Edit: Maybe I should start this as a new thread and move the discussion about this to that thread instead of continuing here?
![Very Happy :D](./images/smilies/icon_biggrin.gif)
~Phil
Moderators: pompeiisneaks, Colossal
~PhilR.G. wrote: ↑Wed Oct 24, 2018 5:49 pm Yeah, it's better to go micro-fine on matching. However, there is the problem of having things to match, as you note with "Wouldn't that be cool? ... When we're done dreaming...".
Here's different thing that's less of a dream. We have computers now. It's not really all that tough to make a tube matching tester. Why not set up a tube test stand with appropriate loading (which will vary a little from tube type to tube type, but is no worse than one amplifier output stage per tube type - maybe three or four) and some current and voltage sensors. Hall effect sensors with multi-thousand voltage isolation between the measured current and the sensor output exist, and can easily enough be set up to measure the plate, screen, grid, etc. currents on a tube being tested, with >100kHz responses. Voltage measurements are easy - use resistor dividers to move the voltages down into A-D range.
Modern PIC (and other uC family) controllers generally have 4-12 channels of A-D available, with sampling times easily able to capture sub-1kHz signals. It's also easy enough to feed the tube an AC signal modulated by a digital pot for amplitude, and to convert the output to digital as well. You'd also need a high voltage opamp to feed the grid bias 0 to -70V or so.
All that sounds fussy, but is probably sub-$100 in parts, exclusive of the output loading, which will involve an OT if you try to do a transformer loading. I think you might be able to use a simple inductor-resistor plate load and fake the tube being used single ended if you like. The whole setup could easily enough be run by a $20 Arduino or a $35 Raspberry Pi.
What you can measure with this (and a little programming) is:
-Curve of gm for Vgk = 0 to full cutoff for DC conditions. Simply stepping the DC bias from 0 to cutoff gives you the full curve, not single point measurement, of the bias sensitivity of the tube.
-It also tells you screen current, as you're measuring that as well.
- AC gain sensitivity as voltage out/voltage in.
- internal "kinkiness" of the tube, by doing distortion analysis on the captured output voltage
- a whole lot of other things that I'm skipping just because my fingers get blunt from typing![]()
Testing one tube at a time gives you some better idea of the tube's operation than trying to filter out the confounding issues of PI matching, and so on.
I've paper-designed this thing a couple of times, and always stopped before building it, starting back in the 2000s. I'm pretty sure there is at least one of these things available commercially, and maybe a DIY kit or so.
But in a computer era, getting data is easy.![]()
Bring it on!R.G. wrote: ↑Tue Oct 30, 2018 4:44 pm...
Hmmmm... I don't know why this didn't strike me before - you could build a sampling probe head into an octal-socket standoff so you could do the actual sampling for a tube inside a functioning amp. The grid supply would be carried along on the measurement system, so you could insert the sampling head. Hmmm... no, wait - you build two sampling heads, so you can use it in a push pull amplifier and actually test the output stage separately from the rest of the amp. That saves you from having to build the power supply, OT and other appurtenances needed that are not the measurement system. Of course, it still works with a test bed separate from an amp, too.
Dang. More design to do.![]()
That's a good point. There are a bunch of libraries that can be used for visualization purposes in a web-based software (besides the benefit of not worrying for client OS, etc.).pompeiisneaks wrote: ↑Wed Oct 31, 2018 3:10 pm
Graphing it all could be done in software, or setup to export into a format that people could import easily into a third party software setup. I would love to load it into elasticsearch and see it in apps like Kibana or Grafana.
I'm sure I'll think of more, but gotta get coffee etc. ;D
~Phil