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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 instru-
ment 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.
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 bizarre noise, it is considered normal and functional.
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