Hi Bob,
I looked at these products at an engineering level for the past owner of
Dentron. I have a time recalling specific components, but can give you some
general guidelines. When I get back fro a trip, I might be able to dig out my
old notes.
In a message dated 96-11-18 15:17:09 EST, you write:
>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?
As I recall it was an almost unnecessary padding cap for higher bands.
>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.
The MLA does NOT need more Q, once Q is larger than one plus the sqrt of
Rplate/RLoad any further increase in Q lowers the efficiency. The MLA already
has more Q than necessary.
The quickest cure, short of correcting the entire tank, is to add padding
capacitance on the loading capacitor. The problem is almost aleways the
loading cap being out of range.
>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.
Even a Q of eight should be ok when calculated at full pate input. I think
not enough C in the loading cap is being confused with not enough Q. The coil
needs **more turns** because it is jammed against the sheetmetal, and the
lack of inductance raises the capacitance required.
Think about it. Look at the plate C on 80, and how the amp works. It only
needs twice that value on 160 (plus a tiny bit because of shunt reactance
from the palte choke).
>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.
And add an electronic grid overload circuit, and a 20-40 ohm energy absorbing
resistor in series with the HV lead.
73 Tom
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