2008 Retrofit to an MLA2500 amp using the Eimac 3CX800 ceramic triodes
done by Lawson, W4EMFhttp://w4emf.gs35b.com/mla2500-retro-3cx800-2008/index.html
Tony King, W4ZT came up with a schematic diagram (July 2007) for converting the Dentron MLA-2500 to use TWO Russian surplus GI-7b ceramic triodes.http://gi7b.com/mla2500/mla2500mod.pdf
W4EMF and K4POZ provided assistance and suggestion.http://w4emf.gs35b.com/mla2500-retro-gi7/index.htmlhttp://gi7b.com/mla2500/index.html
Modifications, in no particular order, that are shown on the schematic above:
GI-7B tube retrofit.
Cathode choke move from the ALC board to under chassis cathode connection.
Input coupling capacitor change.
Cutoff resistor and bias change. Adjustable Bias Board and bypass caps added.
Glitch resistor change from a fuse resistor to a true glitch resistor.
New grid shunt and protection diodes. Resistor value not specified on schematic.
New plate current shunt and protection diodes. Resistor value not specified on schematic.
Metering calibration resistors and pots added.
Harbach SS-100 Step Start module added.
Antenna Relay change to 3PDT Relay to allow keying the amp only after Antenna contacts are made. The third set of contacts is put in series with the bias keying relay and prevents hot switching the amp.
Filament transformer added.
Filament choke and bypass caps added.
New and added filter capacitors, equalizing resistors and reverse protection diodes added.
Dropping resistor in series with delay tube filament to make up for the unloaded filament winding. This value is found experimentally.
Meter switch changes to properly switch the metering and calibration.
Based on this work, other amateurs have successfully documented this conversion.
Dentron MLA-2500 GI7B Modification
by N3OC, February 2013http://n3oc.dyndns.org/GI7B.htm