My vacuum tube amplifier provides the following characteristics:
1. Class A, output stage – pentode
2. Push-Pull schema
3. Lineal power supply
4. One channel (mono)
5. Tone control: LF, MF, HF
6. Bandwidth: 30 – 30 000 Hz
7. Output power: 5 Wt
I use Soviet VT: 6N2P and 6P14P. I propose my own design of the circuit.
Interesting? Press “Read More” link below :-)
Unusual solution (which is striking, to my own opinion) is to use power supply for the grid negative bias. Commonly used solution is - to place the resistor between cathode and the zero line; so, if the electronic tube is open cathode will be upper than the grid. Disadvantage of the resistor is - inaccurate mode. Using the power supply we get perfect accuracy.
Principal circuit presented on fig. 1-2.
Fig1 – preamplifier and tone correction block
VT 1-4 triodes are 6N2P. VT 5-6 : 6P14P.
Fig 2 – voice frequency amplifier
I suppose, at the beginning, you should learn two patterns:
1. How low-pass filter (LPF) works. You need this filter to avoid self-extraction caused by the wide frequency range.
2. How High-pass filter (HPF) works. Interstage separation circuits are HPF at the amplifier
Let’s describe LPF and HPF after preamplifier VT1 (fig.1). The resistor R6 and the capacitor C18 are the circuit for grounding high frequency signals. Point 5 is the input for LPF and point 14 is the filter output.
The next circuit that consists of C12 and R10 is more interesting, since it has three functions:
1. Filtering circuit
2. Cutting the constant value
3. Current limiter
In our case, it is important to take into account all of above mentioned functions.
VT3 and VT4 have the same (R-C, C-R) filters as VT1.
VT5 and VT6 has only LPF, in this case, LPF is the audio transformer.
After VT2, only HPF circuits are designed, since VT2 does not amplify signals.
Tube VT2 is the accurate phase inverter, it produce two signals 180 degree rotated one to another. It happens by the following reason – when VT2 opens:
R20 (on point 29) goes up and R19 (on point 28) goes down.
Electrolytic and in parallel ceramic С41,С42, С45,С46, С44, С43 are used to suppress power pulsations and radio interference.
Sound transformer is specially selected for 6P14P pentodes and for push-pull mode (I’ll better write a special article about it).
Fig 3 – My amplifier on the breadboard
Fig 4 – building the printed circuit board (PCB)
Fig 5 – My power supply
Fig 6 – Power supply
P.S. I’ll be so gratify to get some feedback about my article. Thank you!