If a fuse blows, the  problem is most likely a shorted power tube, Shorts can either be mild or severe. In a mildly shorted tube the electron flow has overcome the control grid and excess current flows to the plate. You will usually hear the amp become distorted and begin to hum slightly. If this occurs, quickly look at the power tubes as you switch the amp to STANDBY and try to identify one as glowing red hot. It is likely that two of a pair will be glowing since the "shorted" tube will pull down the bias for its adjacent mates, but one tube may be glowing hotter - and that one is the culprit. The other two are often fine - unless they've been glowing bright red for several minutes.

Because there is no physical short inside the tube (just electrons rioting out of control) merely switching to STANDBY for a few moments then back to OPERATE will usually cure the problem... at least temporarily. Watch the tubes carefully now. Should the problem recur, the intermittent tube will visibly start to over heat before the others and thus it can be identified.  It should be replaced with one from the same color batch, shown on its label.  Call us and we will send one out to you.

The severe short is not nearly so benign.  In the worst cases, a major arcing short occurs between the plate and the cathode with visible lightning inside the glass and a major noise through the speaker.  If this is seen to happen, IMMEDIATELY turn the amp to STANDBY.  By this time the fuse probably will have blown.  Such a short is usually caused by a physical breakdown inside the tube including contaminate coming loose or physical contact (or near contact) between the elements.  Replace it and the fuse with the proper slo-blo type and power up the amp using the power up procedure as we described earlier in this manual.

The power tubes should be all but quiet when they are tapped.  If crackling or hissing changes with the tapping, you have probably found the problem.  To confirm a noisy power tube, merely put the amplifier on STANDBY, remove it from its socket and turn it back on.  It will cause no damage to run the amp briefly with one power tube missing. You may notice a slight background hum, however, as the push-pull becomes unbalanced.

Whenever you are trying to diagnose a suspect tube, keep your other hand ON the Power and Standby switches ready to shut them off instantly in the unlikely case you provoke a major short.

If you think you've located a problem tube but aren't sure, we recommend substituting the suspect with a new one just to be sure of your diagnoses.  You will be doing yourself and us a big favor by just following the simple guidelines previously mentioned  regarding tube replacement. You'll probably be successful with much less effort than is required to disconnect everything and haul the unit to a technician who will basically perform the same simple tests.  If the tubes are still within their six-month warranty period, we will happily send you a replacement.  Just note the color designation on the tube label so that we can send you the appropriate match.    

Let us begin by saying;  It is a "very good" idea to keep at least a couple of spare pre-amp tubes on hand at all times to insure uninterrupted performance.  These minor pre-amp tube problems can take many forms but can generally be described in two categories:  Noise and Microphonics,  Noise can be in the form of crackling, sputtering, white noise/hiss and/or hum.  Microphonic problems usually appear in the form of a ringing or high pitched squealing that gets worse as the gain or volume is increased thus are more noticeable in the higher gain Lead modes. Microphonic problems are easily identified because the problem is still present even with the instruments' volume off or unplugged altogether - unlike pick-up feedback which ceases as the instrument is turned down.  Microphonic noise is caused by mechanical vibration and shock:  think of banging a microphone around and you'll understand where the word came from.

The best way to approach a pre-amp tube problem is to see if it occurs only in one specific mode or channel.  Then refer to the Tube Task Chart found on the sidewall of your cabinetry and it should lead you to the tube needing replacement.  Then all that remains is to swap the suspect tube for a known good performer.

If you cannot narrow down the trouble to a specific mode or channel, the problem may be the small tube that drives the power tubes which is operational in all modes and channels.  Though rare, a problem with the driver tube would show up in all aspects of performance - so if you can't narrow the problem down to being mode or channel specific, you may want to try replacing the driver tube.  Driver problems generally show themselves in the form of crackling or hum in all modes of performance and/or weak overall output from the amplifier.  Occasionally an anemic driver tube will cause the amplifier to sound flat and lifeless, but this is somewhat uncommon, as worn power tubes are a more likely suspect for this type of problem.

Sometimes making the diagnosis is more trouble than it's worth and it's faster and easier to merely replace the small preamp tubes ONE AT A TIME with a replacement known to be good.  But MAKE SURE you keep returning the tubes to their original socket until you hit the one that cures the problem.  You'll notice that tubes located nearer to the Input jack always sound noisier...but this is because they are at the start of the chain and their noise gets amplified over and over by the tubes that follow.  The tube that goes into this "input socket" (usually labled V1) needs to be the least noisy of the bunch.  The tube that goes at the end of the preamp chain - just ahead of the power tubes - can be quite noisy without causing any problem at all.  The tubes in your amp have already been located in the most appropriate sockets and this is why you should NEVER pull them all out at once and ALWAYS swap them one at a time.  ALWAYS return a perfectly good tube to its original socket.  Also it's a good idea to put the amp on Standby when swapping tubes to reduce the heat build up in the tubes themselves and to prevent explosive noises (which can still occur even if you are pulling the tubes away from their sockets gently) from coming through the speaker. 

Remember, take your time, be patient and chances are real good that you can fix your amp yourself by finding and replacing the bad tube.  It kills us to see someone who has shipped their amp back to us...and all it needed was a simple tube replacement!