Mesa/Boogie OOGIEBMESA Uživatelský manuál stáhnout pdf (Strana 34)

ON TRIODES, PENTODES & IRISHMENT: (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 stricly

enforced at Irish pubs: During normal operation, the traffic flow is similarily 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 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 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 similarily 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 speaking though, the electronic Bias

of the grid is easiert 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 seperated 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 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 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 customers 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 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 fly on past - and once they’re out that far, there’s no stopping them. The

influence of the plate takes over and - being solid metal and of the highest positive attraction - it is at this final destination that the electrons

congregate.

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