[AMPS] Re: MLA-2500B

[Steve Thompson]

-----Original Message-----
From: Phil Clements <philk5pc@tyler.net>
To: amps@contesting.com <amps@contesting.com>
Date: 29 July 2000 19:46
Subject: [AMPS] Caveat on Dentron Filaments + MLA-2500B Notes


>As you can probably guess by my posts, I am refurbishing
>a Dentron MLA-2500B amp.


Heater voltage problems are even worse in the UK, where mains can be 240+V.

I modified the wiring to run the fan all the time mains is on, so the tubes
get cooled down fully after use - I wasn't happy with burning my fingers on
the pins even 10 minutes after shutting it down.

I also changed the grid meter shunt to get 200mA FSD.


I've been told that Svetlana can fit a scrap 8875 cooler onto their 8874 at
production, although I suspect that it needs the right personal contacts.
Presumably the 'core' diameter is the same on both, so it might be possible
to do it aftermarket.

Steve
This model, and most of the Dentron power transformers
were wound for 234 volt mains. (Alpha's were also, for that matter) I
measured 6.75 volts on the tube pins of
those expensive 8875's in the 2500B. I wound six turns
of #22 teflon covered wire on a one inch form and placed it in series with
the side of the filament winding that goes to ground. This reduced the
voltage at the tube pins to a comfortable 6.18 volts. This will also serve
as a "bump" for any in-rush current that may or may not be there. (I don't
have anything to measure inrush with)

I am very interested to chat with anyone that has played with MLA-2500B's.
The padder door knob caps vary from model to model, and even within a model.
Since the load cap is only 500 pf, this presents a very narrow "window"
for matching various loads on 160 meters, as most of the
capacitance is fixed in the padders. Also, various values
of padders on the tune cap are used from model to model
depending on what value it is...140 pf on the older models and 170 pf on the
"B" model.

All in all, the MLA-2500B is a sweet, compact, legal-limit
amp with no bad habits and very smooth tune-up. Too bad
that the 8875's are becoming extinct!

(((73)))
Phil, K5PC


Phil Clements
410 VZ CR 4903
Ben Wheeler, TX 75754




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