Vacuum Tube Preamp
Note: Click on images to enlarge
VACUUM TUBE PREAMP
The kit came with an unpopulated printed circuit board and a bag of parts - but no documentation. Fortunately, I was able to find a schematic online, as well as a few articles about the circuit. The PCB was good quality and was easy to solder the parts to.
Power comes from a 12 VAC 1000 mA power
adapter, bought separately.
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Assembly
in-progress, showing PCB component markings
|
 |
| Side view |
Schematic
Diagram
 |
| courtesy of: https://www.circuitlab.com/circuit/9vnehk/fever-6j1-tube-pre-amp-circuit/ |
Operation
The preamp
worked the first time I turned it on. The filaments glowed orange within
seconds. (The blue lights are from LEDs that serve no purpose other than to look cool for dramatic effect.)
I applied a
sine wave to the inputs and observed an amplified sine wave on the outputs. The
output signal was 180 degrees out of phase with the input signal. It was clean
and its level remained remarkably constant throughout the audio range of
frequencies.
I connected the
preamp’s inputs to a cell phone and its outputs to a pair of headphones. The music
came through clearly with good fidelity. It was listenable, but not very loud.
 |
| Headphone
configuration |
I then
connected the outputs to the AUX inputs of a stereo amplifier (56 Kohm input
impedance). There were no volume issues and the sound quality was very good.
It is difficult to say for certain if the tube circuit introduced any sound-altering qualities (i.e. the legendary warm vacuum tube sound), though.
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| Stereo AUX input configuration |
Amplifier
Gain
|
Gain vs Load |
|||
|
(tested using 1 Vpp 1 KHz input
sine wave) |
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|
R Load |
Vin |
Vout |
Gain |
|
open circuit |
1.0 |
8.1 |
8.1 |
|
10,000 |
1.0 |
6.0 |
6 |
|
1000 |
1.0 |
1.9 |
1.9 |
|
100 |
1.0 |
0.232 |
0.232 |
|
30 |
1.0 |
0.088 |
0.088 |
 |
| Open circuit gain (input red, output yellow) |
This
amplifier appears to have a high output impedance. It has good gain driving a
high impedance load, but it is not as good driving a low impedance. This
explains why the volume was low driving low-impedance (37-ohm) headphones. When
connected to the AUX input of a stereo amplifier (with a 56,000 ohm input
impedance), there are no volume issues.
Frequency
Response
This
amplifier has excellent frequency response - flat through the entire audio
range.
|
Frequency Response |
||
|
(tested using 1 vpp input sine
wave, |
||
|
Frequency |
Vin |
Vout |
|
5 |
1.0 |
0.8 |
|
10 |
1.0 |
2.0 |
|
20 |
1.0 |
4.5 |
|
30 |
1.0 |
6.4 |
|
40 |
1.0 |
7.6 |
|
50 |
1.0 |
8.0 |
|
100 |
1.0 |
8.1 |
|
500 |
1.0 |
8.1 |
|
1000 |
1.0 |
8.1 |
|
5000 |
1.0 |
8.1 |
|
10000 |
1.0 |
8.1 |
|
15000 |
1.0 |
8.1 |
|
17,000 |
1.0 |
8.1 |
|
18000 |
1.0 |
8.1 |
|
19000 |
1.0 |
8.1 |
|
20000 |
1.0 |
6.4 |
Parts
List
|
Board# |
Marking |
Value |
Voltage |
Notes |
|
C1 |
470
uF |
470uF |
35V |
|
|
C2 |
470uF |
470uF |
35V |
|
|
C3 |
470uF |
470uF |
35V |
|
|
C4 |
470uF |
470uF |
35V |
|
|
C5 |
470
uF |
470uF |
35V |
|
|
C6 |
470uF |
470uF |
35V |
|
|
C7 |
470uF |
470uF |
35V |
|
|
C8 |
470uF |
470uF |
35V |
|
|
C9 |
1uF |
|
|
|
|
C10 |
1uF |
|
|
|
|
C11 |
1uF |
|
|
|
|
C12 |
1uF |
|
|
|
|
C9 |
470
uF |
|
16V |
schematics
show C13=680uF, 25v |
|
D1 |
4007 |
1N4007 |
|
|
|
D2 |
4007 |
1N4007 |
|
|
|
D3 |
4007 |
1N4007 |
|
|
|
D4 |
4007 |
1N4007 |
|
|
|
D5 |
4007 |
1N4007 |
|
|
|
J2 |
|
|
|
power
jack |
|
LED(1) |
|
|
|
blue |
|
LED(2) |
|
|
|
blue |
|
POT |
50K |
|
|
|
|
R1 |
2R2 |
2.2 |
|
|
|
R2 |
4K7 |
4.7K |
|
|
|
R3 |
1K |
1K |
|
|
|
R4 |
47R |
47 |
|
|
|
R5 |
47R |
47 |
|
|
|
R6 |
4K7 |
4.7K |
|
|
|
R7 |
1K |
1K |
|
|
|
R8 |
47R |
47 |
|
|
|
R9 |
47R |
47 |
|
|
|
R10 |
2K2 |
2.2K |
|
|
|
R11 |
2K2 |
2.2K |
|
|
|
R12 |
4K7 |
4.7K |
|
|
|
R13 |
470K |
470K |
|
|
|
R14 |
200R |
200 |
|
|
|
R15 |
4K7 |
4.7K |
|
|
|
R16 |
100K |
100K |
|
|
|
R17 |
4K7 |
4.7K |
|
|
|
R18 |
470K |
470K |
|
|
|
R19 |
200R |
200 |
|
|
|
R20 |
4K7 |
4.7K |
|
|
|
R21 |
100K |
100K |
|
|
|
TR1 |
D667 |
|
|
NPN
transistor |
|
TR2 |
D667 |
|
|
NPN
transistor |
|
TR3 |
B647 |
|
|
PNP
transistor |
|
TR4 |
B647 |
|
|
PNP
transistor |
|
6J1(1) |
|
|
|
vacuum
tube (pentode) |
|
6J1(2) |
|
|
|
vacuum
tube (pentode) |
 |
| Tube closeup |