Ve6fi Amateur Radio
8877 Linear Amplifier
This linear amplifier utilizes two 8877 tubes. These are 3cx1500 tubes designed for grounded grid operation. They are indirectly heated tubes and are run in a parallel configuration. The filaments are warmed up for three minutes before high voltage is applied. The filaments are fed by a transformer located in the bottom chassis of the RF amplifier. On the left is the upper chassis layout during construction. The vacuum variable is a 10 to 300 ppfd. The coil is used for 80 through 10 meter operation. The band switches shown are linked by a slotted belt.
The input band switch is a phi network and it is located in the bottom chassis adjacent to the bases of the tubes. The adjustment of the L portion of the phi network can be done from the outside at the rear of the RF chassis. There is no fine tuning of the input circuit when operating the amplifier. The input relay is a standard 24v dc potted relay. It is designed to energize after the K2 in the output circuit and to release quickly. Protection for the tubes and sequencing for the power supply is provided by the Triode board. It is housed in the lower chassis away from the RF. It protects the grids. The filament transformer is also located on the lower chassis.
The output of the main HV transformer is 2400 AC. There is a boost transformer which has 240 volts on its input and 240 volts on the output. This is in series with the 2400 volt transformer to provide a total secondary voltage of 2640 volts AC. With this the output voltage from the rectifier assembly is a 3600 volts.
The rectifier assembly is rated for 6 amps at 10 kV. The filter capacitor is rated at 4 kV. Bleeder resistor are across the HV and will discharge the capacitors in 30 seconds after shut down.
The high voltage transformer, the boost transformer, the rectifier board and the filter capacitor are shown in the picture on the left. The boost transformer is a 240 to 240 transformer that is in series with the output of the 2400 volt transformer. Together that provide 2640 volts AC or 3600 volts DC.
On the picture on the left one can see the adjustment for the input circuit, the high voltage connector, the 'in and out' coaxial cables, the connector for the controls, the blower assembly and the ALC output.
The ALC circuit generates a negative voltage as more drive comes from the exciter. It is set up to feed back a more negative voltage as more drive is applied. This feedback will cut down the output power of the 1000MP.
The single tone grid current to an 8877 is 75 ma. The grid current meter on the front of the RF chassis reads 250 ma full scale. The grid trip on the system was set at 330 ma. So if there is even a spike in current that reaches this level, the PTT line is disabled and the red LED on the front panel comes on. (Grid Current trip). As soon as the PTT is energized again the tubes will be driven.
The plate current is protected such that if the plate current goes above 3 amps for any reason the HV power supply relay will drop out.
The front panel shows the grid and plate current meters as well as the vertical HV meter. There are two band switches, one for the input and one for the output.
LED's on the front panel show the status of the amplifier.
The amplifier was tested for linearity on all bands and the settings reflect the point of good linearity.
The Business end of things!
8877 Output Phi Network
RL = (E supply/2) * I plate
= 3600/2 * 1 amp
= 1800 ohms for one tube
Two tubes in parallelR L = 900 ohms
8877’s Plate Circuit Design & (Actual)
Max voltage across plate capacitor at 3 KW at Z of 900 ohms is E= Sqrt( 3X10*6 X 900 = 2.3 kv peak
C2 measured as 140 – 1200 mfd (measured with AADE)
Tube has 10 pfd output C and wiring has 20 pfds so C1 design values should be 30 pfd less than shown above.
8877Input network two tubes
.5 inch diameter coils with # 20 wire. Insulated wire
C2 has 80 pfd on output and 25 ohm so the design values above should be 80 pfd less
-** It is crazy but C1 needs a 4 inch length of wire to get low SWR!