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

[Chris Miller]

I tried a couple of programs to see if the values in the existing circuit 
were even in the right area.  PiEl seemed to be the easiest to play with the 
values to get a result that seemed to match what was in the amp originally. 
That at least made me feel that there was some hope for the original 
component values.  I tried another program, SimSmith.  It seemed to be 
useful for assembling a number of components together and then generating a 
smith chart of the result.

As far as the actual tank that I prototyped up, the thinking was 'don't 
destroy the original tank playing with the turns and tap points'.  Match it 
as best as I could, replace it, and then move the tap points and add turns 
as needed until the results looked better on the miniVNA.  Then, duplicate 
it with out all the fiddle points so that it was cleaner looking.  I hate 
just hacking something in to make it work, but I had to start somewhere.

I would have liked to have gone through the process of engineering up the 
system from scratch, math wise, as a training exercise.  However, I found 
little teaching material that would help me through the process so far 
(limited tube experience and in linear amp design).  Regrettably, college 
stopped short in the area of linear amp design.   Any designs were of micro 
power at best or in a non RF environment and tubes had pushed off the 
training books as, well, technology was marching on.

I may try to build a single band amp with some smaller tubes I have kicking 
around here at some point just to put math to practice.  I don't want 
everything to be brand name in here :-)






--------------------------------------------------
From: "Carl" <km1h at jeremy.mv.com>
Sent: Monday, December 31, 2012 8:26 PM
To: "Chris Miller" <c_miller_1 at hotmail.com>; <amps at contesting.com>
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit rework

> What program did you use to calculate values?
>
> Carl
> KM1H
>
>
>
> ----- Original Message ----- 
> From: "Chris Miller" <c_miller_1 at hotmail.com>
> To: <amps at contesting.com>
> Sent: Friday, December 28, 2012 10:49 PM
> Subject: [Amps] looking for assistance with MLA-2500 tank circuit rework
>
>
>> 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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>
>