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

[Carl]

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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