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

Chris Miller c_miller_1 at hotmail.com
Wed Jan 2 12:07:13 EST 2013

Pretty sure I have it.  I pulled as much info on the various conversions to 
see what the history of the changes were.  Mainly was looking for any 
significant differences to see if maybe there was a preferred method.  The 
actual conversion has already been completed by the previous owner.

The tank circuit originally seemed to progressively get worse and worse as 
you went down in frequency, for load capacitance being too small, until you 
got to 40m - 160m, at which point you reached full mesh.  It was not way way 
off though, as there was some peaking of power output just not enough to 
actually reach the resonance point.  Pretty well the same behavior I have 
now except I managed to make it happen on all bands.  Oddly enough, when it 
is swept with the VNA it looks ok.

After staring at the spec sheets for the tubes again, I tried to change the 
value of the tube resonance load resistance during sweep tests to 1350 ohms, 
if my thinking is right two tubes in parallel, to see if it would produce 
the same operation I was seeing during my dummy load tests.  It actually had 
the opposite effect.  Most bands had way too much load capacitance for 
resonance.  Fully reinstalling the covers with screws rather than just 
placing them in place had little effect on the sweep tests.

Another user of this old gear reported that, both modified and not, these 
amps were shy on capacitance and that extra capacitance had to be added for 
80 and 160.  I am beginning to think that the values of the RF caps are 
pulling under power, but I would have expected the pull to change with the 
level of power.  It is more likely though I am not understanding the 
interpretation of the spec sheet completely.

I was reminded that the 3cx800 is twice as capable as the original tube, 
even more when you read the fine print on the 8875.  The 8875 shows a duty 
cycle on some of the values of 50% so there is less power available for 
modes like RTTY etc.  I am going to try, for curiosity sake, to see if it 
will run with just one tube.  Thinking is, if the values work for one tube I 
end up with a spare.  The downside is I throw away headroom.


From: "Adrian Rees" <rees.a at btconnect.com>
Sent: Tuesday, January 01, 2013 3:39 PM
To: "Chris Miller" <c_miller_1 at hotmail.com>
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit rework

> Hi Chris
> Have you got a copy of the Svetlana document about retro fitting 3CX800's 
> to the MLA2500 ? I have a copy kicking around (in PDF format) if you 
> havn't got it. Also note that some guys have also swapped in 8877's in 
> place of 8875's. Identical in every way, except for cooling requirements. 
> 8877's are still available cheaply.
> Additionally a chap down Florida way has done the same conversion, 
> successfully I understand. If you can't find him (I suppose a Google 
> search would yield the info), I can dig out his email address. He did 
> subscribe to the Amps forum at one time, but left a while back.
> I was in correspondence with him regarding my 2500, although that was 
> Power supply related.
> My 2500 also had a problem with the door knob capacitors in the tank 
> circuit, going open circuit. Difficult to spot, but caused by Fan 
> vibration over the years, opening the tabs & also dry joints.
> Cheers
> Adrian MW1LCR / GW9X
> On 29/12/2012 03:49, Chris Miller wrote:
>> 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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