The voltage regulators can be somewhat sensetive in high voltage circuits so be sure to review my notes regarding The Care and Feeding of LM317 and LR8 Integrated Circuit Regulators, Particularly in Valve Circuits. What was a sort of a mediocre output stage thus becomes highly accurate and quite excellent. Using a CCS in the cathode circuit of the SIPP stage forces it to operate in class-A. A simple application of a common LM317 IC voltage regulator converts it to a very accurate constant current source (CCS). The advent of solid state electronics provided a solution to the problem. The reason this circuit is attractive is that it eliminates the need for a phase splitter stage. It can only function in a class-A mode with both tubes always conducting. The problem with early applications was that it was difficult to force it to operate in a linear manner. The basic design has been around for a long time (50 years or so). The output stage is a self-inverting push pull (SIPP) design after the Compact Hi-Fi Power Amplifier by Melvin Leibowitz. The gain of the 5751 SRPP driver stage is about 32 times or 30dB.įigure 2: 5751 SRPP / EL84 (6BQ5) Tube Amplifier Schematic (October 2009) I chose one here for simplicity, gain, linearity, drive capability, and power supply noise rejection. SRPP drivers are particularly good as power tube drivers. This is sufficient to ensure linear response. The bias point for the SRPP is approximately 1.25 mA. The 5751 JAN Philips tubes used in the larger amps did not perform as well in these amps. I tried other brands of 5751 vacuum tubes and the Sovtek brand had lower distortion than the others. The 5751 is pin compatible with the 12AX7 but not directly interchangeable as some parameters are different (5751 gain is about 30% lower than the 12AX7). For those who are not familiar with this tube, it is similar to a 12AX7. The driver stage is a fairly straight forward SRPP design using both sections of a new production Sovtek 5751 vacuum tube. I want to hear music, not buzz.įigure 1: EL84 (6BQ5) Poddwatt Power Supply Schematic (October 2009) I hate hum and noise and take special care to minimize them. The end result of this arrangement is an amplifier that is extremely quiet. Three of the four sections are bypassed with polypropylene caps. The four sections start with fast recovery solid state rectifiers and a capacitive input. I do this so that there are minimal interactions between the two channels. The final filter section is split into two halves so that each SRPP has its own filter. The first three sections are in common with both channels. The amplifier uses a single power supply with four filter sections. I want everyone to enjoy music and not have anyone harmed in the process. If you are not familiar with high voltages and how to safely build high voltage equipment, then please do not attempt this project. Contact with such voltages can cause injury or death. Warning: This project uses high voltages. Photograph 1: Poddwatt Class-A Stereo EL84 (6BQ5) Vacuum Tube Amplifier If you don't exceed the power limits it will provide sound quality that is easy to listen to with great detail and clarity. I was very surprised how well this amplifier sounds. There is plenty of bottom end, just not as much as the bigger amplifiers like the KT88 Odd Blocks which use much larger audio output transformers. The amplifier is quite clean from 25 Hz to past 20kHz (see the measurements in Tables 1 and 2 below). The end result is a great DIY project for those of you who need 5 to 7 watts of high quality sound. The earlier project with the ECC802S driver tube did not have sufficient gain for this. This was done to allow the direct connection (via headphone jack) of MP3 players to the tube amp. One area of particular note is the higher gain of this new tube amp. A number of circuit refinements have been included as well. This alone makes for a $40 to $50 savings. The rather large output transformers used in the original project have been replaced by ones that are considerably less weighty and less costly. Most notably it is easier and less costly to build. The basic circuits are rather similar, but the new project has a number of differences. Both tube amplifier projects use EL84 / 6BQ5 tubes and the earlier one used a ECC802S driver while this one uses 5751 driver tubes. It has more than a casual similarity to the first Push-Pull EL84 (6BQ5) Oddwatt project from nearly two years ago. This project is related to the other tube amplifier projects I have posted. Poddwatt: 5751 SRPP / Push-Pull EL84 (6BQ5) Tube Stereo Amplifier
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