Mesa/Boogie Mini Rectifier Twenty-Five Owner's Manual page 33

ON TRIODES, PENTODES & IRISHMEN:
(Continued)
But back to the electron flow. If the electrons are strongly attracted to a positively charged plate, then it follows that they are
strongly repelled by a negatively charged plate and they are. Thus, if an alternating current - such as comes from a transformer
- is applied to the plate, electrons will flow only during the times when the plate is positively charged. During periods of negative
plate charge, electron flow is stopped and the space charge of electrons remains compressed in the area around the cathode.
Thus a diode tube - one with a cathode and an anode - is mostly used to rectify alternating current into direct current by pass-
ing it without restriction, but in one direction only. This also explains why closing time is strictly enforced at Irish pubs: During
normal operation, the traffic flow is similarly unimpeded and uni-directional toward the bar and this process rectifies 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 flow can be a necessary and useful advantage. This led to the invention of those swinging lou-
vered 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
negative 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 influences controlling the access. This
negative influence 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 difficult to
understand. The effect is similar though: The more negative the Bias, the more it impedes forward progress. Generally speak-
ing though, the electronic Bias of the grid is easier 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 field but mostly it's empty space in
between widely separated windings of wire. It's very easy to control the electrons as they pass through the grid's force field:
Changing the grid voltage only slightly will have an enormous effect on how much current flows through...and that's what
AMPLIFICATION is: a small change in voltage at the grid causing a large change in current flowing 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 de-
cide 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 influence is felt outside thus amplifying the customer flow 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 cus-
tomers 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 influence of the control grid. So another grid
coil of fine wire was inserted between the first 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
between windings of the inner control grid... only to find that they have been fooled! Once past the control grid and drawn toward
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