Tony,

I appreciate the comments. For sure there is something awry in this
pi-net. I looked at your tutorial on your web page, and wow, you know your
stuff!
The schematics of the 2500, and 2500B are at the following link:

ftp://bama.sbc.edu/downloads/dentron/mla2500/

Another aspect of this pi-net is that the tank coil consists of tight
wound silver plated stranded Teflon insulated wire of about 10 awg. Users of
healthy 2500's report 1100 to 1300 watts key down (output), and 1500 watts
PEP out with a pulser. This is very good for the size and weight of this
amplifier.

The loading variable cap is about 50-500pf with a 100pf Centralab in
parallel with that. The bandswitch is not a shorting type, so it adds 200pf
on 40m, 1000pf on 75, and 1000pf on 160 (all on 2500B schematic and parts
list). These do not add up due to the type of rotary switch used. I'd prefer
a shorting switch arrangement where on 160m, all the fixed loading caps are
in parallel and each unit has less capacitance.

I will look more carefully at the capacitors used to see what the part
number is. Maybe the correct values are in there, but of the wrong type. I
plan to use my impedance bridge from the 50 ohm amp output looking into the
amp, and replace the 8877 with the calculated plate load RL going into the
pi-net. Hopefully a 1/2 watt carbon comp resistor around 2K (a guess) will
represent the plate side impedance. My hope here is that I can evaluate the
pi-net 160-10m at a low level (0dbm?) to see what the impedance matching
capability is. Maybe the values used are correct, but with high power, the
capacitors get lossy (hot), drift down in value, and cause the efficiency of
the amp to really stink.

I'll let you'all know what I find.

Thanks,
Jim
WD5JKO



"Antonio Vernucci" wrote in message
...
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




..


"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