It is unlikely that the fixed loading capacitor is as high as 1000 pf, except for 160m. However if Dentron has really used a Centralab 1000-pf capacitor as fixed loading capacitor, that is a sign of bad design, because those capacitors are not suitable for that purpose.

As a matter of fact the Centralab 1000-pF capacitors use a different ceramic material than the 100 pF units, and they therefore have a very different behavior, as explained below:

- the 1000-pF units can withstand high RF current, but cannot withstand high RF voltage at the same time (DC voltage does not matter). A more rigorous way to express that concept is to say that the 1000-pf units cannot withstand high "reactive power" (reactive power = RF current times RF voltage). In practice they are only suitable for bypassing or plate-to-pi coupling (high RF current but very little RF voltage).

- on the contrary, the 100-pF units are suitable for operation in resonating circuits, where both the RF voltage and the RF current is high (= high reactive power). So they are suitable as fixed loading capacitors.

If one uses a Centralab 1000-pF capacitor as the fixed loading capacitor of a high-power linear, the capacitor will heat up and change its capacitance, so detuning the circuit. Prolonged high-power operation may result in capacitor failure.

73

Tony I0JX




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"JC" ha scritto nel messaggio . com...
Hi Group,

I have a 2500 that had one bad 8875, and as-is EBay buy, and I am in the
process of a conversion to a single 8877. This may not be the best choice of
a tube for the application, but I had a good one in a box for nearly 20
years, so I decided to give it a try. Before the conversion I powered it up,
and with one tube I could get 450 watts on 80M with 50 watts drive. I did
notice however that the variable loading capacitor had to be at full mesh on
any band I tried. I blew off that issue because with only one tube the
PI-Net was not seeing the as-designed input impedance.

My question is from my blurry schematic and parts list. The fixed
Centralab loading capacitors in my rig are all 1000pf, and the fixed plate
tuning side capacitors are all Centralab 100pf units. The parts list does
not show any 1000pf HV Centralab capacitors. Is there a typo in the parts
list, or does my amplifier have the wrong capacitors installed in the
loading area?

I powered up the amplifier last night, and I can get 600 watts RF out on
80 meters with 50 watts drive. The cathode current reaches 0.8 ampere at
1700 volts Plate. So that is 1360 watts in for 600 watts output. That is
about 44% efficiency. The variable loading control always wants to be at
maximum mesh for maximum RF output. On 160, 40, and 20m meters the loading
variable capacitor must also be at maximum mesh. This makes me think my
PI-Network is not correct for the impedances involved, and then it sure
appears that the Dentron documentation does not match my rig. I might have
to reverse engineer that PI-Net.

So does anyone have a better set of drawings to see if their MLA-2500 PI-Net
parts match the drawings?

Regards,
Jim
WD5JKO