This is the description of the modification I did to the Amptec LA-1200 Amplifier.
This amplifier is made in China and I purchased it in December 2015.
I ordered it directly from the manufacturer.
Their web site is at http://www.amptec.cn
Construction is excellent and the quality of components is superb.
The three (3) 4CX-250Bís were replaced with a single 4CX800A tube.
The installation Is cleaner and performance is incrementally better.
If you have questions you can contact me directly at: firstname.lastname@example.org
Here is the front of the amplifier.
Here is the rear of the amplifier.
The next photo shows the power supply side of the amplifier. Toroid power transformers are used throughout. The large one at the right is the HV transformer.
The smaller one on the left is for filament, bias, screen and 12V. There are two boards mounted above the transformers. The one on the left is the DC board
while the one on the right is for AC and switching.
Here we have a top view of the DC board. It has DC fuses, HVDC rectifiers/filters and the basic screen
and bias supplies.
The AC board is shown below.
Here we have the three 4CX250B/F tubes with the chimney removed. The wire is the air flow temperature sensor. Each tube also has an individual
chimney. Either 4CX250B or 4CX250F tubes can be used as the LV transformer has windings for both.
The Pi-L network. Amptec makes their own variable capacitors
Finally, here is the stock amplifier with the outer chimney in place.
The modifications start here. The 4CX800A has a 12.6V heater so another filament transformer
was needed. The easiest way was to add another toroid on top of the existing LV toroid.
A new metal plate (copper) was made and the 4CX800A socket was mounted to it, making sure the
position of the socket is aligned with the cabinet air flow inlet/exit. Also shown are the cathode resistors.
Here is the PCB that has the swamping resistor, RF input relay, and diplexer for the swamping resistor. I had this board professionally
Designed in the UK and fabricated in China.
Here is the stuffed board. The swamping resistor is not shown as it mounts below the PCB on the copper plate.
Here is the PCB mounted to the copper/plate socket assembly.
Another view from the side. The lead to the swamping resistor and the resistor itself can be seen
near the right stand-off.
The socket, copper plate and RF sub-assembly box is shown in this photo. Please note that here the swamping resistor has not been installed yet.
The two holes for it are clearly visible. In the bottom of this box is a screen regulator, and cutout relay. The small PCB to the lower right
mounts the antenna relay.
Finally, here is the completed PA modification. The Delron rod holds the air flow temperature sensor. A more robust plate choke was
also constructed out silver plated brass. The chimney is a commercially available unit.
This amplifier has an impressive color LCD screen. Here is a photo of it with the amp in operation. The colors
are actually more vivid but are slightly over exposed here.
Here is my Bird 43P showing 800W out.
This page was last updated on 1/22/2018