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

To: "Chris Miller" <c_miller_1@hotmail.com>, <amps@contesting.com>
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit rework
From: "Carl" <km1h@jeremy.mv.com>
Date: Mon, 31 Dec 2012 20:26:44 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>
What program did you use to calculate values?

Carl
KM1H



----- Original Message ----- From: "Chris Miller" <c_miller_1@hotmail.com>
To: <amps@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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