Push Pull Triodes for Class C

Description: The purpose of this document is to explain the rationale behind specific design choices and the selection of certain components in the circuit. The circuit diagram provided serves as a representation for explanation, and with further refinement, it could function as a Class C transmitter. Additional drawings will be included later as refinements for the final product. It is important to note that GIF drawings may lose detail when displayed at reduced sizes on the screen. Users can click on the drawings to view them in a new window and save them for printing. The link coil L1 serves as the input for RF excitation. L1, known as the grid tank coil, is an air core coil featuring a center tap on the main coil, with a few turns wound over the top center of the main coil. The tight magnetic coupling to the main grid coil typically eliminates the need for separate tuning of the link; however, a series or parallel tuning capacitor may be added to achieve a specific load resistance for the driving stage, which can be critical for standing wave ratio (SWR). A small antenna tuner may be utilized between stages for this purpose. The center tap is not intended as an electronic balancing point; rather, it serves as a grid leak path connection point. At the center of a balanced tank circuit, the RF voltage is minimal, making it an ideal point for inserting the DC connection of the grid leak path. Circuit balancing is achieved by ensuring equal capacitances from each end of the main coil to chassis ground. The grid tuning capacitors C1a and C1b provide this balance. It is crucial that the "split stator," as it is referred to, functions as a balanced capacitor, maintaining its balance across the entire tuning range from minimum to maximum capacitance. The RF voltage generated at the coil's ends must be equal and of opposite phase. The grid split stator tuning capacitor is typically mounted directly on the chassis, establishing a ground connection for the common rotor plates. This configuration allows both the grid leak DC bias voltage and the RF voltage to be present on the stator plates relative to the rotor plates. Care must be taken in selecting a capacitor with appropriate spacing between rotor and stator plates to prevent arcing across the capacitor's gap. Determining the exact center of the main grid coil can be challenging; therefore, a non-resonating composition type grid leak resistor (Rg) is used instead of a wire wound (WW) resistor, as the inductance from a WW resistor could disrupt the balance of the grid circuit. Additionally, this approach prevents the possibility of resonating the inductance of a WW resistor with the RF choke in the plate circuit, which could lead to low-frequency parasitic oscillations. Using a composition type resistor also offers a third advantage when the plate supply voltage is modulated with an audio AC voltage. As the plate voltage fluctuates at audio rates, so does the plate current, resulting in slight variations in the electron availability in the cathode space charge. Consequently, the grid current will also fluctuate, along with the grid leak bias. The grid leak bias voltage will follow the modulation voltage in phase across all modulating frequencies. This natural grid modulation is beneficial, contributing to proper modulation. In contrast, employing a WW resistor would introduce a phase shift in the grid modulation voltage at higher audio frequencies due to the reactance of the WW resistor's coil, which is an undesirable effect that could degrade modulation quality at these frequencies. The value of the grid leak resistor is determined using the formula Rg = Eg / Ig, where Rg represents the resistor's value in ohms, Eg is the desired grid bias voltage (typically referenced from the tube manuals for Class C operation), and Ig is the desired grid leak current (also generally derived from the tube specifications).

The circuit presented is a Class C transmitter design that emphasizes the importance of component selection and arrangement for optimal performance. The grid tank coil, L1, plays a critical role in RF excitation, and its design, including the center tap and link coil, ensures effective coupling and tuning. The use of a balanced capacitor configuration with C1a and C1b is vital for maintaining equal RF voltages at the coil's ends, which is essential for achieving the desired performance characteristics. The integration of a composition type grid leak resistor further ensures stability and performance, particularly in modulated environments, mitigating potential phase shifts and resonance issues that could arise from alternative resistor types. Overall, the careful consideration of each component's role within the circuit contributes to the reliability and efficiency of the Class C transmitter design.The purpose of this document is to answer the question of why things are done in a certain way and why certain components are used instead of others. The circuit below is a representation drawing to be used for explanation, but with some refinement could be a working Class C - Transmitter.

After the general explanation more drawings will be shown as refinement for a finished product. Please note that the GIF drawings lose some of their detail if shown reduced on screen. You may click on the drawings to show them in a new window and save them for printing. The link coil of L1 is the input for RF excitation. L1 (known as the grid tank coil) is an air core coil with a center tap on the main coil and a link of a few turns iswound over the top center of the main coil. Because of the tight magnetic coupling to the main grid coil, it is generally not necessary to tune the link separately; however a series or parallel tuning capacitor can be placed on the link if it is desired to produce a specific load resistance to the driving stage that may be more critical of SWR.

A small antenna tuner may be used between stages for this purpose. The center tap is not used as an electronic balancing point but instead it is used to provide a grid leak path connection point. At the center of a balanced tank circuit, there is very low RF voltage so it is an ideal point to insert the DC connection of grid leak path.

The balancing of the circuit is accomplished by having equal capacitances from each end of the the main coil to chassis ground. The grid tuning capacitor C1a and C1b provide this balance. It is very important that the "split stator" as it is called, be a balanced capacitor. It must maintain its balance across the tuning range, from minimum to maximum capacitance. The RF voltage that is produced at the ends of the coil must be equal and of opposite phase. The grid split stator tuning capacitor is generally mounted directly on the chassis providing a connection to ground for the common rotor plates this means that the grid leak DC bias voltage, as well as the RF voltage, will be present on the stator plates with respect to the rotor plates.

Care should be taken in choosing a capacitor with the proper spacing from rotor to stator plate in order to handle these voltages without arcing across the gap of the capacitor plates. The exact center of the main grid coil is somewhat difficult to determine and never exact therefore we use a non resonating grid leak resistor (Rg) of composition type, not wire wound (WW), else the inductance of the WW resistor might cause an imbalance of the grid circuit.

This also eliminates the possibility of resonating the inductance of a WW resistor with the RF Choke in the plate circuit causing a low frequency parasitic oscillation. A third advantage to using a composition type resistor is realized when the plate supply voltage is modulated with an audio AC voltage.

When the plate voltage varies at an audio rate so does the plate current. This causes a slight rise and fall in the available electrons in the cathode space charge. In turn the grid current will rise and fall as will the grid leak bias. The grid leak bias voltage will follow the modulation voltage in phase at all modulating frequencies. This natural grid modulation is a good thing and helps provide proper modulation. If a WW resistor were to be used it would cause a phase shift of the grid modulation voltage at the higher audio frequencies.

This is due to the reactance of the WW resistor`s coil. This phase shift is an undesired effect that will make poor modulation at higher audio frequencies. The Grid leak resistor is determined by the formula Rg = Eg / Ig. Where Rg is the resistors value in Ohms, Eg is the desired grid bias voltage (usually obtained from the tube manuals for the Class C service for the tube that is used), and Ig is the desired grid leak current (usually obtained from the tu