Interestingly enough, after looking at the schematics for the 450/500/550 NT series of amplifiers,? they all use a PI network? on 160-10m.? ?BUT they use a lower, loaded Q.? ( more coil, less tune + load C).?
The alpha 78/ 374A etc, in broadband mode, ( and? also manual tune),? all use a PI-L..... same deal though, a lower loaded Q is used, when switching from 'manual tune'? to ....'broadband'.? ?In broadband mode, more L is used on 80-40-20-15m.?
The broadband feature on the alpha amps only works on 80-15m.? Strictly manual tune on 160+10m.?
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I asked glen, AE0Q, the alpha fellow, why they didn't use the same deal on their newer amps (9500 / 8410).? He said the 78/374A? etc, had lower eff vs manual tuning.? So they switched to motor driven tune + load caps..and the bandswitch.?
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Funny thing is, on one of my older 4-1000 amps, with the roller coil in it? ( and also my 3x3 with a roller).... when normal? Q used ( this is a PI), like 12-15, PO was good.? But then tried increasing the uh on the roller a bit ( and slightly less on the tune + load caps)..and PO increased.? ?So repeated the process again, with more uh, less tune and load C..and again, PO increased.? Repeated several more times, in small increments, ( each time retuning the amp with the same drive level)...and PO kept on increasing, Then no more PO with any further increases in uh.? ?It just flat lined with max PO, regardless of how much uh was dialed in.?
So I backed down the uh, until PO just started to drop off a few watts.... and left it there. This was all on 75m.? ?Same deal on the other bands.? ?The idea here is to use a lower Q...but just low enough to do the job.? ?Go too low, and PO stays the same, (no more increase) even more broad banded, but I highly suspect the harmonic suppression is severely de-graded.?
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There is NO spec for harmonic suppression in the online manual for the 450/500/550? (which also covers the NT versions).? ?
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It would be interesting to know what the actual harmonic suppression is on each band on the NT versions.?
