Re: Dentron MLA-2500 Question


JC wrote:
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.

Good luck on that tube. The early to mid 80's were very bad years for
8877 production. If you get a good tube it will last many years. If you
get a bad one it might fail in hours.

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.

Dentron always had messed up tank values. Just look at the values they
picked. They typically ran excessive Q on 160 meters and 80 meters. As
I recall Q was in the 20's on 160 in many amps they made. They had the
idea "one tank coil fits all".

The only thing that hurts is circulating currents in the tank are a bit
high and the loading control is swamped out by too much shunting
capacioatnce so it has very little range.

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?

Dentron used what they could buy. How do I know? I worked there for a
short period and watched them do that. They would often make parts runs
to surplus stores in Cleveland for components.

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.

It was never engineered in the first place. It is seriously lacking on
inductance.

Be aware also the larger ceramic caps have a high temperature
coefficient. While they will marginally handle the current, any
temperature change seriously affects tuning.

If you really want to fix it, start from scratch.

While you are at it get rid of the carbon resistors across the filter
caps. Get rid of the carbon resistors across the diodes also. Any high
dissipation critical resistor should be a metal component, not carbon.
If you don't change the carbons across the filter caps to metal films
or wire wound they will eventually fail and take out the electrolytics.
If that particular amp has small carbons across the diodes they need to
be removed.

73 Tom

.

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