Pushpull Amp

Description: With all DHT stages (all tubes from Svetlana 1970-s). OK, the input/splitter are not DHT`s but DHP`s - 4P1L - triode wired. Mullard 3-tube input/splitter topology, cap-coupled to cathode bias 6B4G PP. First stage uses battery bias (no cathode resistor). Only paper capacitors everywhere. Gas regulators for the first stage. Later, I replaced Mullard splitter with IT splitter in a Loftin White arrangement. It has gain about 90 at the second stage plate. Next steps: rewire IT to a step-down ration (right now it`s a stock 1+1:1.25+1.25) and replace first stage load for a G.

The circuit described utilizes Directly Heated Triodes (DHT) from Svetlana, specifically from the 1970s, which are known for their warm sound and low distortion characteristics. The input and splitter stages are composed of DHTs, however, the input stage utilizes DHPs (Directly Heated Pentodes), specifically the 4P1L triode wired configuration. This configuration is advantageous for achieving a high linearity and low noise floor, which is critical in high-fidelity audio applications.

The Mullard 3-tube input/splitter topology is employed to provide a balanced signal and improve the overall gain structure of the circuit. This topology is known for its effective signal handling and minimal phase shift, which is beneficial for maintaining audio fidelity. The coupling between stages is accomplished through capacitors, with a focus on using paper capacitors throughout the design. Paper capacitors are preferred in high-end audio applications due to their favorable sonic characteristics and low dielectric absorption.

The cathode biasing of the 6B4G push-pull (PP) output stage is another critical aspect of the design. This configuration allows for a more stable operation of the output stage, providing a richer sound with better control over the output transients. The first stage employs a battery bias system, eliminating the need for a cathode resistor, which can introduce unwanted noise and distortion. This design choice enhances the overall performance of the amplifier by ensuring a more consistent operating point for the tubes.

Additionally, gas regulators are used in the first stage to maintain a stable voltage, further improving the reliability and performance of the circuit. These regulators help to mitigate fluctuations in the power supply, which can adversely affect the sound quality.

Subsequent modifications to the circuit included the replacement of the Mullard splitter with an interstage (IT) splitter in a Loftin White arrangement. This change is expected to enhance the amplifier's performance by providing a higher gain, measured at approximately 90 at the second stage plate. The next steps involve rewiring the interstage transformer to a step-down ratio, transitioning from the current stock configuration of 1+1:1.25+1.25 to a more suitable design for the intended application. Additionally, there are plans to replace the first stage load with a different component to further optimize the circuit's performance.With all DHT stages (all tubes from Svetlana 1970-s). OK, the input/splitter are not DHT`s but DHP`s - 4P1L - triode wired. Mullard 3-tube input/splitter topology, cap-coupled to cathode bias 6B4G PP. First stage uses battery bias (no cathode resistor). Only paper capacitors everywhere. Gas regulators for the first stage. Later, I replaced Mullard splitter with IT splitter in a Loftin White arrangement. It has gain about 90 at the second stage plate. Next steps: rewire IT to a step-down ration (right now it`s a stock 1+1:1.25+1.25) and replace first stage load for a G