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DIAGNOSING PRE AMP TUBE PROBLEMS:
Because your amplifier is an all tube design, it is quite possible that you will at some point experience minor pre-amp tube noise. Rest
assured - this is no cause for alarm and you can take care of the problem yourself in a matter of minutes by simply swapping tubes.
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
“HI” 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 mechani-
cal 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. This 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 pre-amp 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 labeled 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! If you must send back
your amp, remove the chassis from the cabinet by unscrewing the four mounting bolts on the bottom top. The chassis then slides back like
a drawer and comes out from the back. Remove the big power tubes and mark them according to their location from left to right 1, 2 etc.
They need to be wrapped separately with plenty of wadded up newspaper around them and put in a smaller box within the larger carton.
Remove the Rectifier tubes and wrap them also. You can leave the preamp tubes in or remove them and wrap them separately being sure
to label their location. (See Tube Task Chart.)
To wrap the chassis, use plenty of tightly wadded up newspaper so there is at least six inches of “crush space” between the chassis and
the cardboard box. Bubble wrap also works well, but please DON’T use styrene peanuts - they will shift during transit and get lodged inside
your electronics as well as allowing your amp to end up at the bottom of the box unprotected and possibly damaged.
Pre-amp tubes don’t normally wear out as a rule. Therefore, it is not a good idea to change them just for the sake of changing them. If there
isn’t a problem - don’t fix it. If there is no result from your substitutions, it may be possible that you have more than one problematic tube.
Though rare, this does happen and though it makes the troubleshooting process a little more intimidating, it is still possible to cure the
problem yourself.
NOTE: It is normal to hear a slight metallic ringing sound when tapping on the preamp tubes. As long as the tube does not break into
oscillation or start crackling or any other form of bizzare noise, it is considered normal and functional.