Tube Curve Tracer Adapter
After reading the DIY Curve Tracer thread at diyaudio, I decided to build one of these adapters for the oscilloscope for myself. The adapter has a pretty small footprint, using an A4-size PCB that I have laying around, I try to make all the sockets, knobs and switches fit, it is a bit tight, but it should work just fine. For the larger tubes and other tube with odd sockets, they will be hooked up via jumper cables and tested off the board.
Besides the top/front panel that holds all the sockets, pots and switches. The adapter contains:
1) Power Transformer.
2) Power supply board, the on-board power supply provides voltages for the logic circuits and the grid bias. The plate, screen and filament voltages are all supplied by external bench supplies.
3) Grid step generator and driver board.
4) [TBA] gm/THD test rig a al Moglia.
During the October holidays, I made some good progress on building the adapter, of course, there were numerous mis-wirings, in-correct parts that threw me off track, but in the end, I managed to de-bugged it, and may be even improved a bit over the original design…
Anyway, as usual, the biggest problem now remains the mechanical aspect – the front panel and chassis – same old story again – the unit still looks like a high-school science project (may be even worse!) I tried the head-transfer lettering for the front panel, of course, the alignment was off, not only that, the holes for the various connectors also did not line up straight – not surprising, considering I was using a small table-top drill press and just eye-balling the template for the hole locations, a few millimeters off here and there, and that was it…
Before I toss in the towel, I think I will give it one more shot, this time, I will do the heat-transfer lettering first, clear-coat-and-seal, then drill the holes after it dries, hopefully by drill the holes with the texts already in place, I can get a better alignment, also thinking about whipping up a simple jig to help with the hole alignment – I find that even a few millimeter mis-alignment could be easily detected, so getting the holes to line up is a must.
The problem with doing thing this way is that the finished “product” still may look amateurish. But if I spend the money to get either a CNC-mill or send out the front panel/chassis to have them made, I might as well just buy the damn test gear second-hand!
As is, the extra parts that I had to pick up because of the minimum order size and the number of trips that I had to make to the stores locally, probably already exceeded what I could buy without going through all the hassle (of course I do not get the learning experience gained along the way by build them myself, but that’s a separate issue). From a cost-effective angle, it’s at best breaking-even, but much more likely, I am way in the red… That’s why it is called a hobby and not a business 😉
Ran a quick test on the adapter – it worked but the traces look rather bad due to the multiple retraces, need to clean them up first before getting too far along.
Still trying to clean up the traces, but the adapter is usable at least.
I think I finally manged to sort out the issue with double traces, as suggested by the good folks at diyaudio, the problem was with the grid drive – its waveform got distorted as the plate voltage increased. The step voltages were not constant thus creating a loop when the curves were traced. The fix was pretty straight forward – re-biasing the mosfet driver and connecting the source follower to ground instead of 15V, resulted in a more solid grid drive.
Now I just need to clean up the mess of wiring and tidy things up a bit, then I am ready to do some curve tracing… Finally!
Most of the double-traces are gone, fuzziness is due to line interference which seems to occur between the hours of 8AM-9PM!
Still working on the grid drive amplifier, I did not think such simple circuit would require so much time – there are only a handful of parts, for Christ’s sake… Parasitic oscillation is a real problem here, I think I need to move the driver FETs closer to the grid as the current wiring is bit too long, whatever it is, I need to get to the bottom of it, so I can wrap up the project, which has taken far too long than I had expected.
During the trouble-shooting and experimentation, I discovered that a simple unregulated power supply for the FETs just did not work, even though on paper, that’s all it was required, the transformer was already over-rated for the current draw expected. Anyway, with the unregulated supply I got ripples on the grid steps, which of course caused double traces on the curves.
I also tried a simple DC-DC converter for the drain supply, but it generated too much noise and the display was all distorted. So it appears that the current topology is very sensitive on its power source.
What happened was so strange, I just had to post it – when I tried to trace some curves this afternoon, the current kept shooting straight up, so I thought something must be faulty, mis-wired, or shorted, etc. First, I swap out the INA128, thinking perhaps it was burned out during one of the earlier tests… No that wasn’t it. Could it be a bad tube? Swap the tubes, still the same result, WTF! Took everything apart, and discovered that the plate sockets were shorted to ground! Whoa! How could that happen?! The sockets were isolated from the front panel/ground by plastic washers (top & bottom), and tighten down by nuts, so how can they even move around? But obviously they did somehow!
Based on this totally unexpected event and the fact that the wiring needs to be shorten to reduce parasitic oscillation, I think a new build is called for – new PCBs for the controller, new front panel and layout – the works! I will also take this opportunity to make the unit larger, so it is not so cramped…
After moving the grid drive amplifier onto its own board and closer to the grid sockets, the grid drive signal is much improved – clean and free of line interference artifacts. So now it is a matter of final packaging to lock everything into place. Will also wire up the volts/step selector as well as the gm/THD test rig into place. Still need to pick up a few more parts – momentary switches, on-off-on switches, non-polarized capacitor and may be a small +-70V transformer (for the built-in B+ and B- supply for the grid drive amplifier).
New grid driver amplifier on its board:
Nice clean curves (this one of the 6P14P):