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 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.
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.
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