>What are the realistic differences betwen the two....besides the obvious
>extra grid?
>
>I remember the original Penta Company always claimed ~ 10% higher output
>with their PL-175 vs the 4-400A. Hallicrafters used the PL- 172 in the
>HT33A-B. And Svetlana sells the 5CX1500.
>
>73...Carl KM1H
Pentodes have more headroom for the anode voltage to move around in than
tetrodes, so that makes pentodes more efficient. For example, in a
5CX1500A, the anode V can move down to about 250v before nonlinearity
becomes a problem. However, in a 4CX1500A, if the anode V reached 250v,
too many electrons would stick to the screen, the screen current goes
though the ceiling, which means they do not arrive at the anode, so
distortion results. To sum this up, in a pentode, the anode voltage can
cruise well below the screen potential without IMD problems. However, in
a tetrode, for acceptable IMD, the anode voltage can go only slightly
below the screen potential.
So why haven't pentodes completely replaced tetrodes? Pentodes are
relatively difficult to manufacture. Sockets for pentodes are relatively
difficult to manufacture. More difficulty equals more money. How much
money? Eimac's SK-840 socket for the 5CX1500A costs roughly one
kilobuck. Need a tube to go with? One more kilobuck. You can buy a
nice tetrode with handles, socket included, from Svetlana that delivers
8x as much suds, and pocket $500, thank you.
Large tetrodes are apparently problematic to produce. The largest Eimac
pentode is the 5CX3000A. It will do about 8 out in SSB service. The
largest Eimac (single section) tetrode is the 8973. It will do about 800
out in SSB service.
Rich---
R. L. Measures, 805-386-3734, AG6K
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