TopBand: MLA2500

[Bob Eldridge]

Some questions about MLA2500 operation on 160:

I am interested in better efficiency. More output without increasing present
input, or maybe less input for the present output.

1. What is the purpose of C10? Section 2 of the bandswitch puts it (100 pF)
in parallel with 2000 pF in the Pi network output.  Some kind of harmonic or
parasitic suppression?

2. Reason for asking is that maybe it could be transferred to go in parallel
with C9 across the input to the Pi network on 160 to increase the Q.

3. But why is the Q said to be low anyway? C9 is 300 pF across the 140 pF
variable, and <440 pF should give a Q of at least 10 shouldn't it?
I do agree that to resonate properly on 160 this C has to be increased,
because the efficiency goes down if turns are added to the coil.

4. Someone said increasing the "tune" capacitance of the Pi network would
entail adding a switch wafer. What's wrong with just playing with the
capacitance of C9?

5. Has anyone tried relocating the tank coil downwards, away from the top
cover?  Or modifying the top cover? Raising the cover increases power output
(although my top cover doesn't get warm running what I consider to be normal
output - 600 watts).

A comment on MLA2500 threads: Seems to me that running a pair of 8875s at
1000W or more output is asking for trouble. (I do mean 1000W output, not
apparent output across a load that is more than 50 ohms).
Excessive grid current kills 8875s, and I find the optimum condition is when
drive and loading are adjusted to just barely move the grid drive meter. The
extra output resulting from more drive just isn't worth the risk.

73
Bob Eldridge VE7BS,    RR1, Pemberton B.C. V0N 2L0
                        eldridge@whistler.net 


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