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

Don Jones ko7i at comcast.net
Tue Jan 1 19:16:45 EST 2013

In my former past as a hired gun CW contest op I seem to remember the
MLA-2500 was capable of 1,200W out with ease. 
Assuming class AB operation that means the power supply was capable of 2.5
to 3.0 kVa (or so). Therefore the power supply should be able to produce 1.2
to 1.4 ADC @ 2200V. 
Using the formula's in the ARRL handbook(s) I calculate the plate impedances
to be as follows:
For Class AB operation I come up with a plate Z of 1,050 to 1,200 ohms.
For Class B operation I come up with a plate Z of 1,000 to 1,168 ohms.

Bottom line, that power supply is capable of turning that amplifier into a
1st class flame thrower with the correct tank circuit components. 
Given the history you have stated, I would carefully go thru that tank
circuit and check it for every band. As I recall the MLA-2500 did not come
with a tuned input. You should look into adding a pi-network tuned input to
the amplifier. 
Vy 73, Don KO7i


Message: 1
Date: Mon, 31 Dec 2012 21:46:55 -0500
From: "Chris Miller" <c_miller_1 at hotmail.com>
To: <TexasRF at aol.com>,	<amps at contesting.com>
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit rework
Message-ID: <BLU165-ds1926F7CE8E0DA1A195E88A8E230 at phx.gbl>
Content-Type: text/plain;	charset="iso-8859-1"

Before I go too much further, a little framing of my background with tubes.
Basically, I have very limited formal training with them directly.  Any work
has been repairing to bring back to schematic with tube gear so far.  My
training was during the transition from discreet component transistors to
large scale IC and some RF background so I am not totally at a loss,
thankfully.  So please feel free to ask what ever questions you think may be
necessary, especially when it comes to the operation of the tubes.

I was using the value that was shown in the Eimac technical data sheets for
their version of the tube, as that was the tubes in the amp.  However, there
is some doubt for me as to whether or not they were referring to one or two
tubes in the typical operation spec area for the sheet.  It didn't say
specifically two tubes on this sheet, but on another sheet showed the class
AB area as being two tubes ( that may be helping to lead me astray) I
presumed it was for two tubes as the power shown in the typical specs
exceeded the max ratings area specs, if I was reading it right.  If the
specs are for one tube, than I have done my tuning with the resistance twice
what it should be which will throw off everything I did.  The original tube
sheets showed a value of 2140 ohms, but again they were not totally clear on
whether they were using two tubes or one in the typical operation area.  

The CW plate current (0.5 A), plate voltage (2200 V) and bias voltage (8.1
V) listed in the spec sheet seemed to line up well with the limits listed in
the Dentron manual, but I imagine the regulation may not be the best, for
either the plate or bias supplies, as it is done old school from what I have
read so far online.  (I am going to have to watch the drive closely with
this thing, if I get it working as it should)  I am thinking of putting a
board from GM3SEK in it, IF I can get the tank to work as it should.

BTW, thanks to all who have offered up assistance so far.  Everything is


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