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

Carl km1h at jeremy.mv.com
Tue Jan 1 20:57:44 EST 2013

One 3CX800A7 is equivalent to a pair of 8874/8875's in power and load line.

Since the MLA PS wont support a pair of them just remove one and try again 
with the circuit as built.
GM3SEK's software is about as good as it gets.


----- Original Message ----- 
From: "Chris Miller" <c_miller_1 at hotmail.com>
To: <TexasRF at aol.com>; <amps at contesting.com>
Sent: Monday, December 31, 2012 9:46 PM
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit rework

> 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
> From: TexasRF at aol.com
> Sent: Sunday, December 30, 2012 7:59 PM
> To: c_miller_1 at hotmail.com ; amps at contesting.com
> Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit 
> rework
> Hi Chris, 2700 ohms plate load R seems a bit high. What plate voltage and 
> current was that figure based on?
> 73,
> Gerald K5GW
> In a message dated 12/30/2012 2:49:54 P.M. Central Standard Time, 
> c_miller_1 at hotmail.com writes:
>  Hi to all,
>  I have taken on a project that has been bugging me for a bit.  I have a
>  MLA2500 that I picked up that has been retrofitted with a pair of 3cx800
>  intending to have a transportable amp.  From what I have read the tubes 
> are
>  supposed to be a close match for the original tubes, as far as tube specs
>  go.  However, I have found that the amp on 160 through 40 has to have the
>  load control set to 1 to get close to load on power.  20 through 10 
> appeared
>  to work o.k.
>  I decided to sweep the tank through the antenna, with a 2.7k ohm resistor 
> in
>  place on the anode to simulate tube load and found that resonances 
> appeared
>  off on lower bands.  Figuring I had a good handle on the process, I
>  prototyped up a new tank coil to make mistakes on.  I then playing with 
> tap
>  points and turns until I achieved what looked like a good set of patterns
>  for each band, for the range of tuning.
>  When I attempted to test my results, on dummy load conditions, all bands 
> are
>  now showing insufficient capacitance for load on all bands with a little 
> bit
>  of range at near max capacity on 10 near the upper end of the band.
>  Since that seemed to be an epic failure, I must be missing steps in the
>  process.  Is there something I should be watching for or doing so that 
> what
>  I see as tank resonance actually lands where it should on real world
>  testing?  Am I now at the point where I have to do some trial additions 
> of
>  capacitance to the load control?  Is this amp worth the effort to make it
>  work properly (realizing it is a bit of a sardine can in there)?  Has 
> anyone
>  else done the retrofit on these amps to the 3cx800 tubes, like this amp 
> has,
>  and did the amp still resonate as the original tubes did?
>  Thanks for any help
>  Chris VE3CEA
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