“Il Terzo” in English means “The Third”. This vacuum tube amplifier came after the “Il Primo” and “Il Primo Bis“. I used EL84 power vacuum tubes and 12AX7 vacuum tubes as input and phase split. Also here, I designed it using the approach described in my book. More specifically, the design of this vacuum tube amplifier is described in details in “Chapter 6: Step by step design of a push-pull tube amplifier .
Characteristics:
- Push-Pull stereo integrated amplifier
- Max Output Power 2x ~12W
- @ 1 Volt peak input signal, with 16dB global negative feedback
- Power Stage: 2x 2 EL84 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 28 mA anode current bias
- Output Transformer: Hammond 1650E
- Primary: 8K Ohm C.T., 40% screen tap
- Secondary: 4, 8, 16 Ohm
- Power Supply Unit Transformer: Hammond 370KX 500V c.t@322mA
- Global Negative Feedback: 16dB
- THD (measured with Sovtek 12AX7 LPS + JJ EL84)
- at 1W: 0.05%
- at 5W: 0.12%
- at 10W: 0.18%
- Measures of THD with other tubes is reported here
- Two only capacitors in the signal path
- 4, 8 Ohm Speakers
- Input Impedance: 100K Ohm
- Output Impedance: 0.4 Ohm
- Damping Factor: 18
- 3 selectable input sources
Frequency Response
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 | 500V C.T. @300mA, 6.3V C.T. | 1 | Power transformer | Hammond 370KX 500V c.t@322mA, 50V tap, 6.3V@6A 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 | Ground lift | Wima MKS4 0.1uF/400V, polyester | |
C22 | 1uF 100V | 1 | Bias circuit reservoir | Nichicon KW 1uF 100V, UKW2A010MDD, radial electrolitic |
C23 | 8uF 100V | 1 | Bias Smothing Filter | Nichicon FW 10uF 100V, condensatore elettrolitico radiale, UFW2A100MED |
C26, C27 | 6.8nF 100V | 2 | Heater short to ground | Wima FKS2 0.0068uF/100V, polyester |
D1, D2, D3 | UF4007 | 3 | Rectifiers | |
R1, R2 | 330 5W | 2 | Power stage smoothing filter | |
R3, R4 | 5.6K 0,5W | 2 | Splitter stage smooting filter | |
R5, R6 | 12K 0.5W | 2 | Input stage smooting filter | |
R7 | 150K 1W | 1 | Heater elevation | |
R8 | 15K 0.5W | 1 | Heater elevation | |
R33, R34, R35, R36 | 150K 0.5W | 4 | Bias circuit voltage divider | |
R37 | 10K 0.5W | 1 | Bias circuit voltage divider | |
R38 | 27K 0.5W | 1 | Bias Smothing Filter | |
R45 | 120Ω 0.5W | Ground lift | ||
VR3, VR4, VR5 | 10K | 3 | Trimmer for bias circuit | Elecsound 3296W 10K, trimmer vertical multiturn |
Power Stage: | ||||
TR2 | 8k Ohm C.T. 40% U.L. Tap | 2 | Output Transformer | Hammond 1650E 8000 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 | 0.1uF 400V | 4 | Coupling cap | Mundorf MCAP EVO OIL 0.1uF 650V, polypropylene |
R21, R22,R23,R24 | 1K 0.5W | 4 | Grid Stopper | |
R25, R26,R27,R28 | 200K 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 | EL84 | 4 | Power tubes | JJ Electronic EL84 (6BQ5) |
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 | 500pF 500V | 2 | Feedback circuit step net cap | Silver Mica 500pF/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 | 2,2K 0.5W | 2 | Feedback circuit R1 | |
V5,V6 | 12AX7 | 2 | Input and splitter tubes | Sovtek 12AX7 / ECC83 LPS |
VR1 | 100K Log Stereo | 1 | Volume |
Gallery
Thank you, you have inspired me. For some reason I’ve bought a cv2666 off eBay to build an amp. Whilst looking for hints on circuits, I’ve found your article. I like the simplicity of your circuit. I’m thinking of using your prestage to power the dual tetrode. Would you mind please?
Thank you for posting your article.
Cheers Tim
Hi Tim,
Thanks!
Note that the CV2666 can also work with positive grid (Class AB2), according to its datasheet. This prestage circuits is not meant to work with positive grid of the power stage, where the input impedance of the power stage goes down a lot. So, if you use it, configure your power stage to work only with negative grid (Class AB).
Thank you so much, I wasn’t aware of that. I haven’t built any valve circuits for forty years, so relearning so much. Your son clearly has taste!
Many thanks, I am glad you appreciate it. 🙂 🙂
Hello,
Wonderful project! Congratulations, I wish I could build one for myself, but first I have to study and learn to read schematics properly. Built a few guitar amps, but this is a different field of expertise. Is the construction layout accurate?
best regards,
Luís
Hi,
Thaks!
Layout should be fine
Up to now, nobody mentioned issues on it. However, if you have doubts or if you find something weird, do not hesitate to contact me.
Given that you built guitar amplifiers, reprducing it should not be a problem.
Let me know!
Hi Guiseppe,
I’m building your Il Terzo and have good fun with it.
I’ve got hold of a chinese tube amp with blown output transformers. As they are unavailable, of dodgy quality and there are no schematics of the circuit, I decided to gut it and build new. It had 4 EL84 and a pair of ECF82 still in it, so that decided what to look for. I considered Bruce Herans Poddwatt, but I already have an SE amp and wanted to try something PP. I was happy when I found your project as it is so well documented. The old power transformer is still ok, a nice toroidal with enough filament current and a 250-0 winding that I will bridge rectify so it matches your design.
One question, though. You describe the bias PS using a 50V AC transformer where you use 40V DC. Not as easy to find, but a 24V AC is readily available. I sat down and recalculated the divider network and tried it on a breadboard. Works as intended and I get adjustable -12V for all four tubes. Stabilized DC is 25V, The resistance of the divider is now 58k, so still well under the max 100k. Current draw is only 4mA, but thats good, as the transformer is small and can only deliver 14mA. As I see it there is no reason to use the 50V AC transformer? The current draw on the grids should be minimal.
I think I go with my 24V AC solution, or have I missed something?
Greetings Rüdiger
Sorry for the very late reply… To little time to stay behind this.
It should be fine. I used 50V, given that 370KX Hammond transformer has a 50V section for bias circuit