Another piece of the puzzle arrived over the weekend: a relay board that can be driven from a microcontroller. There are eight relays on this board that can pass up to 10A of current at 120VAC, or 10A of current at 24VDC. Fed by an external power source to control the relays and power the indicator LEDs, the eight channels can be switched independently and can be configured as normally-closed or normally-open by selecting a different binding point.
I will be using this relay board to switch into the circuit, different filament voltages for the tube heaters in my Digital Vacuum Tube Tester project. I expect I will be switching 1V, 2V, 4V, 6V, 7.5V, 12V, 50V and 120V to use for the tube heaters in a normally-open configuration as those are some of the more common tube heater voltages. All AC, although I will likely add a switch-selectable option (not programmed) for a rectifier and filter after the relay board to convert the AC heaters to DC. This board came from a Bulgarian eBay store, and took the better part of 2 months to arrive, but looks like it’s decently constructed and should be reliable for some time to come.
The first of my parts for the Digital Vacuum Tube Tester have arrived today – a set of three ceramic tube sockets, the most reliable and strongest type.
I’m waiting on my solid state relays to arrive, and am still keeping my eyes open for a transformer and meter.
I’m in the early stages of brainstorming another project idea that will combine vacuum tubes with modern microelectronics in a useful and innovative way: a computerized vacuum tube tester.
Tube testers themselves aren’t very complicated devices – the simple ones, are just a transformer wired to many different sockets and a meter to measure the tube emissions. Better quality testers employ multiple circuits for each tube element: dynamic testers split the tube plate (high voltage) and grids (control voltages) into separate circuits to provide a more detailed reading, and transconductance testers take it a step further to measure certain tubes, like audio amplifier tubes, under actual operating conditions to allow for matching, a feature highly desired by audiophiles. But ultimately no matter how many separate circuits are employed in the testing, they’re all just a series of switches that are engaged or disengaged depending on the requirements for the tube under test.
This Precision tube tester, a dynamic tester, is my current bench model. It’s somewhat difficult to operate, though – look up the lever positions in a chart on the computer, set the levers, turn it on and wait a few minutes, engage a series of switches sequentially then hold the button to read the meter.
My idea is to, at first, construct a simple emission tester – except instead of switches, use a microcontroller to trigger triacs and relays based on either a lookup table or supplied values. There’s already been one attempt at a computer-driven tester, but that model is a bit less automated than I’d really like. Not to mention, it looks like it’s no longer in production.
Once I’ve worked out the logic, I’ll expand the design – add multiple sockets, enable USB communication, and hopefully work up to producing a fully-automated transconductance tester capable of measuring every tube used from 1920-1970, on full auto. I’ve placed a parts order for a basic microcontroller (the Arduino), a few small tube sockets, and am looking into substitute meters – my current thoughts on that are a digital multimeter with serial output – the multimeter handles the measurement of the electricity and sends it to the Arduino, which will handle communication from the computer regarding settings and back to the computer with the resulting data. There are hacks out there to enable serial on multimeters that don’t come with it, but I might just buy one with the protocol built in for ease of use.
I expect to start designing the circuit schematics for this tester in the next month or so, once I’ve cleared out my backlog of radios, and get into construction of a product over the summer.