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

Carl km1h at jeremy.mv.com
Wed Jan 2 22:00:57 EST 2013

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.


----- Original Message ----- 
From: "Don Jones" <ko7i at comcast.net>
To: <amps at contesting.com>
Sent: Tuesday, January 01, 2013 7:16 PM
Subject: [Amps] looking for assistance with MLA-2500 tank circuit rework

> 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
> helpful.
> Chris
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