music, guitars & other obsessions

DIY Valve Curve Tracer Adapter v.2

While working on the grid drive amplifier, I discovered that the plate sockets were arcing to ground when the plate voltage exceeded ~150V, so it’s time for a new panel! In addition, I replaced all the mini-banana jacks with standard size, recessed banana jacks that that are fully insulated from the front panel – now no more arcing!

Here are some pictures of the new front panel with the completed adapter in action:

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while the front panel did turn out better than the first one, it still has numerous blemishes and defects – some letters did not transfer properly or were rubbed off during the drilling/cleaning/assembly stage. I will try and get one made professionally next time, since I do not think the diy heat-transfer method is ever going to get the kind quality that I would like to have…

While the adapter is working ok, I was never entirely happy with the grid drive amplifier – especially the fact that I have to use large grid stop resistor to prevent parasitic oscillation during the tests. I tried several circuit topologies, layouts, wiring, etc. But none really did the trick… Weeks passed and little progress was made, so I finally decided to go with a power opamp – the design is already proven by Alfredo in his adapter, so I don’t see why not give it try. Instead of making my own board to mount the LME48910 like Alfredo did, I ordered a pre-made PCB driver board from Taobao, so it should take no time to put the whole amp together with some associated parts.

In addition, I will put the rest of the adapter circuit on a PCB – this one has to be DIY and the whole assembly once it is finished in a shielded chassis – that ought to get rid of most if not all the problem that I had with the EMI interference. I am expecting perfectly flat grid drive to the tube and nice, clean curve traces on the scope. We will see if it will turn out that way soon…

Finally got the PCB for the LME49810 yesterday, had to make a bunch of changes to the wiring with jumpers – no trace was cut which is a good thing, in case, I want to use it for a real amplifier one day…

Unfortunately, I messed up the power supply, which may have kill the chip, will need to take it apart and run some tests to see if it is really dead. The part isn’t expensive, it is more of a time-wasted issue, oh well.

I think I should be able to fit everything under the existing footprint, so no modification need to be made to the existing boards or the “chassis” – but it all depends on whether the EMI can be reduced to an acceptable level and if the grid drive signal can be cleaned up with the new grid drive amplifier, but I think I am pretty close to completing this project, finally…

Got the LME power amp working, unfortunately, the result was no better than the simple MOSFET driver. Since the board also took up more space and requires a separate power supply, the ROI simply does not add up. So I put back the MOSFET driver board and wrap up the project – just have to live with the grid drive as is.

After the tribulations of the past few months, I have learned a lot about the inner working of the curve tracer, while the DIY effort did not pay off fully as I had hope, the experience was still worth it (a bit hard on the pocketbook though). But to proper measurements, I decided to purchase an used transistor curve tracer with some mod for tube curve tracing. More to come on my new purchase…


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