Yamamoto Sound Craft A-08S WE101D/104D Single Stereo Amplifier

A-08S is a sister model of A-08S for WE101D and WE104D with smaller output than WE205D, which has been highly evaluated. Although the output power is less than 1W, the sound is as good as WE205D, and this model was commercialized because the sound quality is better than expected at the prototype stage and the output power is sufficient for practical use.
The low output of approximately 0.25W of the WE101D is used as the plate voltage, and 13.5mA is applied to the 250V load to reduce the power consumption to approximately 3.5W. The load impedance at this time is 5k ohms, resulting in an output of approximately 0.6W. On the other hand, the WE104D has a maximum output of approximately 0.8W at a plate voltage of 190V, as specified in the standard table. For both types, the plate current value is determined by fine-tuning the cathode resistor to find the optimum value, and an output of about 0.8 W is obtained as per the standard value. In addition, both vacuum tubes can be used simply by switching the cathode resistor.

WE101D and WE104D look so similar that they hardly differ in appearance, but their characteristics and sound are very different, and it is easy to see that they are completely different vacuum tubes. The WE104D has a feminine, gentle, and soft sound. When comparing the two, the volume position of the preamplifier must be changed drastically due to the large difference in gain. If you listen to them at the same position, the volume is so different that the WE104D sounds less powerful. My impression is that the WE104D has a little too little gain, so using an output transformer of about 2.5k ohms would give better results.

Item	WE101D(ST)	WE101D(Ball)	WE104D(ST)	WE104D(Ball)
Heater voltage	4.5VDC	4.5V DC	4.5V DC	4.5V DC
Plate to cathode voltage	253V	251V	189V	189V
Plate load impedance	5k ohm	5k ohm	5k ohm	5k ohm
Cathode resistance	2k ohm	2kohm	3.5kohm	3.5kohm
Cathode voltage (bias voltage)	-27V	-27V	-58V	-58V
Plate current	13.5mA	13.5mA	16.7mA	16.0mA
Maximum output(THD:10%)	0.69W	0.63W	0.94W	0.81W
gain	21.9dB	21.5dB	15.3dB	15.8dB
Residual Noise	0.62mV,0.82mV	0.44mV,0.91mV	0.44mV,0.63mV	0.44mV,0.62mV
Frequency response(-3dB)	21-27,000Hz	22-29,000Hz	12.6-38,000Hz	12.6-39,000Hz

A bit unbelievably sensational!
The 101D is so much better than the 300B. This is the best amp I have ever had.
My impression of the amp with the WE101D is that it is the most dynamic, clear, and articulate sounding of all the company's WE205D, 45, 300B, and other amps, except at high volumes. This is largely due to the amplifier's non-choke power supply. Ordinary amplifiers use a choke coil to eliminate ripple in the output stage B power supply, but this amplifier does not use a choke coil because the current used is small, and a filter is formed using only CR to supply the B voltage.

The cathode bypass capacitor in the output stage is a lug terminal type polypropylene capacitor made by De Aborn of the U.S. and sealed in a Bakelite case with high-strength epoxy resin, similar to ours, which also contributes greatly to sound quality. In addition, a Siemens C3m vacuum tube with the shield case removed is used as the driver tube in this unit, a pentode tube of a relatively new design released around 1968.

Both WE101D and WE104D were tested with the ST type and the tennis ball type. The ST type had a tight and tight sound, while the tennis ball type had a generous and relaxed sound. The tennis ball type has an interesting shape and is fun to look at, but it is expensive, so we felt that the ST type would be sufficient if you choose from the standpoint of sound quality. The speaker terminals are 8-ohm output terminals only, but there is no problem using 4-ohm or 16-ohm speakers. However, in either case, the maximum output power will be reduced, so a maximum output power of 8W will be reduced to about 4-6W.

Tubes used	WE-101D/104D*, C3m,80
Maximum output	WE101D: approx. 0.6W/8Ω WE104D: approx. 0.8W/8Ω
Frequency response	22Hz-28kHz(-3dB)
Output transformer impedance	5kΩ on primary side, 8Ω on secondary side
Residual noise	0.4-0.8mV
Drive method	CR coupling, self-bias
Size	400(W)294(D)204(H)m
Weight	14.3 kg