[Amps] looking for assistance with MLA-2500 tank circuit rework
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
> 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
> BTW, thanks to all who have offered up assistance so far. Everything is
> 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
> Hi Chris, 2700 ohms plate load R seems a bit high. What plate voltage and
> current was that figure based on?
> 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
> 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
> to work o.k.
> I decided to sweep the tank through the antenna, with a 2.7k ohm resistor
> place on the anode to simulate tube load and found that resonances
> 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
> 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
> now showing insufficient capacitance for load on all bands with a little
> 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
> 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
> 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
> else done the retrofit on these amps to the 3cx800 tubes, like this amp
> and did the amp still resonate as the original tubes did?
> Thanks for any help
> Chris VE3CEA
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