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

To: "Don Jones" <ko7i@comcast.net>, <amps@contesting.com>
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit rework
From: "Carl" <km1h@jeremy.mv.com>
Date: Thu, 3 Jan 2013 13:21:52 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Subject: RE: [Amps] looking for assistance with MLA-2500 tank circuit rework


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.

** Well, just a FT-901 couldnt be the reason for 1800W so maybe the resistor was bypassed, a Dahl transformer was used, meter reading combined Fwd and Ref power, or various combinations. I could barely get 1800W out of my 76PA at 100W and that is 3X 8874's with the low drive mod. Im much happier at 1200W considering replacement cost.


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.


** Guaranteed Im correct there, that is right from Dentron when I used to have transformers built locally and is also in the Dahl catalog.


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


** I get 1300-1400 RL usings Ians spread sheet and moving efficiency around a bit since the MLA output circuit isnt the best in the world. Its not worth even arguing about since there are multiple variables and its well withing range of what was built. The single 3CX800 should work just fine as long as the drive is reduced to 30-50W and live with whatever output results. Trying to push too hard is just going to cause problems in the tank components and PS. At 1000W out that tube is far from stressed.

Carl
KM1H





-----Original Message-----
From: Carl [mailto:km1h@jeremy.mv.com]
Sent: Wednesday, January 02, 2013 7:01 PM
To: Don Jones; amps@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.

Carl
KM1H



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