MLA2500B/4CX800A Conversion

(prepared 18 April 1996, by Robert Schetgen, KU7G)

Refer to "New Life for Dentron MLA2500s," in May 1996 _QST_, 
pp 45-48.

Tube Cooling Chimney (Figure F)

     A 2.80-inch-diameter hole was bored through the Teflon 
to provide a loose slip fit on the anode cooler (cooling-fin 
assembly) of the 4CX800A. A smaller diameter would be too 
tight to allow for some thermal expansion, or not fit at 
all; any larger might allow too much air to escape _around_ 
the anode cooler, rather than going _through_ the cooling 
fins. The diameter on the socket end of the chimney can be 
anywhere between 2.9 and 3.0 inches. A smaller diameter 
would not clear the extensions for the socket mounting tabs; 
a larger hole would be too close to screw holes for 
satisfactory socket mounting. The height of the 3-degree 
tapered section should be about 1.5 inches, to match the 
distance between the bottom of the anode cooler and the 
socket. The length of the overlap of the chimney and anode 
cooling structure is not critical. My chimney has a 
rectangular external shape, which provides room to drill and 
tap two #8-32 holes. Mount the chimney on the inside of the 
back panel with two #8-32 screws to hold it in place.
     If you choose to mount the tube vertically (as were the 
original 8875s), gravity will adequately hold the chimney in 
position, and a thin-walled (about 0.1 inch) circular 
chimney would be fine. In this configuration, however, the 
cooling scheme would have to be different from that 
presented in the article.

Screen-Supply Capabilities (Figure K)

     The voltage/current characteristics of the regulator 
are shown in Figure K. The circuit can supply a maximum of 
about 13 W to the screen grid. (An hyperbola representing a 
screen dissipation value 15 W is shown on the figure at the 
upper right.) The "knee" in the curve depends upon a 
combination of Q2's beta and the value of R11 in Figure 1 of 
the article. Choose a value for R11 that places the "knee" 
at about 35 mA (try values between 10 and 18 ohms.) In any 
case, make sure the voltage is essentially constant from 0 
to 30 mA, and that no combination of output voltage and 
current is above the 15-W line.

Lab Test Results

     Figures L through R are spurious-emissions plots from 
ARRL Lab tests of the converted MLA2500B. Figure S is a plot 
typical of the IMD test. Regretfully, plots for the 30-m and 
15-m bands are not available. The amplifier was tested on 
those bands, and the worst-case spurious emissions were -42 
dBc for a 10.125-MHz test and -50 dBc for a 21.225-MHz test.

            ====================================
Figures

     All photos and line drawings are stored in .JPG 
(pronounced J-PEG) format; Lab plots are stored in .PCX 
format, as used by PC Paintbrush, which is distributed with 
Microsoft's Windows operating system. These formats were 
selected to conserve space. Both formats are supported by 
many graphics viewers, such as Paint Shop Pro, version 3.11.
     All of these graphics have been produced and displayed 
on a Gateway 2000 4DX-33 system with 8Mb RAM, running 
Windows for Workgroups Version 3.11. If you have trouble 
viewing these files, contact the ARRL Technical Secretary* 
and request the printed version of these files. Ask for the 
MLA2500/4CX800A Conversion package from May 1996 _QST_. If 
you are an ARRL member, send $5 (nonmembers, $7.50) to cover 
shipping and handling. (This does not constitute a 
recommendation of any software, format or product by ARRL.)

* ARRL contact information appears in the current issue of
  QST magazine, which is available at most public
  libraries.


Figure Captions

Figure
Letter    Title          Points of Interest
-------   -------------  ---------------------------------
  A       Front View     Grid-current indicator

  B       Rear View      Muffin fan, socket box, screen-and
                         control-grid bias box

  C       Chassis        Grid-current detector,
          Bottom View    indicator and PC board, R7,R3,
                         spiral-wrapped HV lead, screen-bias
                         supply (regulator in Figure I)

  D       Socket         Template for aluminum plate
          Plate          (0.050 inch thick) to cover
          Template       original fan cutout on back panel.
                         This template is full-size when
                         printed so that cutouts are round
                         and 1-inch scale on drawing
                         measures 1 inch on print.

  E       Socket         Socket pins and functions, R4 (A
          Assembly       through C), RFC (Z2), coax
                         grid lead connections, heater lead,
                         screen-bias lead, R5 (A through D),
                         C3 and C6

  F       Tube Chimney   Elevation view of cooling chimney
                         for 4CX800A tetrode. Make the
                         chimney out of Teflon, Delrin, or
                         nylon. Dimensions are in inches.
                         Only those dimensions that are
                         important for fit to the 4CX800A
                         tube and the SK1A socket are given;
                         choose other dimensions to suit.

  G       Top Rear       RF deck with relocated plate choke,
          View           tube chimney and parasitic
                         suppressor, high-voltage and
                         metering deck with spiral-wrapped
                         HV lead, PC1002 and time delay.

  H       Time Delay     Macromatic time delay on PC1002
          and PC1002

  I       Grid Bias Box  Control-grid bias circuit,
                         screen-grid bias regulator circuit

  J       RF Deck        Leads to socket assembly, chimney,
          (looking       cooling fins, parasitic suppressor,
          rearward)      relocated plate RF choke, plate
                         bypass cap, tank inductor

  K       Screen-Grid    Plot of voltage-versus-current
          Bias           characteristics of the screen
          Character-     supply. The hyperbola represents a
          istics         screen-grid dissipation of 15W, the
                         maximum value allowable for the
                         4CX800A.

  L                      160-m Spurious emissions plot

  M                      80-m Spurious emissions plot

  N                      40-m Spurious emissions plot

  O                      20-m Spurious emissions plot

  P                      17-m Spurious emissions plot

  Q                      12-m Spurious emissions plot

  R                      10-m Spurious emissions plot

  S                      Typical IMD plot

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Robert J. Schetgen, KU7G/1      American Radio Relay League
Assistant Technical Editor             225 Main St
QST Team                           Newington, CT 06111-1494
Hints & Kinks Editor  
ph: 860-594-0277 (direct)
ph: 860-594-0200 (switchboard)
fax: 860-594-0259                 email: rschetgen@arrl.org
