Analog Metric C22 User manual

C22 Vacuum Tube
Preamplifier Complete
User Manual
Analog Metric
www.analogmetric.com
[email protected]
Copyright ©2009 All Rights Reserved

[C22 VACUUM TUBE PREAMPLIFIER COMPLETE USER
MANUAL]
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[C22 VACUUM TUBE PREAMPLIFIER COMPLETE USER
MANUAL
]
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INTRODUCTION
The circuit design is with referenced to the classical McIntosh C22 pre-amplifier. It uses
three 12AX7 vacuum tubes for amplification and uses one pentode-triode vacuum tube
6F3P (6BM8) as voltage regulation. Vishay 1N5062 forming a bridge provides for full-
wave rectification. The resulted ~300V DC. A low dropout regulator LM1084 is used to
supply the filament. As all resistors and capacitors are low-noise with good channel
matching, the sound quality should have low distortion.
FEATURES
•Three vacuum tubes 12AX7 and one 6F3P (6BM8/ECL82)
•Two single-ended inputs and two single-end outputs for stereo
•Voltage gain: 22dB
•Dynamic range: output voltage max. 20Vrms
•S/N ration: >90dB
•Built-in voltage regulation circuit.
•Optional mounting holes for delay protection module.
•Power requirements one 260V AC(200mA), one 15V AC(1A), and one 6.3V AC(1A)
•PCB dimension: 110mm (W) x 250mm (L)
•PCB thickness: 2.5mm, double layer, 2oz copper.
•
PRECAUTIONS
•Do not use finger or any body parts to touch the components or board! It is hazardous,
since the high voltage capacitors may not be fully discharges after switched off the
power supply.
•Turn off the power supply if the transformer is getting hot or some smoke is observed
or strange buzz sound is heard.
•Fuse should be used either in power transformer or main socket to avoid accidentally
large current drawing.
•Always contact technicians or experts to seek help.
PROCEDURES
1. Solder all the components according to the schematic, part list, and photos. Notice to
the polarity of the high voltage capacitors and the direction of the Zener diodes.

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2. Either connect delay output protection module (optional) to the jumper J6 and J7 or
short it.
3. Connect the power transformer 280-300V AC, 12-15V AC, and 6.3V AC to the board,
without plugging the vacuum tubes.
4. Measure the AC voltages at the connectors to see whether the voltages are maintained
that are 280V AC, 15V AC and 6.3V AC.
5. If the voltages are correct, plug in the tubes and turn on the main power.
C22 Preamplifier with Tube Voltage Regulator
1. Connect an IEC power socket and a power transformer T120 as shown in Figure 1.
Two secondary windings (115V) of the transformer T120 are connected either
serially or parallel for main power supply of 220V or 110V, respectively.
Figure 1: Connection diagram between IEC power socket and the power transformer.
2. Connect the secondary windings (two 360V, 5V, and 6.3V) of the power transformer
to the connectors (J1, J2, and J3) of the tube voltage regulator respectively as

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illustrated in Figure 2. Then, use a metal wire to short the connector L1 (displayed as
yellow line)
3. Plug in the vacuum tube EL84 and 6J8 (EF86), and turn on the power supply.
Measure the output DC voltage at J4. Adjust the resistor trimmer VR1 of tube
regulator board so that the output voltage reaches 290V DC. This 290V is the B+
voltage for C22 preamplifier.

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Figure 2: Connection diagram between the power transformer T120 and the tube power
voltage regulator.
4. Connect the B+: 290V DC and ground (J4) of tube voltage regulator board to
positive and negative terminals of C15 of C22 preamplifier board respectively, as
shown in Figure 3. Also, connect the secondary winding (15V AC) of T120 to the
J3 of C22 preamplifier board for tube filament supply.

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Figure 3: Connection of power supply by the tube voltage regulator to the C22 preamp.
5. To power up the input selector board and the delay protection board, the power
supply ~12V DC can be obtained from the positive terminal of C13 or from the
low voltage part of the tube voltage regulator, either J6 or J7 of tube voltage
regulator board.
6. Connect the output (marked with Ro+ and Lo+) of input selector to 23 step
attenuator and the output of 23 step attenuator to C22 preamplifier as in Figure 5.
7. Finally, wiring all the connections according to Figure 5

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.
Figure 5: Full C22 preamplifier connection diagram.
8. Plug in all the tubes (EL84 and 6J8 to power regulator board, and three 12AX7B
to C22 preamplifier board)
9. Switch on the power supply without applying any input signals and check again
the B+ voltage (~290V) and filament voltages (~12.6V). Also, a ‘click’ sound
from a delay protection board will be noticed after a few minutes. This means the
delay protection module is functioning.
10. If the B+ voltage and tube filament voltage are correct, then apply audio signal to
the input of input selector and selector the channel by the rotary.
11. Enjoy it.
If you have any problem in assembly, please contact us by email to
[email protected]