6 - 2 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.
Thus the proper cocktail waitress - visible through the louvers - is scantily clad so as to be all the more effective at reinforcing the attractive influence of her bar and by being located in between the door and the bar, she serves as bait to lure customers past the door’s negative influence. Once through the door however, it is the rare Irishman who actually comes in personal contact with the cocktail waitress as, for all intents and purposes, she - like the screen grid - turns out to be a vanishing illusion. Yet, having come this far, the solid influence of the bar itself now takes over and attracts the customers to congregate, having happily reached their destina- tion.
Since each channel of the TIGRIS uses three pairs of power tubes, the Pentode / Triode switch is configured in three positions so you can hear any combination of characteristics and sample for yourself the extra juice provided when those wenches lay bait to the plate. If you’re still following this and haven’t lost track of the count, you’ll know we’re still one element short of the five needed to make a Pentode. This last part is a pair of
Once at the bar though, the circuit is complete and the process of
With appreciative thanks to the inhabitants of the Land of the Leprechaun, we have now concluded our little diversion into the mechanics of proper bar
Chapter 7 - TUBE MAINTENANCE & TUBE ROLLING:
7 - 1 TUBE MAINTENANCE:
Just as light bulbs have a limited life span - from a few hundred hours to automobile head lights rated for thousands - so all tubes eventually wear out and need replacement. The tubes used in the TIGRIS are of the industrial type and highly rugged and reliable. They should provide years of
Eventually, even the hardiest of tubes will have to be replaced, and to support your maintenance, we offer the original owner a unique plan in the industry: a 50% discount on replacement tubes for as long as you own our amplifier. At the time of this writing, a complete retube kit consisting of 4 x 12AX7A, 4 x 6V6 and 8 x EL84, is available
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