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 Post subject: The great curve tracer experiment
PostPosted: Dec Wed 13, 2006 9:07 am 
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I was getting all depressed looking at the prices Hickock testers are selling for on ebay, and I was regretting that I traded the children for a bag of crack, because they would have made a good down payment on a 539C. Anyway, that's water under the bridge, so I decided to content myself with a crappy Superior TV-11 emission tester.

Then, I got to wondering what it would take to convert it into something better. After all, it already has the hard to find components built-in: multi-tap filament transformer, meter, lots-o-sockets. Here's a simplified schematic of an emission tester
Image
This one puts AC across the tube, and the plate voltage can be sensed at point B, and the plate current is proportional to the voltage between B and C. So, if you connected these points to the X and Y inputs to a scope you should be able to get a trace of the tube's characteristic curve.

Here is my tester setup to do this with a triode tube.

Image

And here is the trace on the scope.
Image

Note, the vertical axis isn't really giving a proper current trace, because it's not the difference between B and C, but it's a start.

Anyway, a curve tracer needs a circuit to cycle the grid bias through a bunch of different values, so some extra circuitry is needed. Here's my grid voltage step generator circuit.
Image

And here is a trace of the triode with the grid voltage circuit in operation giving multiple grid voltages.
Image

Here is the output of the grid waveform generator.
Image
The top trace is the grid voltage step waveform, and the bottom is the 60 Hz line frequency that it's synchronized to.

This is the schematic of the grid voltage step generator.
Image

Anyway, I just though some of you might find this interesting. I don't know how practical this is, but it was a fun experiment.


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 Post subject:
PostPosted: Dec Wed 13, 2006 10:56 am 
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Just realized that I had one channel of the scope set to DC and the other set to AC. That explains why the traces are a bit loopy. Here is a proper trace.
Image

Edit:
Also note, the 7915 is a negative voltage regulator producing -15 volts. The schematic shows the 2000 mfd. electrolytic backwards. Positive should be to ground. I didn't actually wire up the regulator part. I just used my bench power supply, so the value of the filter cap may require some tweaking.


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 Post subject:
PostPosted: Dec Wed 13, 2006 1:40 pm 
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Although this thought is outside your original homebrew tracer project (which I consider to be a great one), I might note that most high-end curve tracers such as the Tektronix 571, 576 and 577 can actually do vacuum tubes if you add an additional isolated heater/filament supply for the tube. The dual-switching mode makes it easy to check/compare dual tubes such as the 12AT7. The 576 can handle up to 1000v on the plate and 10a of plate current at lower voltages .... not robust enough to check an EIMAC transmitting tube at full ratings, but still .....

Dean


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 Post subject:
PostPosted: Dec Wed 13, 2006 11:10 pm 
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Just to clarify, this tracer is for tubes, but yes I had considered the idea of putting some kind of switch & socket arrangement into the circuit to allow quick changeover for tube matching. At the moment though, I'm trying to keep down the number of parts.

I was well into testing this out when I realized that when my scope is in X-Y mode, you can't use differential mode, so I wasn't able to properly display plate current on the vertical axis. So, now I will have to build a little differential amp. I think there is a fairly simple one in the application notes for the LM3900 IC. So, I will try one of those.

The other obvious next step is to increase the available plate voltage. My tester only puts out 117 volts maximum.

BTW - If anyone wants to try this, make sure your tester is isolated from the AC line. Some, like mine, use an autotransformer rather than a regular transformer. I added an isolation transformer to mine.


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 Post subject:
PostPosted: Dec Thu 14, 2006 12:30 am 
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It's great to see someone trying new ideas. Looks as if you could put a 10 or 100-ohm current-sensing resistor from cathode to ground, if you set the cathode lever to "open" and ran the resistor from another socket with the same pin number. Depends how much gain the scope has.

I turned my Hickok TV-7 into a curve tracer (sort of), at least temporarily. Dedicating a Superior to this, looks a lot more practical.


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 Post subject:
PostPosted: Dec Thu 14, 2006 12:31 am 
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It's great to see someone trying new ideas. Looks as if you could put a 10 or 100-ohm current-sensing resistor from cathode to ground, if you set the cathode lever to "open" and ran the resistor from another socket with the same pin number. Depends how much gain the scope has.

I turned my Hickok TV-7 into a curve tracer (sort of), at least temporarily. Dedicating a Superior to this, looks a lot more practical.


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 Post subject:
PostPosted: Dec Thu 14, 2006 1:46 am 
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Yes, the Superior is a pretty good candidate because the lever switches can be left in the "no connection" position to allow feeding in grid signals through the adjacent sockets, or connecting a cathode resistor like you mentioned. I used a banana plug plugged into the large 7 pin socket to make the grid connection to the test tube. The connection is shown in the photo above (highlighted with the green circle). My other goal is to do this in the least invasive way possible. So far I've only had to solder to 4 points in the tester and I brought the cable out through the same hole that the power cord uses. I'm just going to solder a connector onto the cable, so I can either plug it into the curve tracer circuit, or continue to use it as a regular emission tester. The only modification that I may make is to add a SPST switch to the plate bus so that I can switch over to my own plate load and plate voltage supply.


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 Post subject:
PostPosted: Dec Thu 14, 2006 2:32 am 
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Another good donor box candidate is the Weston 798 a.k.a. military OQ-3. Not much value as a tube tester any more, because they're ancient things that don't even have nine-pin sockets, but they use jumpers to connect to the socket bank, for the ultimate no-cutting, noninvasive experience. They also sell for next to nothing.


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 Post subject:
PostPosted: Dec Fri 15, 2006 11:07 pm 
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Very nice, Bob :D

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 Post subject:
PostPosted: Dec Sat 16, 2006 2:08 am 
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WOW!!!,is all I can say Bob!!!
How the heck did you figure that out???

If they ever invent a machine that can transfer ones brain knowledge to anothers,please let me be that another.

Leaky Caps.aka Leaky brain cells


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 Post subject:
PostPosted: Dec Sat 16, 2006 3:26 am 
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I fixed the plate amps display problem by building a little differential amplifier using an LM3900. I'm still playing around with some component values, but it is now displaying true plate amps on the vertical axis. Here is what it looks like now.

Image

As you can see it now looks like a proper triode characteristic curve.
So, this could be used to match tubes, and determine transconductance at any operating point within the range of the instrument. I'm going to add a screen bias circuit and see what results I can get with a pentode.

I'll post a updated schematic when I get a chance.


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 Post subject:
PostPosted: Dec Sat 16, 2006 7:28 am 
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I thought it would be relatively easy to add a screen grid bias circuit for doing pentodes, but when I tried it, there was enough screen current on the tubes I was experimenting with (6AQ5 and 6AU6) to affect the voltage significantly. It seems to require a really stiff supply. Anyway, after tinkering a bit, I got a reasonable trace of a 6AU6. This is it.

Image

And this is the current version of the circuit I used.

Image

The LM3900 works very well as a high voltage differential amp, but the two 10 meg. resistors need to be fairly well matched to eliminate common mode signals. It's easy to tell when they aren't right, because the scope trace should be completely horizontal when there is no tube in the tester. A trim pot can be put in series with the lowest value resistor and then adjusted until the trace is flat. It may also require a small gimmick capacitor in parallel with one of 10 meg. resistors, if there is any looping in the scope traces.

The screen grid supply still needs work though. As you can see, I added a lot of filtering, thinking that I had a ripple problem, but it turned out, as mentioned above, to be the high screen current causing the problem.

<Edit:
I went back and removed the second 1 k resistor and 22 mfd. cap from the screen grid bias circuit, and it seemed to fix the problem.>


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 Post subject:
PostPosted: Dec Sat 16, 2006 2:11 pm 
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Hi Bob,

Wow, that's a very clever way to get a lot of functionality for a handful of parts! Another future project for my list.

Question on the LM3900 diff amp. I would think the mismatch between the 680K & 1.5M resistors would make the circuit unbalanced and prone to common mode noise or nonlinearity. I think using a single ended power supply for the op amp can make things a little dicey. I've got a pdf of a TI booklet on single ended op amp circuits which might help you optimize the diff amp. If you'd like it let me know and I'll email it to you.

Great project!

Tim


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 Post subject:
PostPosted: Dec Sat 16, 2006 10:26 pm 
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Thanks Tim. Yes I'd like to see the TI booklet. I'll send you a PM with my email.

As for the differential amp, as long as both inputs have matching resistors (10 Meg. in this case) there should be no common mode problem that can't be balanced out with a bit of trimming. I used a 100 k trimmer in series with the resistor that measured lowest, and that worked fine. The 680 k resistor sets the overall gain. The gain is 680 k divided by 10 Meg. which gives .068. So, in fact, it isn't really amplifying, it's attenuating in order to keep the output within the range of the DC supply.

LM3900's are designed to work from a single supply, although not normally a negative one like I have here. National Semiconductor calls them Norton amps rather than op amps, and they are a little different, as far as biassing goes. They take a bit of getting used to, but I'm quite impressed with them.


Leaky Caps wrote:
How the heck did you figure that out???


Hi LC,
The only part of the circuit that's a bit out of the ordinary is the step generator part. There are a number of circuits for them floating around, but they all work pretty much the same way. They act similar to a voltage doubler circuit. On the positive half cycle, C1 is charged, and then on the negative half cycle the charge is dumped onto C2. C2 is much larger than C1, so the voltage on it only rises by small increments. By connecting D2 to the emitter of Q2, this compensates for the rising voltage on C2 and keeps the voltage increments constant.

BTW, I just noticed that I have D2 and D3 shown backwards in the schematic. Oops.

The circuit I based this on, originally gave a positive going step waveform, so I changed to a negative supply, swapped the PNP and NPN transistors, and reversed the diodes. I also added Q1 to square up the sinewave input.

Q3 and Q4 discharge C2 once the voltage reaches a set value. The original circuit used a unijunction transistor to do this, but they aren't available in an opposite polarity version. I fried 2 unijunctions trying to come up with a way to make them work, but eventually I subbed in Q3 and Q4 wired back to back as a trigger circuit, and they seem to perform even better. You could probably use a small SCR in place of Q3/Q4.


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 Post subject:
PostPosted: Dec Sun 17, 2006 1:49 am 
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Bob, you really did something great here. I have a good transistor curve tracer that (one day) I want to convert for tubes. Your design is probably better, have not looked at stepped output on my tracer yet.

I would think that by curve-tracing, a whole new level of understanding would open up.

Thanks for sharing your interesting and eloquent design work.

ds

edit: eloquent - elegant? I don't know - it's nice.


Last edited by ds bk on Dec Sun 17, 2006 2:20 pm, edited 1 time in total.

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PostPosted: Dec Sun 17, 2006 3:22 am 
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Thanks for the explanation Bob.

Hats off to you for coming up with this curve tracer.

Don


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 Post subject: Re: The great curve tracer experiment
PostPosted: Apr Thu 24, 2014 8:59 am 
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8) Bob this is really cool!! Did you formalize it into a finished project or did it stay at the breadboard stage?

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 Post subject: Re: The great curve tracer experiment
PostPosted: Apr Thu 24, 2014 6:40 pm 
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Nice project. I had not seen before.

I have thought that my EICO model 443 semiconductor curve tracer would take care of the step generator and scope interface parts of a triode tube tracer, as an add-on to my Precision Apparatus 612 emission tester. The Eico has fairly high voltage ability.

The combination, plus scope, would need a bunch of bench space to experiment with, however. And then to add a screen power supply and a meter or two, the size is a problem.

My BK 501A is another nice semiconductor curve tracer, and a couple of other brands show up on Ebay periodically, like Leader, Williams/Sprague, Heathkit

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 Post subject: Re: The great curve tracer experiment
PostPosted: Apr Thu 24, 2014 6:57 pm 
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If you can your hands on a Tektronix 576 curve tracer, it will do tubes very nicely with its internal 1000V collector (plate) supply. The only external piece of equipment needed is a power supply for the heater. I keep thinking that it would be fun to build up some vacuum tube test fixtures for the beast so I could easily curve-trace tubes without having to resort to a whole mess of jumper wires, but you know how it goes... I mostly use it for transistors and ICs these days anyway.

-Matthew

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 Post subject: Re: The great curve tracer experiment
PostPosted: Apr Thu 24, 2014 7:07 pm 
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Didn't Hickok's 440 semiconductor tracer have an optional output and filament supply on the side to plug something like a CA-5 to sweep tubes ?


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