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

Carl km1h at jeremy.mv.com
Thu Jan 3 13:21:52 EST 2013

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.


> -----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.
> Carl
> KM1H
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