SINGLE ENDED 50EH5 TUBE AMP
Giovanni Militano 
Page 1: Background, Single Ended Amp Design
Page 2: Construction, Sound
BACKGROUND
Since having completed a S-5 Electronics K-12M Tube Amplifier Kit, I have become hooked on tubes. I decided to take a step back from the Push-Pull kit and build a small and simple Single Ended (SE) Tube Amplifier. The main goals for this project were to use as few parts as possible and to keep costs down.
This little amp is based around the small 50EH5 power pentode and is similar to other "spud" amps that I have stumbled across. Primarily, Dave's 5-Buck Amp and Dad's 50EH5 Single Ended Amplifier.
The 50EH5 is a miniature power pentode which was primarily designed and widely used in the audio power output stage of radio and television receivers and in phonographs. The tube features high power sensitivity at low plate and screen voltages. Tube characteristics and the typical operation for a Class A1 Amplifier is summarized below.
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Plate Voltage Screen Voltage Cathode-Bias Resistor Peak Grid Voltage Plate Resistance Transconductance Zero-Signal Plate Current Maximum Signal Plate Current Zero Signal Screen Current Maximum Screen Current Load Resistance Total Harmonic Distortion (THD) Maximum-Signal Power Output |
110 Volts 115 Volts 62 Ohms 3.0 VOlts 11000 Ohms 14600 Micromhos 42 Milliamperes 42 Milliamperes 11.5 Milliamperes 14.5 Milliamperes 2500 - 3000 Ohms 7 Percent 1.4 Watts |
To keep costs down on this project, the majority of the parts used have been salvaged from an old Fleetwood Stereo console that was purchased from a local thrift store for $7CDN. Photograph 01 shows the salvaged console and a map of the circuitry from the console is shown in Drawing 01 below.
Photograph 01: Salvaged Fleetwood Stereo Console
Drawing 01: Map of Fleetwood Stereo Console
(Click for Larger Image)
SINGLE ENDED AMP DESIGN
The 50EH5 pentode has been widely used in many audio applications in past, so there is no need to develop a new design around this tube. On that basis, I decided to stick to the single ended amp design shown in the typical application notes. While the 50EH5 Single Ended Typical Operation Schematic shows an output transformer with a load resistance of 3K, the 50EH5 Datasheet indicates that a load resistance of 2.5 to 3K is suitable for single ended operation. My design preference was not to use a cathode bypass capacitor so I tried the circuit with and without a cathode bypass. Without the bypass capacitor, there is a clear loss of power and a loss of depth to bass and mid-bass. In an effort to determine the optimal size of the cathode bypass required, I experimented with several different types and sizes of capacitors. Naturally, if I had to use a capacitor, my preference would be to use polypropylene, but I quickly determined that they are not practical given the amount of capacitance required. 82uF does not quite get you there, and no sonic improvement was realized using greater than 100uF of capacitance. With a requirment for this much capacitance, electrolytic seems to be the only practical choice which is unfortunate.
Drawing 02: 50EH5 Power Supply Schematic
Drawing 02 shows the schematic I used for the power supply. Obviously there are several improvements that can be made, but this supply has been designed around parts I had salvaged and had on hand. The caps used for filtering were salvaged from a dead computer monitor and do not need to be this big. The transformer I used is a little on the large side and was salvaged from the Fleetwood tube console. Almost any transformer that is the 100V range should be suitable. Obvious improvements to the power supply would be tube rectification, a choke and polypropylene capacitors. With the small current draw and relatively low voltages, 200V poly caps in the 30-50uF range should do the trick. I used PSU Designer II to model the power supply and it got me very close to final design that I eventually settled on. I was aiming for 50V on the heaters and ended up with 52V which is close enough for me. Trying to stick to the application note, I was shooting for 110V on the plate and 115V on the screen. I ended up with 110.5V on the plate and 115.5V on the screen! In addition to indicating power, the LED is also a (very slow) bleeder once the power has been shut off.
The final single ended amp design that I settled on is shown Drawing 03 below. I used a 100k pot because that is what I had on hand.
Drawing 03: 50EH5 Single Ended Amplifier
All the parts required for the 50EH5 Tube Amplifier project are available from TubeDepot.
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Construction, Sound |