The right on the photo is the R core output transformer in bare condition.
The product is enclosed in the square case on the left.
RW405 is R core audio output transformer for single ended type amplifier. Excellent magnetic properties of R core as well as precisely wound construction of the coil by the computerized manufacturing system achieve the wide bandwidth, low distortion, and low magnetic loss of the transformer.
Type  SingleEnded 
Output capacity  40W at 35Hz 
Primary impedance  3,000 ohms, 5,000 ohms, with UL tap 
Secondary impedance  4 and 8 ohms 
3rd.  cathode feedback winding 16 ohms X 2 
Frequency bandwidth  25Hz to 63Khz (2dB), input=4V, signal source impedance=3,000 ohms 25Hz to 60Khz (2dB), input=4V, signal source impedance=5,000 ohms 
Primary inductance (H) 
12H(min.), 15H(max.) (3,000 ohms, DC current 80mA) 
Primary permissible DC current  140mA(3,000 ohms), 120mA(5,000 ohms) 
Recommended primary DC current  110mA or less (3,000 ohms), 90mA or less (5,000 ohms) 
Power loss  0.23dB 8ohms 
DC resistance  Primary B to 9.5K : 168ohms Secondary 0 to 8 ohms : 0.17ohms 3rd. 0 to 16 ohms : 4.18 ohms 
Dielectric withstanding voltage between primary, secondary and 3rd. .  2KV AC 
Maximum permissible voltage of primary B to 9.5K  1KV AC 
Core  Type R160 160W core 
Shape  Enclosed in square case, the same mounting dimensions as those of Tango XE60,FC30 
Connections  Lead wires 
Overall dimensions and weight  W: 110mm, D: 100mm, H: 150mm, Wt.: 3Kg W: 4.33inch, D: 3.94inch, H: 5.91inch, Wt.: 6.6lbs. 
Case color  black, silver 
Price  US$220 (excluding tax and shipping) 
The table below describes the selection of load impedance and the relation of the phase between the primary and secondary and 3^{rd}. You can select noninverted connection between the primary and secondary and 3^{rd} for twostage amplifier and inverted connection for threestage amplifier. When the load is selected to 5,000 ohms, two taps for UL are available; its feedback factor is 50% or 25%. The higher factor makes the plate resistance and distortion lower, however, the sensibility and output power will be reduced.
Connected to 
Noninverted 5,000 ohms 
Noninverted 3,000 ohms 
Inverted 5,000 ohms 
Inverted 3,000 ohms 
Plate 
Yellow 
Blue 
Orange 
Green 
Screen grid (UL tap factor) 
Red(50%) Green (25%) 
Green (33%) 
Red (50%) Blue (25%) 
Blue (33%) 
Power source 
Orange 
Orange 
Yellow 
Yellow 
When
using the cathode feedback winding, the connection of primary winding and cathode
feedback winding is as follows.
There are two cathode feedback windings in RW405. The cathode feedback ratio can be
selected by two windings being used in parallel or in series.
The cathode feedback ratio decreases if it uses in parallel. The cathode
feedback ratio increases if it uses in series.
In addition, a different wire connection adjusts to RW405 by the Noninverted
or the Inverted. The table below shows the connection of each situation.
Noninverted  Inverted  
cathode feedback winding Low 
Noninverted  Inverted  
cathode feedback winding High 
Calculating the closedloop gain and feedback ratio
It explains here as 5K Primary and in parallel cathode feedback winding.
The gain and ratio for 3K Primary and in series
cathode feedback winding is described in the last table.
The
load of the output tube is both the primary winding and cathode feedback
winding in the cathode feedback circuit.
The load impedance of the output tube is not 5K ohms.
The load impedance is as follows.
Rl = ( SQRT(5000) + SQRT(16) ) **2 = 5582 ohms
丂丂丂丂 SQRT : Square route operation
丂丂丂丂 ** : Factorial operation
The openloop gain on the primary side before feedback
丂丂 Aopen = gm * ( 5582 // Rp )
丂丂丂丂 gm : Transconductance of the output tube
丂丂丂丂 Rp : Plate resistance of the output tube
丂丂丂丂 // : Parallel value operation
The feedback factor 兝 is a voltage ratio (a winding ratio) of load Rl and the
cathode winding .
丂丂 兝 = SQRT ( 16 / 5582 ) = 0.054
The closed loop gain on the primary side after feedback
丂丂 Aclosed = Aopen / ( 1 + Aopen * 兝 )
丂丂 Feedback ratio = 20 * log( Aopen /
Aclosed ) dB
Example
Value that uses EL34 for output tube
gm of EL34 = 11000uS
Rp of EL34 = 15000 ohms
The openloop gain on the primary side before feed back
丂丂 Aopen = gm * ( 5582 // Rp )
丂丂丂丂丂丂
= 11000 * 10**6 ( 5582 // 15000 )
丂丂丂丂丂丂
= 11000 * 10**6 * 4068
丂丂丂丂丂丂
= 11 * 4.068 = 44.75
The closed loop gain on the primary side after feedback
丂丂 Aclosed =
Aopen / ( 1 + Aopen * 兝 )
丂丂丂丂丂丂丂 丂 = 44.75 / ( 1 + 44.75 * 0.054 )
丂丂丂丂丂丂丂 丂 = 44.75 / 3.42 = 13.08
The feedback ratio
丂丂 Feedback ratio = 20 * log( Aopen / Aclosed ) dB
丂丂丂丂丂丂丂 丂丂丂丂丂 = 20 * log( 44.75 / 13.08 ) = 10.68 dB

Primary 5K 
Primary 5K 
Primary 3K 
Primary 3K 
Impedance of cathode feedback winding 
16 
64 
16 
64 
load ImpedanceRl 
5582 
6195 
3454 
3940 
Feedback factor 兝 
0.054 
0.101 
0.068 
0.127 
Openloop gain by EL34 
44.75 
48.22 
30.8 
34.32 
Closed loop gain by EL34 
13.08 
8.21 
9.97 
6.4 
Feedback ratioBy EL34 
3.42 
5.87 
3.09 
5.36 
1. Frequency Characteristics
2. Frequency Characteristic: with different signal source impedances
The frequency characteristic of the audio output transformer varies with the signal source impedance that drives the transformer.
Frequency bandwidth by signal source impedance
Signal source impedance  Bandwidth 
5K ohms (Standard)  25Hz to 60KHz 2dB 
1.2K ohms (Triode tubes)  11Hz to 80KHz 2dB 
20K ohms (beam tubes)  42Hz to 44KHz 2dB 
3. Phase Characteristic
4. Primary impedance Characteristic
5. Cathode feedback winding Frequency Characteristics
6. Cathode feedback winding Phase Characteristic
7. Primary Inductance Characteristic
The primary inductance of the singleended output transformer has a characteristic of constant inductance. With this, the output transformer can drive steadily the loudspeaker without being influenced by the level difference of the superimposed DC current or AC input signal. The less the inductance fluctuates under various conditions, the more solid and stable sound quality can be obtained.
Primary inductance
Primary 
Minimuminductance 
Maximuminductance 
DC current 
3K ohms 
12H 
15H 
80mA 
5K ohms 
19H 
23H 
80mA 
The primary coil of RW405 is perfectly symmetrical the center of which is set at red lead wire. This construction enables RW405 to be used as constant inductance type pushpull output transformer, fairly suitable to build a DC unbalance current proof, maintenancefree type pushpull amplifier. The table below describes the connections and specifications as constant inductance type pushpull output transformer.
Type  Constant inductance type PushPull 
Output capacity  80W at 50Hz 
Primary impedance  5,000 ohms, with UL (50%) taps 
Secondary impedance  4 and 8 ohms 
3rd.  cathode feedback winding 16 ohms X 2 
Frequency bandwidth  40Hz to 47KHz (1dB), input=4V, signal source impedance=5,000 ohms 
Primary inductance (H)  19H(min.), 23H(max.) 
Primary permissible DC current  280mA (for 2 tubes) 
Primary permissible unbalance DC current  120mA (recommended within 90mA ) 
Power loss  0.23dB 8ohms 
Dielectric withstanding voltage between primary and secondary  2KV AC 
Maximum permissible voltage of primary PP  1KV AC 
Core  Type R160 160W core 
Shape  Enclosed in square case, the same mounting dimensions as those of Tango XE60,FC30 
Connections  Lead wires 
1.Cathode feedback winding of pushpull use
When using the cathode feed back winding, the connection of primary winding
and cathode feedback winding is as follows. In the case of selfbias, a
selfbias resistance and a bypass capacitor are connected between K1Brown
and a ground and between K2+ white and a ground.
Calculating the closedloop gain and feedback ratio
The load of the output tube is both the primary winding and cathode feedback winding in the cathode feedback circuit.
The load impedance of the output tube is not 1.25K ohms of a usual pp circuit
(1/4 of 5K ohms).
The load impedance is as follows.
丂丂Rl = ( SQRT(1250) + SQRT(16) ) **2 = 1548 ohms
丂丂丂丂SQRT : Square route operation
丂丂丂丂** : Factorial operation
The openloop gain on the primary side before feedback
丂丂Aopen = gm * ( 1548 // Rp )
丂丂丂丂gm : Transconductance of the output tube
丂丂丂丂Rp : Plate resistance of the output tube
丂丂丂丂// : Parallel value operation
The feedback factor 兝 is a voltage ratio (a winding ratio) of load Rl
and the cathode winding .
丂丂兝 = SQRT ( 16 / 1548 ) = 0.102
The closed loop gain on the primary side after feedback
丂丂Aclosed = Aopen / ( 1 + Aopen * 兝 )
丂丂Feedback ratio = 20 * log( Aopen / Aclosed ) dB
Example
Value that uses EL34(6CA7) for output tube
丂丂丂丂gm of EL34 = 11000uS
丂丂丂丂Rp of EL34 = 15000 ohms
The openloop gain on the primary side before feedback
丂丂Aopen = gm * ( 1548 // Rp )
丂丂丂丂丂丂 = 11000 * 10**6 ( 1548 // 15000 )
丂丂丂丂丂丂 = 11000 * 10**6 * 1403
丂丂丂丂丂丂 = 11 * 1.403 = 15.43
The closed loop gain on the primary side after feedback
丂丂Aclosed = Aopen / ( 1 + Aopen * 兝 )
丂丂丂丂丂丂丂 丂= 15.43 / ( 1 + 15.43 * 0.102 )
丂丂丂丂丂丂丂 丂= 15.43 / 2.57 = 6.07
The feedback ratio
丂丂Feedback ratio = 20 * log( Aopen / Aclosed ) dB
丂丂丂丂丂丂丂 丂丂丂丂丂= 20 * log( 15.43 / 6.07 ) = 8.2 dB
2. Frequency Characteristic of pushpull use
3. Phase Characteristic of pushpull use
4. Other Characteristics
The impedance and inductance characteristics are the same as those of the
singleended connections. Please refer to those characteristics.