Modifications for the MLA-2500
Using the GI-7B Tube
The Dentron MLA-2500 amp is a very good amplifier to retrofit using the GI-7B tubes. This retrofit requires some metal cutting and moving of the RF sampling board and antenna relay. Look at the links to these retrofits at to see lots of pictures.
Here are schematics of the MLA-2500:
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.
Many of these modifications can be done as stand alone mods. Others have to be done in concert with each other. Be sure that you understand the modifications before you start.
73, Tony W4ZT