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

[Chris Miller]

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