Update:

The fixed loading capacitors on the MLA-2500B for 75, 160 meters are both
1000pf per Dentron parts list whereas they are 100pf on the MLA-2500 parts
list. It looks like a typo in the documentation. Then I measured my 1000pf
capacitors in my amp, these both measured about 820pf on my capacitance
bridge at 4 Mhz. For sanity sake, I also measured a 820 pf silver mica
capacitor, and the bridge read 850pf @ 4 Mhz. So it seems that I uncovered
two problems:

* Dentron MLA-2500 parts list has typos in it

* My two loading capacitors for 75 and 160 are out of tolerence on the low
side. This explains why my variable loading capacitor needs to be at full
mesh.

Anybody have a 8877 Teflon chimney they want to get rid of?

Jim
WD5JKO

"JC" wrote in message
. 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