Mesa/Boogie II - Manual (Page 35)

PAGE 31
ON TRIODES, PENTODES & IRISHMEN: ( Continued )
Thus a diode tube - one with a cathode and an anode - is mostly
used to rectify alternating current into direct current by passing it without restriction, but in one direction only. This also explains why
closing time is strictly enforced at Irish pubs: During normal operation, the traffi c fl ow is similarly unimpeded and uni-directional to-
ward the bar and this process rectifi es the work-day negativity. It goes without saying that no one leaves as long as the atmosphere
around the bar remains positively charged.
TRIODES:
This section is a continuing technical treatise on the workings of Irish Pubs but to make it easier for the layman to
understand, it is explained in terms of vacuum tube technology. Enter the original bar - free beer and no doors. Well, it turns out that
some control over the fl ow can be a necessary and useful advantage. This led to the invention of those swinging louvered saloon
doors which are open at the top and bottom. They are patterned after the control grid of the vacuum tube, which is a loosely wound
coil of thin wire located between the cathode and the plate.
In a Triode the plate is always positively charged with high voltage D.C. and even though the grid is blocking the path, those nega-
tive electrons can still FEEL the strong attraction - just as the Irishmen can see in through the louvers of the bar doors. They know
what pleasures lie beyond, but to get there requires overcoming the negative infl uences controlling the access. This negative infl u-
ence is typically called a Bias. In electronic terms that means the grid is supplied with a voltage which is slightly MORE NEGATIVE
than the already negative electrons. The more negative the Bias, the more it tends to neutralize the attraction of the plate and repel
the electrons back toward the cathode.
The Irish can be similarly charged with Bias, but unless you are Irish yourself, this type of Biasing may be more diffi cult to under-
stand. The effect is similar though: The more negative the Bias, the more it impedes forward progress. Generally speaking though,
the electronic Bias of the grid is easiest to overcome, and for two main reasons: First, the Bias is set - like the bar doors - to allow
some passage. Second, the grid is mostly NOT THERE, like the louvered doors which are mostly open spaces. Unlike the plate
which is solid, the grid is like a coiled bed spring. It can create a repelling fi eld but mostly it’s empty space in between widely sepa-
rated windings of wire. It’s very easy to control the electrons as they pass through the grid’s force fi eld: Changing the grid voltage
only slightly will have an enormous effect on how much current fl ows through... and that’s what AMPLIFICATION is: a small change
in voltage at the grid causing a large change in current fl owing to the plate.
The purpose of the louvered bar doors is similar to that of the grid, namely, to give momentary pause while still revealing the
promise within. Hesitation mostly gives way to temptation, but there are those few stalwart Irishmen who think twice and decide
to come back later. Most just pause slightly then go on through. That is the purpose of the bar doors: to prevent everyone from
crowding in all at once - and as the door is made less of a barrier, wider spaces between the louvers, more of the bar’s attractive
infl uence is felt outside thus amplifying the customer fl ow and increasing the crowd at the bar.
PENTODES:
Occasionally though, bar doors - even the louvered type - were found to be too effective, and too many custom-
ers turned away. Something further was needed to increase the attraction of the bar and overcome the resistance created by the
door. Thus the cocktail waitress was invented.
Once again the idea was inspired by the vacuum tube. It had been discovered in some tubes, often large power types, that the
distance to the plate was too great to attract enough electrons past the negative infl uence of the control grid. So another grid coil of
fi ne wire was inserted between the fi rst grid and the plate. This was called the screen grid and carrying a highly positive charge, it
functioned as a “bait” for the plate.
In a properly designed power tube such as an EL84 or a 6V6, the windings of the screen grid are precisely aligned to fall in the
shadow of the control grid. This way the electrons responding to the pull of the screen grid are lined up in sheets as they pass be-
tween windings of the inner control grid... only to fi nd that they have been fooled! Once past the control grid and drawn toward the
screen grid, they discover...there’s almost nothing there. The path they’re on has them aligned to zing straight through the spaces
BETWEEN screen grid windings. So rather than a close and personal encounter, they just fl y on past - and once they’re out that
far, there’s no stopping them. The infl uence of the plate takes over and - being solid metal and of the highest positive attraction - it
is at this fi nal destination that the electrons congregate.