from using it, at least until I get some new fuses and figure out what the problem is. I haven't tried the old pencil tapping trick on the preamp tubes yet, I planned on doing so this weekend.

KG6_Steven The SL-X is a single channel amp, so troubleshooting is a lot easier than if I had a problem with my Triple Recto. As of the last time I played the SL-X, everything worked perfectly fine. No issues with gain, volume, or the effects loop. I was sure to switch from pentode/triode when the amp was totally off.

EDIT: After doing some research, it looks like I needed to switch the head to 8 ohms when going from pentode to triode.
Last edited by Guitarmiester at Sep 9, 2010,
Regarding the pencil tapping trick... Your problem is not associated with a microphonic tube. Also, remember that most tubes are microphonic to some degree. Just because a tube is microphonic, doesn't make it bad. I'm an electronics technician and this isn't something I've ever worried about. As long as the microphonic tube doesn't break into oscillation (feedback), I wouldn't worry about it.

Edit: Also, remember that it's possible to damage a tube by tapping it too hard. You won't damage the glass with the pencil, but tapping it too hard can cause the hot components inside the tube to deform, ruining it.
Last edited by KG6_Steven at Sep 9, 2010,
Even though I haven't checked the preamp tubes, I'm skeptical that any of them need to be replaced since they're only a few months old and should last longer than the power tubes. I edited my previous post concerning ohms and running it at half power.
Just an update for anyone who was interested...

I bought a pack of fuses, replaced the blown fuse, played the SL-X for an hour, and everything looks to be good. The fail LED did not come back on. I'm thinking either the fuse wasn't replaced by the previous owner when he changed the tubes or it blew from an impedance mismatch.

I'm glad it didn't turn out to be the typical bridge rectifier or blown capacitor.