Western Electric had a banner year in 1936. The introduction of the 300A, the mother of all triode power tubes, and the 91A cinema amplifier, a single-ended design based on the 300A, meant commercial success in the 1930s and 40s. The essentially identical 300B was introduced in 1938. It would have been impossible to predict then that these products would play pivotal roles in the directly heated single-ended-triode (SET) renaissance that began in the 1970s, mainly in Japan, and slowly moved westward. All things Western Electric (WE) became valuable in Japan. Old movie theaters were scoured for 91A and 91B amps that would then depart the USA. I would guess that just about all the extant original WE amps now reside in Japan.
It’s not surprising therefore that when Yamazaki-san, Triode Corporation’s designer, decided to design a reference-caliber 300B amplifier, he looked toward the WE 91A for general guidance—a decision that deserves a standing ovation. Think of Triode Corporation’s new M300 as a modern version of the WE 91A. This explains a tube complement identical to that of the original: a 274B rectifier, a pair of 310A receiving pentodes, and of course a 300B. However, paying homage to a vintage 1930s design should only go so far. For example, the gain of the 91A amplifier was colossal at 92dB due to the use of an input transformer and the cascading of two pentode gain stages. But that’s way too much gain for an audio amplifier nowadays partnering a typical preamp, which is capable of delivering at least a 2V signal. The M300 addresses this by dispensing with an input transformer and by using local feedback from anode to grid of the first pentode stage to decrease its gain and increase its effective headroom. In addition, the second 310A is connected as a triode in order to further reduce the overall gain. As a result, the input sensitivity is a reasonable 0.8V. The power supply has also increased in sophistication, following a high-end trend started by Audio Research in the 1970s. The 300B’s 5V filament supply is DC, because that’s the most effective means of lowering residual hum to acceptable levels when driving a high-sensitivity speaker. And for the record, the M300 is about as quiet as possible for an SET amplifier. Residual hum was simply not an issue even when driving a 96dB-sensitive loudspeaker. Likewise, the 310A filament voltage is rectified and actively regulated at 10V.
No global negative feedback (NFB) is used. The option of adding a 3dB global NFB switch was considered during the final design stage but was ultimately rejected on the basis of sound quality. There is usually no major incentive for using global NFB in the context of an SET amplifier as the output-stage source impedance is already fairly low and decent bandwidth and distortion figures can be achieved using a good-quality output transformer. The measured source impedance off the 6-and 8-ohm taps was 2.7 to 2.8 ohms, within the range of 2 to 3 ohms typical for such designs. The situation is of course considerably different for push-pull pentode and ultra-linear tube amps where large amounts of NFB, usually on the order of 20dB or more, are required to obtain a low output impedance and decent damping factor.
Both RCA unbalanced and XLR balanced inputs are provided. However, keep in mind that this is, after all, a single-ended amplifier. Hence, the XLR input should be considered strictly as a convenience in interfacing with a preamp or linestage lacking a single-ended output. The XLR input is routed through an op-amp buffer stage in order to reduce it to a single-ended input, so it’s best to avoid it if possible. Build-quality and finish are exceptional and speak to the care and reverence bestowed on what is intended to be a reference amplifier.
The M300 is shipped with a Psvane (aka Pavane) WE300B. The Psvane brand name is owned by a consortium of audiophiles within and outside of China and has gained a fine reputation for sound quality. This 300B is said to be a faithful replica of the original WE 300B in all respects, including materials and structure. That was good news since I’m the proud owner of two pairs of the Westrex Corporation 300Bs manufactured in the U.S. in 1997. (They are now temporarily unavailable, but Westrex is said to be resuming production sometime during 2014.) I was looking forward to a sonic shootout between these two purported exact replicas of the original. To that gladiatorial mix I added a TJ-Full Music mesh plate from the TJ factory in Tianjin province, China. This is a 300B variant that I’ve enjoyed very much over the years due to its vivid tonal colors and dynamic sound.