I’m a heavy user of fixed-bias output stages. Yes, I do prefer them despite the additional complexity. However, I’m not looking to open a can of worms around this subject. On the contrary, I wanted to report a fantastic product developed by Situbes.
SiDPM Digital “Panel” Meter
Here is the brief description from the website. I suggest you take a look at the datasheet as well:
The SiTubes DPM is a digital “panel” meter packaged in a standard octal tube envelope.
It measures a DC input voltage from 0 to 2V full-scale. Several selectable legends (V, mA, A, etc.) can be selected by an external programming resistor. It is particularly useful to measure supply voltages (such as plate voltage) and tube bias levels (normally plate current) in tube amplifiers.
The DPM can be powered from an AC or DC voltage. The power input is isolated from the measurement input, so floating measurements can be made up to 1500V above or below the power input. This allows high-side current measurements – for example, sensing plate current directly at the tube plate – or the ability to be powered from a supply that is not referred to ground.
Looking at their construction you will realise instantly the high-quality of this product. Impressive finish and presentation. The tube is made of glass and fits very tightly to the 8-pin plastic base.
I did a simple and basic test on my work bench to test this device and its accuracy. In 5 minutes I wired it on my curve tracer to access the pins easily without soldering a test rig. With a 1Ω 1% resistor I configured the device to current mode and placed my 5½ digit bench meter in series for reference. The refresh cycle is very good, more than what you’d need in normal operation. Accuracy with the reference resistor was great. It’s calibrated as provided by the seller and error was below 1mA up about 200mA which is the planned use case for me. The OLED display has the right brightness for day operation. It’s just great.
You can use them easily to measure anode/cathode current, grid bias or supply voltages in multiple configurations.
They are pricey, but worth every penny. A top quality product which I’m keen to use shortly in one of my next builds.