Stealth Max
Monobloc Power Amplifier
Users' Manual
Rev. July 16/08
Mapletree Audio Design
Lloyd Peppard
R. R. 1, Seeley's Bay, Ontario, Canada, K0H 2N0
(613) 387-3830
i nf o@mapletreeaudio.com
http://www.mapletreeaudio.com
© Copyright Lloyd Peppard 8
Introduction _
The Mapletree Audio Design
Stealth Max
vacuum tube monobloc power amplifier
features:
♦
A low distortion Class AB1 ultra-linear output stage employing easily obtained KT88
beam power tubes delivering a conservative 50 W continuous power. A triode/ultralinear mode switch to permit experimentation with different sonic presentations. The
triode power rating is about 20 W.
♦
An output transformer that is an exact replica of the Dynaco A431S used in the
original Mk III amplifier. The power transformer is conservatively rated and can
deliver the current required for maximum power output. Soft startup is assured
through use of a 5AR4/GZ34 rectifier tube.
♦
A driver module based on the Mapletree
Special Red SR-70
board for the Dynaco ST-
70. It employs octal tubes in a Mullard-type circuit with 6SJ7 pentode voltage
amplifier and a 6SN7 long-tail phase inverter.
♦
Wide frequency response: 20 Hz–20 kHz –1 dB at 50 W output.
♦
Adjustable fixed bias for optimum performance with any matched pair of output
tubes.
♦
Stainless steel chassis and high quality components and hardware used throughout.
2
Poweri ng up _____
Please read this instruction manual thoroughly before powering up your
Stealth Max
for
the first time. The location of the tubes on the driver board and chassis are shown below.
When installing tubes, locate the key on the central spigot and make sure it matches with
the indent on the tube socket. Push the tube into the socket with a slight rocking motion
to ensure it seats properly. Removal is also easier of the tube is rocked slightly as it is
pulled out. Don’t over-do the rocking as it can cause the spigot to break off. The KT88
output tubes are installed as matched pairs as indicated on the boxes supplied with your
amplifier. Matched quads are not required as individual bias controls are used for each
channel.
With the tubes installed, connect the IEC power cord to the AC input receptacle on the
rear of the chassis. A proper sized fuse (3 A/250 V) has been installed (a spare is
provided). Plug the cord into an ac outlet rated for at least 200 W (110-125 VAC or 230240 VAC for the export version, 50-60 Hz). The ac power switch is on the front panel.
The red pilot light indicates the AC power is on. You will see the heaters light up,
particularly in the KT88 tubes. It takes approximately 60 sec for the plate voltage to build
up to the operational level and stabilize. This “soft start” maximizes the lifetime of the
tubes and capacitors in your amplifier.
3
Biasing the output tub es________
Stealth Max
The
is designed to run the KT88 output tubes at a cathode (combined plate
and screen grid) current of 110 mA per push-pull pair. With a plate voltage of 500 V, the
plate dissipation is about 25 W for each tube (the KT88 is rated for 35 W). The correct
bias condition is pre-set for the tubes supplied but will require adjustment if the tubes are
replaced. It is a good idea to check the bias when first powering up as it may have shifted
during shipping. A matched pair of KT88s is recommended for lowest distortion. The
procedure for adjusting the grid bias is as follows.
The bias adjustment potentiometer is located on the rear apron next to the green bias
measurement jacks. When installing a new pair of output tubes, start with the
potentiometer at the position it was for the previous tubes. A multimeter capable of
reading at least 2 VDC is required, connected with the red lead to the top jack and the
black lead to the lower jack. Set the multimeter to read dc Volts (most meters have a 2 V
scale which is ideal). Turn the ac power switch on. Once the tubes reach their operating
conditions the meter reading should rise to around 1 Volt. Adjust the bias potentiometer
using a blade screwdriver counterclockwise (CCW) if the voltage is greater than 1.1 V
and clockwise (CW) if the voltage is below 1.1 V. Adjust to achieve a reading close to
1.1 Volt. The combined plate and screen current for both tubes is being sampled across a
10 Ohm resistor connected between the green jacks so 1.1 V corresponds to 110 mA.
Input/Output conn ect ions______
The input RCA jack is located on the rear apron of the chassis. A level control is next to
the input jack and should normally be set full on (full CW). If you have higher than
normal preamp gain or if your speakers are very efficient, you may find that it is
desirable to adjust the level control to allow use of the preamp volume control near its
mid position. The input resistance is 100 kΩ. An input voltage of 1.2 V is required to
drive the amplifier to full output.
Binding posts for connection to a speaker system are located on the rear apron of the
chassis and are appropriately labeled. Speaker interconnect wires can be terminated as
bare wire, spade lugs, or banana plugs. The latter are most convenient, especially if
different speaker systems are to be connected from time to time. Outputs are provided for
both 4 and 8 Ohm speakers, unless otherwise specified.
4
Triode Mode
– The toggle switch located next to the
KT88 tubes permits changing the output tube
configuration from ultra-linear to triode mode. You
can switch “on the fly” and determine which mode
suits your sonic preferences. The maximum power
output in triode mode is around 20 W.
Tube sub st ituti on ____
Any vintage or currently manufactured pair of KT88
beam tetrode power tubes can be used in the
Max
. Lowest distortion is obtained using a matched pair to achieve identical bias currents
in both tubes. The 6SJ7 metal small signal pentode was designed for audio use, offering
low noise and high gain. The glass 6SJ7GT can be substituted without change in
performance. The special red series 5693 can also be directly substituted and offers the
potential for very long life (10,000+ hrs). No currently manufactured versions are
available but NOS tubes are readily available from large tube suppliers and on ebay. The
6SN7GT is a dual high mu triode. It is currently manufactured in Russia and available
under the Electro Harmonix brand and others. NOS versions are not hard to find but the
supply is limited. The special red equivalent is the 5691. The rectifier tube can be any
GZ34/5AR4 type.
You may be told that the 6550 power tube can be substituted for the KT88. With its lower
screen grid voltage rating, the 6550 is not recommended for the
V screen potential operating in either ultra-linear or triode mode.
Stealth Max
Stealth
with its 500
5
Specifications___
Tube complement:
Rectifier
Output configuration:
Phase inverter:
Heater supply:
Rated power output (4/8ΩΩΩΩ):
Frequency response at 1 W output:
Frequency response at 50 W output:
Gain:
Input resistance:
Noise:
Dimensions:
Weight:
Power consumption
Fuse:
: tube full-wave rectification with 5AR4/GZ34 cathode-type rectifier tube for soft startup.
24 dB (16 V/V) or 1.2 V input for 50 W output
less than 1 mV ( greater than 86 dB below rated output)
20 lb
3 A fast blow type rated at 250 V
1 x GZ34/5AR4, 1 x 6SJ7, 1 x 6SN7GT, 2 x KT88 (matched pair).
Class AB1 ultra-linear push-pull with fixed bias or triode mode.
cathode-coupled (long tail)
ac
50 W
15 Hz–44 kHz –1 dB
20 Hz–20 kHz –1 dB
100 kΩ
10" W x 10" D x 8" H overall
: 100 W, 115–125 VAC (230-240 VAC for the export version) 50-60 Hz
Frequency response at 1 W output
2
0
-2
-4
dB
-6
-8
-10
10 100 1000 10000 100000
Freq uen cy (Hz)
6
Inp ut
chassis
Level
100K
ground bus
Mapletree Audio Design
V1
6SJ 7/6SJ7GT
10K
511K
117
270 p
+ 97 V
8
3
4
5
+3.1 V
3.57K
+ 69 V
1
6
0.047/400
2.7K
511K
100K
+163 V
Stealth Max
240 VAC for Murray Smith
© Co py r ig ht Ll oy d Pe pp ar d 200 8
Rev. July 12/08
1/400
10K
1M
150p
V2
6SN7GT
1
3
6
4
2
+107 V
5
+348 V
332 V
11K
2 W
+437 V
20K
2 W
22K
2 W
0.1/400
-54 V
0.1/400
Monobloc Power Ampl ifie r
V3
KT88
+495 V
3
10K
Bias set
1.1 V
–
1
5
10
5
1
5
+
5
KT88
4
8
8
4
3
V4
10K
221K
Bias
221K
+493 V
UL
+498 V
UL
G/W
G
1K
T
B/W
Y
16
OR
8
B
4300
A431S
60 mA
BR
4
BK
Ω
R
T
1K
240 VAC
3A.
Power
P782Sexp
BR
BR
6.3 VAC 6 A
GR
GR/Y
GR
Y
5VAC/3 A
Y
R
400
R/BK
60
R/Y
0
200 mA
R
400
KT88s
2
7
4
6
MUR180
6SJ7
2
2
7
787
2
8
V5
GZ34/5AR4
-72 V
470/100
+
6SN7
15K
50K
560
100/400
100/400
CCW
+507 V
+
+
+
470/100
1.5 H 200 mA
150K
150K
-54 V
Bias
55 Ohms
C354
100/400
100/400
+498 V
500
+437 V
5.1K 2W
+
150K
100/400
+
150K
100/400
all 1 W
+
+
330K 1 W
150K
150K
+163 V
+
100/400
7
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