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Super Stealth
Monobloc Power Amplifier
Special Edition
Users' Manual
Rev. Oct. 24/12
Mapletree Audio Design
R. R. 1, Seeley's Bay, Ontario, Canada, K0H 2N0
http://www.mapletreeaudio.com
© Copyright Lloyd Peppard 2002-12
Lloyd Peppard
(613) 387-3830
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Introduction _
The Mapletree Audio Design Super Stealth Special Edition Monobloc Vacuum Tube
Power Amplifier provides 8 W of clean output power within a compact chassis and offers
a number of desirable features:
? ? The exclusive use of high quality new old stock (NOS) or current production tubes
that can be obtained at modest cost from many suppliers.
? ? Class AB Ultra-Linear push-pull beam power tetrodes delivering 8 Watts output
power at low distortion.
? ? Wide frequency response: 20 Hz – 20 kHz –1 dB at full power output.
? ? DC Balance and Bias adjustments for lowest distortion even with unmatched output
tubes.
? ? DC heater power supply for low noise, with switch for 12.6 or 6.3 V driver tubes.
? ? The exclusive use of polypropylene film signal capacitors and Nichicon Muse high
performance electrolytic cathode bypass capacitors.
? ? Cathode type rectifier tube for soft startup, prolonging tube and capacitor lifetime.
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Adjustments____
Level control (optional) – The sensitivity of the Super Stealth is 0.45 V to produce full
power output. For some high-output preamps, it may be advantageous to reduce the input
level in order to shift the preamp volume control nearer the mid-position for normal
listening. The Super Stealths can also be used without a preamp, directly from a source
(e.g. CD player) using the level control for volume adjustment.
Heater voltage – The switch located adjacent to the two driver tubes can be set for use
of 6SJ7 + 6SL7 tubes or 12SJ7 + 6SL7 tubes. It has no effect on the heater voltage of the
6V6 tubes.
DC Balance adjustment – With the dc voltage between the two red test jacks
monitored on a 2 VDC meter scale, the DC Balance control is adjusted to produce a
reading of close to 0 VDC. It is also possible to measure the plate voltages of the 6V6
tubes by connecting a meter between each red test jack and the black speaker binding
post. Use a meter scale of at least 400 VDC. Refer to the schematic diagram for target
voltages.
Bias Adjustment – The design value of total plate and screen current for the two 6V6
output tubes is 70 mA. The Bias control should be adjusted to produce a meter reading of
7.5 VDC (across 100 Ohms) between the two green test jacks. If you are using an analog
meter, the red meter lead should be inserted in the green jack nearest the front of the
chassis in order to obtain a positive reading. After adjusting DC balance, it may be
necessary to re-adjust the bias for lowest distortion.
Speaker impedance – The standard wiring is for an 8 Ohm loudspeaker. If 4 or 16
Ohm outputs are required, the wiring will need to be changed under the chassis. The
diagram below shows the connections to be made.
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Specifications___
Tube complement: 1 x 6/12SJ7, 1 x 6/12SL7GT, 2 x 6V6/6V6GTGT and 1 x 5AR4GT
Output configuration: Class AB1 ultra-linear push-pull with self bias (6V6GT matched
pair recommended)
Phase inverter: cathode-coupled
Heater supply: dc heater voltage for all tubes
Rated power output (8? ): 8 W
Frequency response at 2 W output: 20 Hz–20 kHz –0.5 dB
Frequency response at 5 W output: 20 Hz–30 kHz –1 dB
Gain: 25 dB (17.8) or 0.45 V input for 8 W output
Input resistance: 470 k???????k? with level control)
Noise: less than 0.7 mV (81 dB below rated output)
Dimensions: 6"W x 11" D x 5" H overall (SE version is 9"W x 13" D x 6" H)
Weight: 10 lb
Power consumption: 65W, 115-125 VAC 50/60 Hz
Fuse: 3 A fast blow type rated at 250 V (located inside chassis)
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Mapletree Audio Design Super Stealth Monobloc Power Amplifier
Special Edition
© Copyright Lloyd Peppard 2004-12
Rev. Oct. 2612
Input
470K
3.5K
+2.5 V
220/25
Nichicon
Muse
3
4
+
107
6/12SJ7
8
5
3.5K
+102 V
+72 V
6
1
0.047
400V
511K
C
+182 V
200p
10K
0.22/250
110K
511K
6/12SL7GT
+232 V
5
4
6
4.7K
+3.2 V
3
1
2
+232 V
154K
B
+286 V
154K
0.047/400
DC Bal.
0.047/400
470K
100/2W
100/2W
100/2W
470K
2.2K
819p
1K
Bias
100/2W
CW
1K
6V6/6V6GT
5
8
37 mA
100/2W
39/1W
37 mA
8
5
6V6/6V6GT
3
(7.5 VDC)
3
+291 V
4
Green
Test
Jacks
A
+295 V
4
Red test point
(DC Bal.)
See User’s Manual
for secondary connection
1K
BR
1K
details for other
speaker impedances
8000? /10W
Hammond 1608
BL
BL/Y
R
BR/Y
Red test point
(DC Bal.)
Y
G
G/Y
BK/Y
BK
Spkr
8 Ohms
500
Hammond 270DX
120 VAC 50-60 Hz
275-0-275 VAC 90 mA
3A
Power
BK
G
WH
G
G
R
R/Y
Trad
PP10-2000
6.3 VAC
R
Y
5 VAC
Y
3 A
2 A
5AR4/GZ34
4
6
5VAC
4 A
+295 V
470/10W
8
2
+331 V
50/500
6A
+
50/500
10000/25
+
13 VDC
–
12 V
Heater Voltage
+
+
6 V
6/12SL7
5.1K/2W
7
8
6/12SJ7
+286 V +182 V
+
100/400
7
2
110K
6V6GTs
2
7
2
7
CA B
+
100/400
1.8K
LED
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6V6GT
6V6GT
Spkr
Power
Pilot
6
12
DC Bal
Heater Voltage
Tube substitutions______
The Russian-made 6V6GT output tubes supplied are an excellent sounding, highreliability design, replaceable at modest cost. However, users may wish to substitute NOS
(new old stock) versions from a variety of manufacturers made over the past 70 years.
The metal 6V6 is also a possible substitute and may offer a different sonic character than
the glass versions. Matched pairs are recommended but not necessary if the dc balance is
adjusted accordingly.
The 6/12SJ7 is a pentode developed for audio and low frequency RF circuits and exhibits
very low noise. It rivals the EF86 in most respects. It is also available in glass as the
6/12SJ7GT. The metal “special red” 5693 is a premium substitute for the 6SJ7.
The 6/12SL7GT is a high gain dual triode well suited to the phase splitter role in the
Super Stealth. The “special red” 5692 is a substitute for the 6SL7GT. The 6SU7GT is
also a substitute for the 6SL7GT and offers matched triode sections, although this is not a
requirement of the circuit used.
The 5AR$/GZ34 rectifier tube is a cathode type that offers a soft startup. Other
compatible cathode types include the 5V4/5V4G/GT and the 5Z4. You can also use
directly heated types such as the 5Y3GT and 5U4G/GT with loss of the soft startup
feature and a reduction in supply voltage (which reduces the output power and may
increase distortion).
Bias
AC
5AR4
In
Power
Xfmr
OPT
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