The empirical approach that I used tu build “Il Primo” was replaced by a more rigorous approach in “Il Primo Bis”. Here, I tried to use a more formal and rigorous process to the design and value settings of the various parts of the vacuum tube amplifier. Especially, the Power Supply Unit was significantly revised, and component values were chosen with more confidence and appropriateness. This more rigorous approach is the one described in my book.
Characteristics:
- Push-Pull stereo integrated amplifier
- Max Output Power 2x ~35W
- @ 1 Volt peak input signal, with 6dB global negative feedback
- Power Stage: 2x 2 EL34/6CA7 power vacuum tubes in Push-Pull Ultra-Linear configuration and fixed bias
- Phase Split Stage: DC coupled Concertina with 12AX7 vacuum tubes
- Input Stage: 12AX7 vacuum tubes with self bias and bypass capacitor
- Class AB with 40 mA anode current bias
- Output Transformer: Hammond 1650HA
- Primary: 6.6K Ohm C.T., 40% screen tap
- Secondary: 4, 8, 16 Ohm
- Power Supply Unit Transformer: Hammonnd 373FX 650V c.t@460mA
- Global Negative Feedback: max 6dB
- THD (measured with Sovtek 12AX7 LPS + JJ EL34)
- at 1W: 0.058%
- at 5W: 0.11%
- at 10W: 0.17%
- Two only capacitors in the signal path
- 4, 8 Ohm Speakers
- Input Impedance: 100K Ohm
- Output Impedance: 1.45 Ohm
- Damping Factor: 5.6
- 3 selectable input sources
Frequency Response
THD
Vacuum tube amplifier schema
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Power supply schema for the vacuum tube amplifier
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Construction layout
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List of components
Component: | Design values: | Quantity: | Description: | Chosen component: |
Power supply: | ||||
TR1 | 650V C.T. @460mA, 6.3V C.T. | 1 | Power transformer | Hammond 373FX 650V c.t@460mA, 50V tap, 6.3V@9A section, 5V@6A Section |
C1, C2 | 100uF 450V | 2 | Power stage smoothing filter | F&T Fischer & Tausche 100uF/450V, axial electrolitic |
C3, C4 | 47uF 450V | 2 | Splitter stage smooting filter | F&T Fischer & Tausche 47uF/450V, axial electrolitic |
C5, C6 | 4.7uF 450V | 2 | Input stage smooting filter | F&T Fischer & Tausche 4.7uF/450V, axial electrolitic |
C7 | 22uF 500V | 1 | Reservoir | F&T Fischer & Tausche 22uF/500V, axial electrolitic 18x30mm |
C8 | 10uF 100V | 1 | Heater elevation | Nichicon FW 10uF 100V, radial electrolitic |
C21 | 0.1uF 400V | 1 | Ground lift | Wima MKS4 011uF/400V, polyester |
C22 | 4.7uF 100V | 1 | Bias circuit reservoir | Nichicon KW 4.7uF 100V, UKW2A4R7MDD, condensatore elettrolitico radiale |
C23 | 8uF 100V | 1 | Bias Smothing Filter | Nichicon FW 10uF 100V, condensatore elettrolitico radiale |
C26, C27 | 6.8nF 100V | 2 | Heater short to ground | Wima FKS2 0.0068uF/100V, polyester |
D1, D2, D3 | UF4007 | 3 | Rectifiers | |
R1, R2 | 120 5W | 2 | Power stage smoothing filter | |
R3, R4 | 10K 0,5W | 2 | Splitter stage smooting filter | |
R5, R6 | 82K 0,5W | 2 | Input stage smooting filter | |
R7 | 180K 2W | 1 | Heater elevation | |
R8 | 15K 0,5W | 1 | Heater elevation | |
R33, R34, R35, R36 | 33K 0,5W | 4 | Bias circuit voltage divider | |
R37 | 10K 0,5W | 1 | Bias circuit voltage divider | |
R38 | 15K 0,5W | 1 | Bias Smothing Filter | |
R45 | 120Ω 0.5W | 1 | Ground lift | |
VR3 | 220k | 1 | Trimmer for bias circuit | Elecsound 3296W 200K, vertical multiturn, 3296W-1-204 |
VR4, VR5 | 25K | 2 | Trimmer for bias circuit | Elecsound 3296W 25K, vertical multiturn, 3296W-1-253 |
Power Stage: | ||||
TR2 | 6.6k Ohm C.T. 40% U.L. Tap | 2 | Output Transformer | Hammond 1650HA 6600 c.t. 40% screen 4,8,16 Ohm |
C13, C14,C17,C18 | 0.47uF 100V | 4 | Grid leak short to ground cap | Epcos TDK 0.47uF/100V, MKT metal polyester |
C15, C16,C19,C20 | 0.22uF 400V | 4 | Coupling cap | Mundorf MCAP EVO OIL 0.22uF 450V, polypropylene |
R21, R22,R23,R24 | 4.7K 0,5W | 4 | Grid Stopper | |
R25, R26,R27,R28 | 82K 0,5W | 4 | Grid leak | |
R29, R30,R31,R32 | 10Ω 0,5W | 4 | Cathode Probe | |
R39, R40,R41,R42 | 1K 1W | 4 | Screen resistors | |
V1, V2,V3,V4 | EL34 | 4 | Power tubes | JJ EL34, Matched Quartet |
Phase splitter stage: | ||||
R17, R18,R19,R20 | 100K 0,5W | 4 | Cathode/Anode Resistors | |
Input Stage: | ||||
C9,C10 | 0,22uF 100V | 2 | Coupling cap | Mundorf MCAP EVO OIL 0,22uF 450V, polypropylene |
C11,C12 | 150uF 100V | 2 | Catode bypass cap | Sprague Atom 150uF/50V, electrolitic axial |
C24,C25 | 220pF 500V | 2 | Feedback circuit step net cap | Silver Mica 220pF/500V |
R9,R10 | 47K 0,5W | 2 | Grid Stopper | |
R11,R13 | 150Ω 0,5W | 2 | Feedback circuit R2 | |
R12,R14 | 680Ω 0,5W | 2 | Cathode resistor | |
R15,R16 | 220K 0,5W | 2 | Anode resistor | |
R43,R44 | 6.8K 0,5W | 2 | Feedback circuit R1 | |
V5,V6 | 12AX7 | 2 | Input and splitter tubes | JJ 12AX7, matched pair |
VR1 | 100K Log Stereo | 1 | Volume |
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