NOTE: ALC as used in the 30L1 means Automatic Load Control ---not Automatic level control!
1. The voltage divider as seen in note (4) above consists of the plate to grid inter electrode capacity (5pf) and the (220pf) capacitor that is added from grid to ground. The divider couples negative feedback to the grid that will be 1/40th of the plate peak to peak voltage swing. The photo above schematic is a more logical depiction of the 30L1 and can be found in (Sideband Principals And circuits) which was authored by the Collins engineers who participated in the 30L1 design review. It is also, described in the users manual 3.7
2. The voltage divider provides a fraction of the plate voltage which is used to:
a. Generate negative feedback to improve MDS and linearity.
b. Effect neutralizing. At max plate PP 1300 volts, it applies 22 volts of negative feedback to the grid. This balances the 20 volts of destabilizing positive feedback from plate to cathode.
The voltage divider used in the 30L1 is the same divider that is used in grid driven triode and tetrode amplifiers.
c. Input to the ALC/Automatic LEVEL Control rectifier line to control the exciter.
d. Input to the ALC/Automatic LOAD Control comparator. Described below.
NOTE: There are a number of people who believe that the current that flows from cathode to grid during the conduction cycle distorts the phase relationship of the drive but it has little if any effect. The 30L1 uses an RF choke to separate the DC current flow from the cathode to grid sinking it to ground leaving superimposed pulsating DC which is coupled through the plate blocking capacitor in the form of AC to the PI section. There is no AC current flowing in the plate or grid chokes, nore is there AC in any other choke in any other amplifier. The energy stored in an RF choke during the conduction cycle is inserted during the nonconducting cycle, effect similar to push pull.
The above schematic is a more logical and easy to follow description of the 30L1 with the negative feedback components including the plate to grid capacity. The voltage divider NOTE (4) provides approximately 22 volts of negative feedback ( compression to the grids). The output of the voltage divider is used by the tune comparator to measure the plate PP voltage which is compared to the cathode drive voltage. When the meter tune comparator is adjusted properly as per 4.7 in the manual, the the tune meter will dip to (0) with 300 ma of plate current: indicating resonance and proper load throughout the power range.
The tune logic enables you to tune the transmitter at reduced power 300 ma plate resulting in correct loading throughout the power range. Tuning the amplifier to max output power will be done once per operators manual 4.7, NOT every time you change bands or antennas. Simply tune the transmitter for dip to (0) with 300ma of plate and the transmitter will be properly loaded and ready to operate at full power --- just increase the drive.
The upper leg of the comparator is driven by the voltage divider representing a fraction of the plate PP voltage and the bottom a fraction of the cathode voltage/drive. The top leg will develop a negative - voltage relative to the plate PP voltage and the bottom will develop a positive + voltage relative to the cathode drive. When they cancel the tune meter will read (0).
NOTE: The comparator was calibrated with the transmitter tuned and loaded to full power then the drive was reduced to 300 ma of plate current where the bridge was adjusted for dip to (0). You can now tune the amp at reduced power a very unique feature of the 30L1 design.
NOTE: If there are any modifications to the grid logic i.e. larger capacitors on the grids, directly grounding the grids, replacing the choke with something else, the comparator and tune meter will be useless.