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WHAT THE HECK IS
A RECTIFIER TUBE????
All of our Marshall and VOX tube amps require DC (direct
current) to work. However, as you know, the power we get from the wall is AC
(alternating current). So, a device known as a rectifier is utilized within the
power supply of an amp and its job is solely to convert AC into DC - hence the
name "rectifier." In most modern tube amps a solid-state device known
as a diode does this “rectification”. However, a few "older" amps
use a tube as their rectifying device and we happen to distribute four (4) of 'em!
They are the Vox AC30 (GZ34 rectifier tube) AC15 ( 5Y3 rectifier tube), the
re-issue JTM45 (GZ34) and the 1962 Bluesbreaker combo (also aGZ34).
Now, as the guitar's signal never actually passes through
this part of the amp's circuit, one would think that a rectifier tube has no
direct effect on the sound of the amplifier… well, that's the theory anyway!
The truth however is different. The thing is, the rectifier tube has a direct
bearing on the power supply tube for the other tubes and, due to its relative
inefficiency (compared to a solid-state rectifier), it can and will “sag”
when the demand for power is higher than it can handle. And, as it happens, a
lot of players like the sound and feel that results from this
"inefficiency." To hear and feel this "sag" in action, plug
into a JTM45 or a 1962, crank the thing up and then start attacking those
strings! Believe me, once you've experienced this "sag" in action,
you'll fully understand why it is so named. I hope this information helps. Be
careful out there, wherever you are…
WHY DO I NEED TO
CHANGE MY TUBES?
There are two main reasons why amplifier tubes need to be
changed. The first reason is that the tube(s) simply burn out, causing a
malfunction with the amplifier. On the JCM900 series amplifiers there is the
fail safe fuse which is built in to indicate power tube failure. The tubes need
to be changed immediately so damage to the transformer is avoided. As I stated
in my previous tip, tubes have no definite life span and can burn out at any
time. Thus the 90 day warranty on them!
The second reason and perhaps the more important is to
improve the performance (tone) of your amp. The effect the tubes have on your
amp is much like the effect that strings have on your guitar. The more often you
play and the more aggressive you play, the faster the tubes/strings will wear
out. Playing loud and hard will cause your tubes to lose power and tone long
before they finally die. This wear down process is very gradual and most people
(including myself) wouldn’t recognize the difference in tone until new tubes
were put in and biased by an authorized technician. I always mention to
customers that if a power tube goes that they should replace all
of them so they will have equal wear over time. This will help provide a more
even tone and back up tubes are always a good thing.
A common problem that we’ve all seen with tubes is a
microphonic pre amp tube. This is the culprit that causes that high end
squealing even without a guitar plugged in. I think Ted does that also….. A
way to check this is to unplug the amplifier, remove the back panel, turn the
amp back on and then let the tubes warm up. Take a pencil and gently tap each
pre amp tube to see which one has a loud ringing or crackling sound. They will
all ring to a certain degree but the “bad” one will be much more noticeable.
Replace this one immediately. A second way to find it is to pull out the suspect
one(s) and replace it with new ones. This is simple because as you know there is
no bias procedure for pre amps tubes and any non –idiot (read: non-keyboard
player) can do this without hurting themselves, the amplifier or others. Plus
it’s always a good idea to have replacements for gigs anyway.
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