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Pete Millet
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Wiring
Tips From
A Pro
Digital
Audio
Breakthrough
“Ultimate”
SE Amp
Design
Volume 12 Number 6 2000
www.audioXpress.com
$7.00
CANADA $10.00
BY SATORU KOBAYASHI
This article was originally
published in Japan’s premier high-end tube magazine, MJ Audio Technology,
May 1999.
PHOTO 1: The SV300B stereo amplifier.
I
built this Svetlana SV300B stereo amplifier using Plitron’s single-ended
toroidal transformers (designed by Menno van der Veen of Holland). The
design includes a high-gain, high-linearity voltage driver circuit, finetuned by simulation (Photo 1).
The results showed an excellent power bandwidth: 70kHz at 1W output,
and 50kHz even at 10W out. The overall gain characteristic is extremely flat
over the full frequency range. The distortion characteristic is also relatively
low at 0.5% at 1W, and 3.5% at 10W. The sound from the amplifier brings a
wide, deep presence with a JBL S3100 speaker system, allowing me to enjoy
to page 9
all kinds of music.
(from page 1)
DEFINING OPTIMAL LOAD
IMPEDANCE
Audiophiles have designed many 300B
amplifiers to experience the pleasant
sound of that tube. There are too many
examples published in the audio magazines to choose the best one, however.
Even Western Electric delivers considerable 300B data via their website.
Therefore, I examined WE300B operating conditions using statistical analysis
methods, with a Microsoft Excel spreadsheet (Table 1). As expected, the following tendencies appear: 1) the average
output power increases by 2W in proportion to the plate voltage increment of
50V; 2) the distortion stays mostly the
same as the supply voltage varies (Fig. 1).
When you vary voltages through 350V,
400V, and 450V and vary the load impedance, the power output and distortion
will decrease as the load impedance increases. With this information, you can
determine the most preferable load impedance from the actual data: 5kΩ is the
optimum load at 450V, 3.5kΩ at 400V, and
3kΩ at 350V, respectively (Figs. 2a, b, c).
This allows a tentative load-impedance
fix of 5kΩ and a plate voltage of 450V for
the project. The simulation then gives a
grid bias of −105V. From this, you can expect approximately 12W maximum output power. To drive this output stage properly requires design of a driver circuit that
can generate 75V RMS (105/√2) with an
input level of 0.5V RMS through 1V RMS.
DRIVER STAGE
The 300B tube needs about −105V grid
bias voltage. It requires more attention
than the conventional tetrode and/or pentode power tubes, which need a grid bias
of only −20 to −50V. A conventional voltage driver circuit usually employs a pentode at the first stage and a triode at the
driver stage, with negative feedback adjusting the total gain of the amplifier, as
well as an interstage transformer to get a
high peak drive voltage.
For this project, I determined that a linear circuit producing a high output AC
voltage is required. Mark Kelly’s “The
ABOUT THE AUTHOR
Satoru Kobayashi is from Tokyo, Japan. He has been
interested in audio and in ham radio since he was in his
teens. After majoring in EE in Tokyo, he joined the
semiconductor industry, designing DRAM circuits for a
living, although he now works in the technical and marketing area. His debut as a writer came in the early ‘80s
in the form of an article about ham radio for CQ magazine. Now he periodically writes on the subject of audio
for a few different magazines.
Search For Linearity” (GA 6/96, p. 42, 1/97,
p. 32) was very useful to me, and allowed
me to make a final decision. The article
shows a number of high gain and high linearity circuits, such as cascode, mu-follower, SRPP, White cathode follower, and
so on. All use a vacuum tube as a current
source in place of either cathode or plate
resistor, enhancing the gain and linearity.
After reading this, instantly I hit upon
an idea: If I used one of those, then I could
drive a 300B easily and simply without an
interstage transformer, and save cost and
space. However, such a high-gain amplifier provides a rather high output impedance (a few kΩ to tens of kΩ). Thus, an
impedance converter is needed to drive a
300B with a low impedance. A White
cathode follower would be a good choice,
rather than a standard cathode follower.
Once I tentatively selected the circuit
type, then I performed a simulation to
determine the total gain with available
tubes, such as 12AX7, 6189W, 12AU7, and
6FQ7. I used TubeCAD software by GlassWare to calculate parameters such as
gain, maximum output voltage, and
other figures very quickly, without need-
ing to draw load lines on characteristic
curves of each tube (Fig. 3). In an SRPP
circuit, the voltage gain of a 12AX7
is more than 90, while 6189W yields
a gain of less than 60. I have simulated
the circuits of the following: standard
SRPP, self-biased SRPP (mu-follower),
and fixed-bias SRPP. Subsequently, the
mu-follower circuit showed the best
performance in terms of voltage gain
and maximum output saturation voltage (Table 2).
The White cathode follower that I chose
for the driver stage is improved from the
standard cathode follower, by replacing
the cathode resistor with an active current
source made from a vacuum tube. This
structure generates a higher peak voltage
output than a conventional cathode follower. To choose the best tube out
of my personal stock, which includes types
6189W, 12AU7, and 6FQ7, I conducted
more simulations using Tube CAD.
The result showed the 6189W to be the
lowest output impedance out of three
tubes. Also it showed superior power-supply ripple rejection, about 10dB better
than the other tubes (Table 3).
TABLE 1
WE300B OPERATING EXAMPLES
PLATE
VOLTAGE
GRID
BIAS
IDLE
CURRENT
LOAD
IMPEDANCE
200
−42
30
2000
200
−39
40
2500
200
−37
50
2500
250
−55
30
2000
250
−55
30
4500
250
−52
40
3000
250
−50
50
2500
250
−48
60
2000
250
−48
60
2700
250
−45
80
1500
300
−65
40
2500
300
−63
50
2000
300
−63
50
3000
300
−61
60
2400
300
−61
60
3400
300
−58
80
1700
350
−76
50
3600
350
−76
50
5000
350
−74
60
2000
350
−74
60
3000
350
−74
60
4000
350
−71
80
2200
400
−91
40
5000
400
−89
50
3000
400
−89
50
4000
400
−87
60
3500
400
−87
60
5000
400
−84
80
2500
450
−104
40
6000
450
−102
50
5000
450
−102
50
6500
450
−100
60
4000
450
−100
60
5500
450
−97
80
2000
450
−97
80
3000
450
−97
80
4500
Source: Western Electric WE300B Technical Note from WE website.
OUTPUT
HARMONIC
2ND
HARMONIC
3RD
HARMONIC
3
2.6
2.5
4.9
3.2
4
4.4
4.7
4.1
5
6.7
7.2
6.1
6.6
5.6
7.5
7.8
6.2
10.2
8.3
7
9.6
8.4
11.5
9.4
10.5
8.3
12.5
9.5
10.7
9
12.5
10.1
17.8
14.6
11.5
20
26
30
18
27
26
26
26
30
26
20
21
26
26
30
26
26
30
21
26
30
26
26
21
25
26
30
25
26
27
30
26
30
21
26
31
31
38
45
27
40
36
39
38
45
41
30
29
37
37
44
37
38
45
30
38
44
39
37
31
38
38
46
37
38
39
45
38
44
30
37
45
GLASS AUDIO 6/00
9
OPERATING POINT
age of the driver stage to the highest in
the circuit; 450V might be adequate in
this amplifier. However, this high plate
voltage might exceed the heater-to-cathode maximum limit of 200V.
In a cascaded pair of
triodes encapsulated
in a single glass tube,
the cathode voltage of
the upper tube is
about one-half of the
supply voltage. Thus, a
supply voltage of over
400V may cause problems with this circuit
structure. So I chose a
350V plate voltage for
simulation purposes,
leaving some room
G-1478-1
below the tube’s maxiFIGURE 1: WE300B output power and distortion
mum rating.
characteristics.
It became obvious
that a mu-follower circuit will generate 250V
It is often said that the higher the plate
voltage on a tube, the higher the output
voltage will be…and the better the linearity. So, I prefer to increase the plate volt-
peak-to-peak (pp) output while holding
180V DC at the cathode on this circuit
under a 350V DC supply voltage. Although the circuit generates a peak output level of 305V, with the output node
swinging from 55V to 305V, the value exceeds the maximum heater-to-cathode
voltage easily. To prevent damage from
this excess voltage, the heater of the tube
must be biased to one-half of the peak
output voltage. A 150V heater bias will be
adequate for this purpose.
After this simulation, I built a prototype circuit whose characteristic I measured with plate supplies of 350V, 400V,
and 450V to verify its operation (Figs. 4
and 5). Consequently, I found the following: 1. The maximum output voltage increases in proportion to the plate voltage, and the distortion then decreases. 2.
The maximum peak voltage on the 300B,
with a 2V RMS signal input, was 325V pp,
356V pp, and 390V pp, respectively. 3.
The overall voltage gain was approxi-
TABLE 2
SIMULATION RESULT: MU-FOLLOWER DRIVER
2A
G-1478-2a
2B
G-1478-2b
2C
G-1478-2c
FIGURE 2A, B, C: WE300B output power and distortion
characteristics by plate voltage.
10 GLASS AUDIO 6/00
CIRCUIT
SRPP
SRPP
FIXED BIAS
SRPP
SELF BIAS
SRPP
SELF BIAS
Tube
Plate voltage
Plate current
Cathode resistor
Plate resistor
Input resistor
Coupling capacitor
Gain
Cathode voltage
(upper unit)
Grid voltage
(upper unit)
Ripple rejection rate
Maximum output
voltage, peak-to-peak
Grid bias
Input impedance
Output impedance
Frequency
response—low
Frequency
response—high
Bias resistor
12AX7
350V
1.5mA
10kΩ
10kΩ
100Ω
1µF
93.49
190V
←
←
1mA
←
←
←
←
←
184V
←
←
0.9mA
←
←
←
←
96.06
188V
←
450V
←
←
←
←
←
←
238V
189V
183V
186V
235V
−24.1dB
−108/
+108V
−1.15V
50.1kΩ
3.99kΩ
0.16Hz
←
−125/
+125V
−1.34V
←
←
←
−28.5dB
−124/
+125V
−1.3V
48.7Ω
←
←
←
−173/
+173V
−1.8V
←
←
←
>1MHz
←
←
←
N/A
1.6MΩ/1.8MΩ
N/A
N/A
FIGURE 3: SV300B loading curve by SE Amp CAD simulator.
G-1478-3
TABLE 3
SIMULATION RESULT: WHITE CATHODE FOLLOWER
TUBE
6189W
6FQ7
12AU7
Plate voltage
Plate current
Cathode resistor
Plate resistor
Input resistor
Coupling capacitor
Gain
Cathode voltage (upper unit)
Grid voltage (upper unit)
Ripple rejection rate
Maximum output voltage,
peak-to-peak
Grid bias
Input impedance
Output impedance
Frequency response—low
Frequency response—high
350V
1.6mA
10kΩ
10kΩ
100Ω
1µF
0.98
191V
167V
−61.6dB
±157V
←
←
←
←
←
←
0.95
←
173V
−48.6dB
±152V
←
←
←
←
←
←
0.94
←
168V
−45.9dB
±150V
−2.06V
5.31MΩ
9.99Ω
1.59Hz
>1MHz
−6.58V
2.15MΩ
31.1Ω
←
←
−7.62V
5.31MΩ
42.4Ω
←
←
G-1478-4
FIGURE 4: Input versus output characteristics and distortion of driver stage.
TABLE 4
SIMULATION RESULT: FINAL STAGE LOAD IMPEDANCE
LOAD IMPEDANCE
Ω
5kΩ
Plate voltage
Idle current
Primary resistance
Secondary resistance
Grid bias
Maximum output power, RMS
Maximum output voltage, RMS
Maximum output current
Grid voltage (upper unit)
Output impedance
Damping factor
2nd harmonic distortion
3rd harmonic distortion
450V
70mA
80Ω
0.1Ω
−98.8V
9.53W
8.73V
1.09A
167V
1.8Ω
4.45
2.9%
0.7%
Ω
3.5kΩ
←
←
50Ω
←
−99.4V
11.6W
9.65V
1.21A
173V
2.24Ω
3.58
6%
2.1%
mately 78. 4. The bandwidth at a −3dB
level was 60kHz or less at 70V RMS output. The distortion at this output was approximately 1%.
With the aforementioned simulation
and experiment, the lineup of the driver
circuit was fixed, with a 12AX7A for the
first stage and a 6189W for the driver
stage, using a plate voltage of 400V. I
chose the 400V plate voltage because it
will generate the maximum output level
and hold the cathode-to-heater voltage
on the driver tubes under the maximum
limit of 200V, given a 150V DC bias at the
heater electrode. The maximum output
voltage was 250V pp with this circuit at
3.2V pp input, which is at the onset of
clipping (Fig. 6).
FINAL STAGE
To define the final stage operating condition, I used SE Amp CAD software, comparing three different load impedances
to determine which one would bring the
best performance under simulation. As
of today, Plitron offers three suitable
models, with primary impedance of
2.5kΩ, 3.5kΩ, and 5kΩ—all suitable for a
Ω
2.5kΩ
←
←
40V
←
−99.5V
12.7W
10.1V
1.26A
168V
2.72Ω
2.94
10.2%
4.3%
G-1478-5
FIGURE 5: Frequency response of driver stage.
TABLE 5
FINAL SIMULATION RESULT:
OUTPUT STAGE
LOAD IMPEDANCE
Ω
4.98kΩ
Plate voltage
Idle current
Primary resistance
Secondary resistance
Grid bias
Maximum output power
Maximum output voltage, RMS
Maximum output current
Grid voltage (upper unit)
Output impedance
Damping factor
2nd harmonic distortion
3rd harmonic distortion
450V
70mA
40Ω
0.1Ω
−98.8V
9.53W
8.73V
1.09A
167V
1.8Ω
4.45
2.9%
0.7%
300B SE amplifier. The simulation parameters assumed a 450V plate voltage
and a 70mA idle current, which is within
the maximum limit with fixed-bias operation for the 300B. Also, the simulator
needs the transformer parameters, such
as load impedance, secondary wiring resistance, and so on (Table 4).
Armed with results from three different models, I gave first priority to lower
distortion rather than maximum output
power, so the final choice for loading was
G-1478-6
FIGURE 6: Driver stage
output waveform.
5kΩ. On the other hand, I had already
placed the order for 2.5kΩ transformers
during the design stage, so I decided to
make the primary impedance 5kΩ by
connecting the 8Ω load to the 4Ω output
tap of the transformer.
Afterwards, I did the simulation again,
using fully defined parameters for verification (Table 5). The result shows 20%
headroom below the maximum plate
dissipation of 40W with 70mA of idle
current, giving maximum output power
of approximately 10W with 3.1% harmonic distortion, at 99V input to the
300B grid.
GLASS AUDIO 6/00
11
POWER SUPPLY
The power-supply circuit (Fig. 7) does
not use the traditional choke transformer
to save money and lessen the total
weight of the amplifier. Experimental results showed better ripple rejection using
a power MOSFET ripple filter than a conventional 10H/200mA choke—the result
was 9.4mV RMS ripple voltage riding on
the 450V DC supply (Fig. 8).
I also installed the final tube-protection circuit, which consists of a 30-second timer relay and a 10kΩ/20W series
resistor in the 340V AC line. After turning
the amplifier on, the plate voltage idles at
approximately 270V DC, holding the
300B tubes in a cutoff state with approximately −100V grid bias for 30 seconds.
Then the relay eliminates this resistor,
raising the plate voltage to the nominal
value of 450V DC. Also, a 0.3A fuse in the
plate supply line ensures additional protection of the 300Bs.
Negative grid bias was generated with
a voltage tripler on the 40V AC tap of the
power transformer, plus a 2SC4233 NPN
transistor controlled by an IC voltage regulator, the TL783C by Texas Instruments
(TL783C Datasheet. Texas Instruments,
SVLS036C—Sept. 1981. Revised April
1997). The IC regulator needs only a reference resistor of 82Ω and an extra resistor of 6.8kΩ to achieve the desired voltage, and give a low ripple voltage of only
0.3mV RMS. The output voltage of −105V
goes to a series-connected 68V zener
diode and 5.6kΩ resistor, so that about 5
to 6mA flows through this resistor.
Two potentiometers, connected in parallel to this bleeder, adjust the grid bias to
the final tubes independently. This structure prevents excessive idle current when
adjusting bias, and it allows use of a
small-size potentiometer (since the current flow in the potentiometer is only a
couple of mA). A 20kΩ pot brings −68 to
−105V to the grid of each 300B tube for
adjusting the bias level.
I mounted the IC regulator and NPN
transistor onto separate heatsinks for
cooling. DC switching regulators, of
12V/0.8A and 5V/2A, respectively, power
the heaters of all the tubes. The power
transformer that I used provides only a
single filament winding of 6.3V/6.8A,
which is not adequate for the 300B amplifier. At the design stage, only this
transformer was available. Plitron has recently introduced a new design of
toroidal power transformer for 300B
amps, 6900-X0-00 (Fig. 9). I strongly recommend that anyone who wishes to
copy this amplifier should try to use this
new model, which provides a couple of
12 GLASS AUDIO 6/00
G-1478-7
FIGURE 7: Circuit diagram.
5V/2A windings and
6.3V/3A for heaters, a
couple of 325V/0.25A
for the plate supply,
and even 100V/0.1A
for the grid bias supply. The heater of the
driver circuit is run
from 12.6V DC biased
above ground to +150V
DC, to lessen the voltage difference from
cathode to heater.
PARTS
G-1478-8
FIGURE 8: Ripple filter I-V characteristic.
First of all, I chose the
Svetlana SV300B, since
a number of audio magazines evaluated
this tube and found it to be as good as the
WE300B, so the performance of the tube
seems to be superior to its name. Also this
tube is much less expensive than the
WE300B. Furthermore, Svetlana has a new
package design for their SV300B that I
prefer, with individual data sheets for each
tube in a pair, as well as a safe and attractive package for the matched pair.
I obtained Plitron transformers via the
authorized dealership, Tec-Sol Inc. in
Hamamatsu, Japan, who started to carry
Plitron in 1999. These transformers were
manufactured in Canada, and were originally designed by van der Veen in Holland (Table 6).
The power transformer (#754709) provides 340V-0.7A AC at 280W. The output
transformer (PAT-3025-SE) features a
maximum output power of 13W, defined
at the 50% current level over the coresaturation current of 204mA. It implies
G-1478-9
FIGURE 9: Plitron 6900-X0-00 transformer pinout diagram.
that this transformer can handle enough
power. Thus, the transformer is quite
heavy—5.4kg.
Both transformers need only three
round mounting holes in the chassis,
compared to conventional E-I-cored
transformers requiring a large rectangular hole. The package uses a 2mm thick
black polyamide plastic shell. Epoxy
plastic encapsulates the bottom. This
structure gives a rigid and solid appearance as well.
Mounting requires only a ⁵⁄₁₆″, 3¾″
length bolt and nut through a hole in the
center of the power transformer, while
the output transformer needs only a 1¾″
length, ⁵⁄₁₆″ bolt through a hole in the
bottom center of the transformer. The
output transformer is encapsulated in a
black spun aluminum can, whose surface is black-coated to resemble the traditional Japanese silk fabric for Kimonos,
called “Chiri-men.” This gives the transformer a gorgeous appearance.
The first-stage driver uses one
12AX7WA and one 6189W by Philips ECG
of the US, which I purchased at a vacuum-tube shop in Akihabara, Tokyo. Others come from the parts dealers in Akihabara, the well-known electronics center where hundreds of shops are gathered on Central Street—like Fifth Avenue
in New York City—within a half-mile
radius.
TABLE 6
PLITRON TRANSFORMER CATALOG
Primary impedance
Secondary impedance
Turns ratio Np/Ns (4Ω secondary)
−0.1dB frequency range
−1dB frequency range
−3dB frequency range
Nominal power, RMS
Full power bandwidth starting at
Total primary inductance
Primary leakage inductance
Effective primary capacitance
Saturation primary DC current
Total primary resistance
Total secondary resistance
Tube plate resistance (300B)
Insertion loss
Q-factor second-order HF rolloff
HF rolloff specific frequency
Quality factor (Lp/Lsp)
Quality decade factor
Tuning factor
Tuning decade factor
Frequency decade factor (note 4)
PAT-3050-SE
PAT-3035-SE
PAT-3025-SE
5060Ω
4 + 8Ω
35.55
12Hz − 20kHz
5Hz − 45kHz
3Hz − 84kHz
13W
20Hz
40H
10mH
1.2nF
143.43mA
80Ω
0.1Ω
0.7kΩ
0.17dB
0.49
134.08kHz
4100
3.6
7.79
0.89
4.5
3490Ω
4 + 8Ω
29.52
16Hz − 21kHz
7Hz − 48kHz
3Hz − 90kHz
13W
20Hz
28H
7mH
1.1nF
172.72mA
50Ω
0.1Ω
0.7kΩ
0.17dB
0.49
142.54kHz
4100
3.6
6.38
0.81
4.41
2490Ω
4 + 8Ω
34.96
23Hz − 22kHz
10Hz − 49kHz
5Hz − 91kHz
13W
20Hz
18H
5.5mH
1nF
204.32mA
40Ω
0.1Ω
0.7kΩ
0.18dB
0.49
147.21kHz
3273
3.52
5.49
0.74
4.26
ASSEMBLY
First of all, using my Macintosh G3/
266MHz and Claris Draw software, I defined the proper chassis size and the parts
locations on the chassis (Fig. 10a and
Photo 2). In this project, Plitron transformers did not allow a wide variety of layout plans because of its big round shape.
So I lined up the transformers in a row
along the long axis of the chassis. Consequently, I needed a big chassis: 480 × 240 ×
65mm. Then, I chose San-Ei Musen in Akihabara to construct the chassis, which
consists of a 1.6mm-thick polished stainless top plate and 1mm-thick steel bottom
box, painted with a matching bottom lid,
thus giving a good appearance.
I assembled the ripple filter with its
MOSFET on a 10 × 7.5cm PCB, and
mounted the other switching power supplies on 10 × 15cm and 10 × 11cm PCBs
with Teflon-insulated pin electrodes for
easy internal wiring and maintenance. I
also mounted a couple of potentiometers on a PCB to adjust the grid bias.
Each 4-pin socket for an SV300B is assembled with a metal plate adapter (Fig.
10b), custom-made by San-Ei Musen.
This adapter sets the level of the socket
14 GLASS AUDIO 6/00
FIGURE 10A: Chassis layout.
to 30mm beneath the chassis, to make
the height of the tubes and other components over the chassis more even.
The internal wiring uses a breadboard
with turret terminals, the so-called PTPboard (Fig. 11a) from International Audio
Group in Tex. I custom-ordered a board
to my specs, with terminals placed as I
wished, via e-mail, which was extremely
convenient. I received my custom board
in less than ten working days. Also, the
toroidal transformers give some extra
room, so this PTP board and the power
supplies (Figs. 11b and 11c) neatly fit into
this custom-made chassis beneath the
transformers.
G-1478-10a
G-1478-10b
FIGURE 10B: 300B socket adapter.
Four 9-pin sockets and all components for the driver stage are mounted
over this board, allowing me to wire
them easily and simply. The parts over
the board are symmetrically placed
against the centerline of the chassis, to
match the wiring topology elsewhere in
the circuit (Photos 3 and 4).
ADJUSTMENT
After assembly, I double-checked the
internal wiring. After correcting any
PHOTO 2: Overhead view inside the chassis.
errors, I turned the power switch on
(before inserting the tubes) and monitored the value of grid bias. The grid
voltage should be set to −105V or so,
by adjusting the potentiometer controls
on the PC board. You can perform the
adjustment process with any digital
multimeter.
Then, I turned the switch off, plugged
the tubes into their sockets, and turned
the switch on again. I checked the heater
line voltage, which should be 5V ±0.2V DC
and 12.6V ±0.4V DC, respectively. After
several minutes warm-up time, I checked
the idle current of the output tubes by
measuring the voltage drop across a series
1Ω resistor placed between the plate supply and the output transformer. The value
must be around 70mV or so. That completes the adjustment.
FINE-TUNING AND
MEASUREMENT
The key area of fine-tuning was in the
power supply (Fig. 12). No further finetuning was required on the amplifier
part, since the design was verified by experiment and simulation, optimized
with the SV300B and Plitron output
transformer.
After completing the assembly, while
working on the aforementioned adjustment process, I turned the amplifier on
with a speaker system attached. A loud
hum came out of the speaker. Originally I
designed a semiconductor DC regulator
for the filament/heater supply of the
300B and its driver tubes by using a single 6.3V AC winding. I was confident
G-1478-11a
FIGURE 11A: PTP terminal board layout.
G-1478-11b
FIGURE 11B: 450V regulated power supply.
16 GLASS AUDIO 6/00
(to page 20)
G-1478-11c
FIGURE 11C: 12.6V DC, grid bias regulated power supply.
PHOTO 4: Rear of the unit.
PHOTO 3: Inside view front to rear.
(from page 16)
about making a low-ripple DC supply
with various IC regulators, thanks to my
semiconductor design background.
However, using only one 6.3V AC
source did not allow for a low-ripple DCregulated supply for the SV300B filaments. I was able to confirm a 5V DC
level monitored by a multimeter, which
is an adequate level to run SV300Bs, yet
the ripple value was about 60mV RMS—
even though I added more than
27,000µF in electrolytic capacitors. It
was difficult for me to eliminate the ripple voltage. I have checked it out using
an oscilloscope and found the ripple
value was more than 150mV peak to
peak.
Thus, I experimented by running the
SV300B tubes with my home-brew regulated power supply (variable from 4 to
18V, 10A maximum, 3mV ripple voltage).
The experiment showed that residual AC
hum voltage at the secondary of the output transformer dropped to 0.2mV. This
result changed my mind about using a
switching power supply. It was tempting,
since these supplies are available in Akihabara at very low cost.
After this improvement, the residual
noise voltage at the speaker terminals de-
FIGURE 13: Input-output characteristic.
20 GLASS AUDIO 6/00
FIGURE 12: Modified power supply circuit for upgrade.
G-1478-13
FIGURE 14: Distortion.
G-1478-12
G-1478-14
FIGURE 15: Frequency response.
G-1478-15
FIGURE 16: Damping factor.
creased to 0.5mV in one channel, 0.6mV
in the other. Obviously I prefer to use this
ripple-less switching power supply, driving 300Bs as well as other tubes.
After this tuning, I measured the following characteristics:
• Input versus output and distortion
(Figs. 13 and 14)
The result shows 10W output with
3.5% distortion at an input level of 0.9V
RMS. This result corresponds with the
simulation results. The sound is very
good for a non-NFB amplifier.
• Frequency response (Fig. 15)
The results showed a good power
bandwidth of 70kHz at 1W and 50kHz
at 10W. The overall gain characteristic
is extremely flat over the full frequency
range.
• Damping factor (Fig. 16)
I used the on-off method at the loading of 8Ω, and applied a 1V RMS output level. The calculated result shows a
damping factor of 6, a good value for a
non-NFB amplifier.
• Waveforms (Fig. 17)
The waveforms show a good power
bandwidth, with a better square-wave
response than that of many conventional amplifiers. The waveform at
100Hz shows less distortion than that
of E-I-cored conventional transformers. I observed no overshoot at the
transition edges at 1kHz and 10kHz,
since the toroidal transformers are
well-tuned and well-damped by their
inherent design.
LISTENING IMPRESSION
I brought this amplifier to my old friend
who owns JBL S3100 speaker systems, to
listen to music and get his impression as
well as my own. First of all, I was impressed with the clarity of the sound
coming from the speaker. The sound of
musical strings, such as cello, violin, gui-
22 GLASS AUDIO 6/00
G-1478-16
MEASUREMENT EQUIPMENT
HP 334A audio analyzer
Kenwood AG-204D audio generator
Trio RA-920 attenuator
8Ω 50W 2 ch. homemade dummy load
HP 1746A oscilloscope
Fluke 8020A digital multimeter
REFERENCES
“300B Power Amplifier Kessaku-sen (Design Pool of
300B power amplifier),” Seibun-do Shinkosha (MJ
Audio Technology Magazine).
WE300 Technical Note, Western Electric/Westrex Co.,
Atlanta, Ga.
Svetlana SV300B Datasheet, Svetlana Electron Devices Inc., Huntsville, Ala.
SOURCES
G-1478-17
FIGURE 17: Output waveforms
(100Hz, 1kHz, 10kHz).
tar and so on, impressed me so much
that I can draw a picture in my mind of
the artists playing their instruments in
front of me. The vocal recordings, including Joan Baez, Ella Fitzgerald, and Natalie
Cole, sounded light and pure. I could hear
even their breathing during each song,
thanks to accurate middle-range tone.
I checked the low-frequency response
with a CD by the Oscar Peterson Trio, We
Get Requests, recorded in 1964. I chose
“You Look Good To Me” for testing. At the
Tec-Sol Inc.
Hamamatsu-shi, Wada-cho 514
Shizuoka 435-0016 Japan
+81-53-468-1201
FAX +81-53-468-1202
http://www.tec-sol.com/
http://www.plitron.com/
e-mail: [email protected]
Svetlana vacuum tubes and Plitron toroidal transformers
San-Ei Musen (This business closed as of August 2000.)
Chiyoda-ku, Soto-Kanda 1-15-16
Tokyo 101 Japan
+81-3-3251-7985
FAX +81-3-3251-2343
Custom made chassis
International Audio Group Inc.
PO Box 10096
Killeen, TX 76547-8702
Phone/FAX (254) 699-8702
e-mail: [email protected]
PTP board
Angela Instruments
10830 Guilford Rd., Suite 309
Annapolis Junction, MD 20701
(301) 725-0451
FAX (301) 725-8823
http://www.angela.com/
Angela capacitors
beginning, the sound of a triangle was
incredibly clear, and subsequent bass
lines by Ray Brown came out with great
power and control. Also, I could hear Ray
whispering as he was swinging.
I was really impressed by the sound
from this amplifier, my first exposure to
the SV300B and to Plitron toroidal transformers. The sound brought me a more
realistic presence than I had heard before, with a wider and deeper atmosphere than conventional amplifiers I
have built in the past.
I have compared the sound of
WE300Bs installed in this amp—though I
could not find or hear any better improvement on what we got with the
SV300Bs.
❖
GLASS AUDIO
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Editorial Dept., PO Box 876
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FAX (603) 924-9467
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TABLE 7
PARTS LIST
AMPLIFIER
Vacuum tube
Vacuum tube
Vacuum tube
Power transformer
Output transformer
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Zener diode
Capacitor
Capacitor
Electrolytic capacitor
Electrolytic capacitor
Electrolytic capacitor
2
2
2
1
2
2
2
2
10
2
4
4
2
4
2
4
2
2
12V—0.8A, 5V—0.7A, 100V AC input
5V—2A, 100V AC input
20kΩ (B) Bourns #3296
2SK719 (NEC) or IRFPC40 (VGS = 600V,
Id = 6.8A, Pd = 150W, RDS = 1.2Ω)
2SC4233 (VCBO = 1200V, VCEO = 800V, Ic = 3A, Pc = 60W)
TL783C (Texas Instruments)
Z6150, 150V 2W, Ishizuka Denshi
Z6120, 120V 2W, Ishizuka Denshi
Z668, 68V 2W, Ishizuka Denshi
6V 1W, NEC RD-6F
1000V 1A RG4C Shindengen
600V 1A, RH1S Shindengen
82Ω ½W
1kΩ ½W
5.1kΩ 2W
5.6kΩ 2W
6.8kΩ ½W
10kΩ ½W
10kΩ 20W w/bracket
56kΩ 1W
75kΩ 5W
100kΩ ½W
0.1µF 600V, ceramic
47µF 160V
47µF 200V
220µF 350V, Nippon Chemicon
100µF + 100µF 500V, Elna Cerafine
14K431U Matsushita, purchased at Akizuki Denshi
OMRON MY-2V 30 sec. w/socket
Yoshikawa Kinzoku 17P23 (for 2SC4233)
Mizutani 20P25-25 (for TL783C)
Mizutani (for MOSFET)
w/0.3A fuse
Teflon insulated
1 ea. 100 × 150mm,
2 ea. 100 × 75mm,
Copper clad epoxy glass t = 1.6mm
1
2
2
1
POWER SUPPLY
Switching regulator
Switching regulator
Potentiometer
Power MOSFET
Transistor
IC regulator
Zener diode
Zener diode
Zener diode
Zener diode
Diode
Diode
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Capacitor
Electrolytic capacitor
Electrolytic capacitor
Electrolytic capacitor
Electrolytic capacitor
ZNR
Timer relay
Heatsink
Heatsink
Heatsink
Fuse holder
Pin terminal
PCB
OTHERS
Chassis
Socket
Socket adapter
Socket
RCA socket
Power switch
AC inlet
Speaker terminal
Pilot lamp
Spacer
Spacer
Spacer
Spacer
⁵⁄₁₆″ bolt & nut
⁵⁄₁₆″ bolt
24 GLASS AUDIO 6/00
SV300B (Svetlana)
12AX7 WA (Philips—NOS)
6189W (Philips—NOS)
Plitron 754709 or 6900-X0-00
Plitron PAT-3025-SE or PAT-3050-SE
1Ω 1W
100Ω ½W
1kΩ ½W
10kΩ ½W
82kΩ 1W
100kΩ ½W
1MΩ ½W
47V, 3W Toshiba, 3Z47
0.1µF 630V Angela (Solen Equivalent)
1µF 630V Solen
47µF 16V OS capacitor by Sanyo
47µF 160V
47µF 450V tubular
480 × 240 × 65mm stainless-steel top custom made by San-Ei Musen
Svetlana SK-4A
custom made by San-Ei Musen
9 pin Chuoh-Musen
San-Ei Musen
125V 10A
w/fuse
San-Ei Musen
12V 150mA
10mm long, 3mm dia, brass base, nickel plated
15mm long, 3mm dia, brass base, nickel plated
20mm long, 3mm dia, brass base, nickel plated
30mm long, 3mm dia, brass base, nickel plated
90mm long
32mm long
1
1
4
1
1
2
4
3
1
3
1
1
1
1
1
1
1
5
8
2
2
2
1
1
1
1
1
1
1
3
1
2
2
4
2
1
1
1
1
12
8
4
8
1
2