I obtained a Dentron MLA-2500B shortly before ARRL SS last fall, and
proceeded to renovate it into operating condition (replaced filter caps and
equalizing resistors, blew out dust etc.). When I finally brought it up,
my TS-450S exciter was unable to drive it properly on any band. The SWR
indicated on the transceiver meter was not extraordinary (<3), but the
power was [apparently] being cut back to about 20 W. by the protective
circuitry. The built-in antenna tuner was unable to find any suitable match.
At this point, I examined the input circuits (both schematic and actual)
and noticed two things:
1. The MLA-2500B doesn't have a tuned input circuit; it relies on the
[usually] low input impedance of the 2 8875's in conjunction with a 100 ohm
resistor across the input.
2. The 100 ohm resistor was not connected.
I concluded that the resistor had been disconnected when the mod to add 10
meters was made. I also concluded that it shouldn't have been
disconnected, and I reconnected it. Because of the part location, I
connected it by running a wire from the input relay output terminal to the
resistor (about 3 in.). Had it been easier to get to, I would have run the
coax to the resistor and then on to the tubes.
So, here's the problem. The amplifier seems to work (sort of). Power
output is about 350 W. CW, 600 W. SSB. This is low (I think), but I don't
really know what to expect from this amp. The real problem is that I still
have an input SWR on 10 meters that is beyond the TS-450S tuner's ability
to fix. 15 meters is much better, but still not <2.
This leads to my questions:
1. Was I correct in concluding that the 100 ohm resistor should be in the
input circuit?
2. Do you think my problem with 10 and 15 meters is caused by excessive
inductance of my 3 in. wire from the input signal to the 100 ohm resistor?
If not, any ideas?
3. What kind of power should I be getting out of this amplifier? The power
in is between 80 - 100 W. (depending on band).
4. Is there any simple modification to the input network that would improve
the match with a driver like the TS-450?
Any other advice regarding this amplifier would be welcome as well. Thanks
for any help on this; and thanks to everyone contributing to the reflector,
it makes for enjoyable reading.
73--
John (KK5MH)
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