[Amps] looking for assistance with MLA-2500 tank circuit rework

Don Jones ko7i at comcast.net
Thu Jan 3 10:47:01 EST 2013

OK now I will drop my proverbal pants the rest the way, I distinctly
remember the station owner loaded up his his FT901 for 100W drive and then
loading up his MLA-2500 for 1800W out on his watt meter. I remember thinking
it was pretty ballsy but I saw it with my own two eyes.
I know this much though, the next I went back to operate his station a
couple years later, the MLA-2500 was long gone. 
So in the name of playing nice, lets assume Carl is right about the
transformer rating of the MLA 2500.
Using his numbers of 2,000V plate @ 0.9A for class AB I come up with a plate
Z of 1,481.48148 ohm. Chris needs to rescale his tank circuit components
from a 2700 ohm plate Z downwards. 
See you-all in the pile-ups.
Vy 73, Don KO7i

-----Original Message-----
From: Carl [mailto:km1h at jeremy.mv.com] 
Sent: Wednesday, January 02, 2013 7:01 PM
To: Don Jones; amps at contesting.com
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit rework

The MLA-2500 runs a pair off 8875's rated at 300W Pd each. As designed it
could do 1200W SSB PEP OUTPUT  when driven hard but it is a stretch for CW,
especially in a contest but when tubes were cheap, and the legal CW INPUT
was only 1000W contesters were particularly known to take advantage of the
extra 3dB that was available on many amps of that era.

Plate current was maxed out at about 800ma which is also what a 3CX800A7 is
capable of.
The transformer is rated at 800VAC at 900ma CCS; that is a long way from an
assumption of 2.5 to 3 kVA. In reality the plate voltage under load is
around 1900-2000 depending on line voltage and when run properly 1000W was
about all it could do.

Many used to bypass the input swamping resistor and get a few hundred watts
more out using their tube rig drivers with the resultant short tube life.

I have a MLA-2500 as a backup, got it from the original owner with original
tubes. I drive it at 60-70W for 1000W output on a 246VAC line so the voltage
is up a bit from the 234VAC design center. The main amps for the 2 stations
are a LK-500ZC and an Alpha 76PA 3 holer so the backups are only used once
in awhile to get exercise. The other backup is a Dentron DTR-2000L.


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