Yamamoto is well known for its Teflon tube socket. Yamamoto has a very wide range of Teflon tube socket, from standar Noval/Octal to very unique like UX5 or UX6 or even 10 pin type. I have ever discussed some here.
But maybe not many people knows that Yamamoto also has panel meter (mostly mA meter). Though we can find VU meter and DCmV meter.
Build quality of this 38 mm size panel meter is superb and detail. One of the finest Japanese product. The needle also reminds me of vintage Triplett or Simpson (but without arrow on the end). The background text also build with attention to detail. Simple but elegant!
According to Yamamoto, the accuracy of this meter is 2.5% according to JIS standard. There is also optional round flange to cover the screw.
The unique factor of this meter is it can be installed “outside” signal loop. Just put this meter in series with additional resistor then paralleled with cathode resistor. Voila, this meter will measure the current flow accurately. The additional resistor in series must follow basic rule: for 50mA meter, use 50x resistance value compared with the cathode resistor. Use 100x for 100mA meter, 150x for 150mA meter and so on.
Below is the schematic taken from Yamamoto Sound Craft website.
As easy as ABC, we can have a meter that will precisely measure the bias current of the tube. Normally people will use a switch to turn it on/off when needed, instead of permanently using the meter. Also usually single 3-way switch is used, so we can monitor left or right channel, then put it on the center to turn it off.