Tube Curve Tracer

[tubesornothing]

One of psychonoodler's threads got me all wire up about tube curve tracers.  I can't seem to find an old tek 370 for under $4000.  The sofia is no longer made, and the closest production one (VacuTrac) is $2000 and only does up to 300V.  I have a few plans kicking around anyone feel like designing one with me?  I would like to spread the pain...

P.S.  Don't ask why - I just think it'd be cool.

[jhadhar65]

I'm interested in the project, even though I don't have the knowledge to contribute in any meaningful way.  There will be a big investment after the fact in terms of tubes for standards.  You'll need enough of each brand and model to make sure you end up with representative curves for a bogey tube.  You can start with tubes that have already been matched by the distributor/reseller, but expect wide discrepancy margins anyway.

Then you'll have to decide on conditions.  For example, I'd start with the same circuit conditions used on common data sheets for that tube.  This would be really revealing and give us good curves to compare to older standards, such as GE or RCA tubes.

I'd also like to see curves with other circuit parameters.  For example, typical performance is usually based on Va & Vg2 = 250VDC.  Plate curves tend to set g2 at 250V as well.  I'd like to see additional curve traces with g2 at different voltages up to max rating, like new 6L6GC curve charts for Vg2=250V, Vg2=300V, Vg2=350V, Vg2=400V, and Vg2=450V.  Maybe there wouldn't be enough difference between these charts and that's why they never made any, but I would think there would be differences.

I'm rambling.  I'm interested, though.  If the results of something like this were published to the web, I would expect a ton of traffic at that site.  Tell me how I can help.

[mackie2]

tubesornothing--

Look at these possibilities:

http://www.tech-diy.com/MCTracer_no_pix_files/MCTracer_opto_ver_feb6_04.htm

http://webpages.charter.net/dawill/tmoranwms/Circuits/Curve_Tracer.gif

http://www.diyhifisupply.com/files/diyhs-tj-factory-tour.pdf

Mackie2

[tubesornothing]

Wow, great links.  I saw the MCTracer stuff - good stuff. I like the simplicity of Tim Williams manual tracer.  Maybe that is a good place to start, just to get the feel for it.

Roughly my goals are as follows:

- make it work with an analog scope (can always use a digital scope to capture images to the computer)
- cheap and simple
- triodes, small signal pentodes and power tubes - full voltage and full current?!?!
- ability to run two tubes at once (or two halfs of a triode or two pentodes) to do a full tube comparison/match
- no need for meters, we can use our DMMs

Here is another link.  Not much use, but interesting:  http://www.techlib.com/electronics/curvetrace.html

I have been reading the following:

(1)  Have a variable B+ supply.  Being cheap, I will have no regulation, so have a PT with a variac behind it.  Use a seperate filament transformer.

(2)  Have step generator that can step through the grid voltage with some degree of accuracy

(3)  Have a sawtooth generator, in sync with the steps that will cycle B+ on and off for each step

(4)  Somehow feed this to the X-Y of the scope (I know this is how they do it, but not too sure)

I have no clue what to due with screen voltage though (for pentodes).  I guess we need some variable way to bring down the voltage.

[tubesornothing]

Tim had an interesting note on an improvised tube tracer:

First you need a scope. Hook up an unrectified B winding (say 300VAC) to the tube's plate. Whatever voltage you want to swing up to. A variac or rheostat (NO LAMP DIMMERS!) will come in handy to change voltage if you don't have a suitable transformer on hand, or want it continuously variable. Make sure the rheostat can handle the tube current!

The tube will rectify on its own. You can put in a diode to protect it (or two or four if you want to be fancy-shmancy and add a negative load so there is little DC in the transformer) if you care. Hook up a C- supply (variable grid bias with voltmeter) and A supply (heater; 6.3V filament transformer or whatever) and you're almost there.

Connect a small (10 ohm or so) resistor in series with the cathode, or transformer return (current that flows in the plate flows throughout the supply winding and back to ground, equal at all points because it's a series circuit). This senses current, connect to the VERTICAL (Y) scale on your scope. It goes 10mV/mA so 10mV/div reads 1mA/div, and so forth. You'll have to invert the display if you use the ground return method. Connect the HORZONTAL (X) scale to the plate, or if voltage swing is greater than you can read, add a voltage divider. 100k to 1k will give 1V/100V (not exactly, but we don't care about the 1% in the divider, let alone 5% in the resistors or ~3% in the scope, depending on what you have), which is 500V full screen (10 div) at 500mV/div.

[mackie2]

tubes or nothing--

I downloaded a copy of Tek 570 curve tracer manual. Large file--need a laser printer!

http://bama.edebris.com/manuals/tek/570/

Mackie2

[tubesornothing]

Very cool. Interetsing things to note:

120 or 240 steps/sec
voltage between steps: 0.1, 0.2, 0.5, 1, 2, 5 and 10
150mA peak grid current
selectable load resistance between 300 and 1M ohms
selectable peak voltage 5-500V

plots a wide variety of things:
- plate current vs plate or grid voltage
- screen current vs plate or grid voltage
- grid current vs plat or grid voltage

only $925!!!!

[tubesornothing]

A buddy from another forum sent these schematics to me.  They have been graciously donated by the Valve Wizard (Merlin Blencoe).    He suggests a couple of improvements:

(1)  a screen supply that is only on when plate voltage is on
(2)  the plate/screen supply is a sawtooth wave to  reduce dissipation

[tubesornothing]

After looking at the main schematic a few hundred times, I think I got what he is doing here:

U1 - 4011 NAND gate - I am figuring this must for the basic pulse.  But I have no clue how this pulses the plate voltage as he shows in the graph.

U2 - this is a decade counter and is used to generate the steps for the bias supply.  I presume SW1 sets some ranges of the bias voltage.

I think this would form a good basis for the tracer.  Thoughts? Comments?

[tubesornothing]

I have been thinking about the power supply and metering (instrumentation).  I would like to make this work with either a variac or a HV power supply.  I have both which is good for me, but for others it might be better to have the option.  Anyways, with the variac option here is what I am thinking:

(1) 120VAC supplies a variac which is connected to a PT for HV B+ and screen power.  Pick your favorite HV fender PT for this job.

(2) 120VAC supplies a second PT for Heaters and Bias.  Just grab any PT from the box.  At most it has to supply 2 power tube for heaters and some kind of bias supply - 100V will do just fine.

Now Merlin's above design uses a single supply for Heaters and Bias, with the heaters being regulated.  I think to cheap out a bit, the heaters could just be run off the winding of the second PT.

As for instrumentation, I figure just a bunch of test  points. $10 DMM can be plugged in at key points:
- bias
- plate
- screen

[Merlin]

Hi,
here is an ultra-simple curve tracer design, so you can see how they work. This one has manual bias adjust of course. The anode is fed with half-wave rectified DC to keep dissipation to a minimum.

 This circuit only tests triodes and triode-strapped pentodes. A 100mA PT should be ok to test bottles up to EL84 and 6V6GT size. For bigger bottles I suspect you'd need a more powerful PT (or do the tests very quickly...)

More clever designs will provide a staircase generator to step the bias down automatically, producing family of curves.

To test pentodes you would need a screen supply of course, but beware! The anode spends half the time at zero volts, so the screen dissipation will be very large during these periods! You really need a circuit that switches the screen supply off while the anode is at zero (I am working on a design that does this).

[FYL]

One of psychonoodler's threads got me all wire up about tube curve tracers.

There's an excellent module sold by Alain Ducrocq, a French solderhead. Works either with a scope or a PC thru a serial port (or coupled with a serial to USB converter). Just add the PS transformers, some glue and case it... You can draw curves, measure current for any given grid bias and of course measure transconductance.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250497917113

He has also devised a simplified portable tester, fitted with DC to converters and nearly everything required (just add a reg for heaters, there's room, pads and traces on the board).

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250497158376

English docs are available, just ask him if you're interested.

[tubesornothing]

Alas, it only supports up to 330V and it does not have anything for pentodes (i.e. separate control of screen voltage).  Any suggestions to modify it?

[tubesornothing]

Ahh, I notice in the addendum to the circuit he has a schematic for testing pentodes.  Now its just seeing if I can get a 500V curve out of it.

[FYL]

Alas, it only supports up to 330V and it does not have anything for pentodes (i.e. separate control of screen voltage).  Any suggestions to modify it?

The actual test voltage is app. 1.3X AC, so 330V AC => app. 430V plate. You can go up to 480V.

Pentode testing is quite easy : just add a variable or tracking grid supply. There are examples in the doc (cf. enclosed pic), but they are quite primitive.

A much better approach is to Mosfet source follower and it's glue in the screen supply. Something like the attached schemo will be fine even for beefy tubes such as KT88's. And yes, it's a basic VRM/VVR/Power Scaling/... module, so you can use it for other applications <WINK> <NUDGE>.

Random notes : the pot should be a 1ML with suitable isolation - standard large Alpha or PEC pots are OK up to 500V, isolated plastic pots up to 1.4KV, the Mosfet should be heatsinked (a compact 12°/W heatsink is OK). You may use a small Mosfet such as the IRFP450 but it'll be marginally spec'ed for 480V plate.

Eagle sch and brd files are available if anyone is interested. Any half decent circuit house should be able to use them.

[tubesornothing]

Merlin,  in your design (the one with the decade counter), one thing I cannot see:  how is the plate voltage cycled on and off - or is it on all the time?

[FYL]

As screen current is quite low (10 mA max for a 6550C), you may also consider using an integrated high voltage regulator such as the Supertex LR8, which works fine up to 450V in/438V out and 10 mA typ./20 mA max.

Datasheet: http://www.supertex.com/pdf/datasheets/LR8.pdf

[Merlin]

Merlin,  in your design (the one with the decade counter), one thing I cannot see:  how is the plate voltage cycled on and off - or is it on all the time?

You can see the rectifier diode which feeds the anode, on the left (diagram of the decade counter). It's fed from the HV transformer.

[tubesornothing]

Yes I see that, but what has me confused is:  how does the anode voltage get cycled on and off with the grid stepping?  Somehow to do with the NAND gate?

[Merlin]

Yes I see that, but what has me confused is:  how does the anode voltage get cycled on and off with the grid stepping?  Somehow to do with the NAND gate?

When the AC swings positive the NAND gate receives a pulse which is phase shifted just enough by the 0.1u cap to produce a square wave that is offset from the pulses applied to the anode. The bias then steps whenever the anode is at zero.

[tubesornothing]

Very cool.  I am definitely going to build this circuit.  I would like to make two changes  any advice would be greatly appreciated:

(1)  The HV is going to be fed by a separate transformer connected to a variac so I can vary the B+ I am guessing this will screw up the nand gate etc.

(2)  If I was to put on a simple screen supply, could I just put in a hi wattage variable resistor off the plate supply?

thanks!

[Merlin]

Very cool.  I am definitely going to build this circuit.  I would like to make two changes  any advice would be greatly appreciated:

(1)  The HV is going to be fed by a separate transformer connected to a variac so I can vary the B+ I am guessing this will screw up the nand gate etc.

(2)  If I was to put on a simple screen supply, could I just put in a hi wattage variable resistor off the plate supply?

thanks!

A variac won't mess up the NAND gate- it should work with any supply voltage.

However, the decade counter design I used is pretty silly- too many resistor values. I have a new design using a 4 bit counter which is a bit simpler if you want.

Also, move the Y-sense resistor to the grounded end of the transformer to reduce error.

However, if you move the sense resistor then you can't take a screen supply off the plate supply (it will create an error), so I guess you have to make a decision there!

[tubesornothing]

Hey thanks for the quick reply. I will get it setup on a protoboard and start playing with the circuit and see what happens (or blows up!).

I am happy to do the decade counter,  but I was just about to order parts, what was the component ID for the 4 bit counter?

[Merlin]

Here's the schem.
The switches are on-off-on type.

[DummyLoad]

this device might be of interest to you.

http://www.mskennedy.com/client_images/catalog19680/pages/files/130rc.pdf

[DummyLoad]

for regulated DC filaments - for all you cork sniffer audiophiles...

10A linear V reg.

http://www.mskennedy.com/client_images/catalog19680/pages/files/5012ri.pdf

20A version.

http://www.mskennedy.com/client_images/catalog19680/pages/files/5021rg.pdf

[tubesornothing]

Hey Merlin,  I put together your first circuit on the proto board - the one with the decade counter.

I am getting a good trigger into pin  14 of the 4017, but the output, as measured into pin 2 to the op amp (U3 OP275) only show a three level stepped staircase (with a bit of noise).  One thing I did not have was a 13k resistor - is this my culprit?

thanks

[Merlin]

but the output, as measured into pin 2 to the op amp (U3 OP275) only show a three level stepped staircase (with a bit of noise).  One thing I did not have was a 13k resistor - is this my culprit?

thanks

The feedback trimpots should be 47k (50k in US?) I think- it wasn't shown on my schem. Adjust RV1 to get  2V bias steps (switch in off position).

Then throw the switch to RV2, you can then adjust it to get 1V bias steps, since it's in parallel with RV1. Then switch to RV3 and do the same to get 0.5V bias steps.

However, pin 2? That's the inverting input. Don't you mean pin 1?

The 13k resistor shouldn't be a problem- I assume you made it up with some other values?

Also, make sure you're supplying the logic chips with a supply voltage that is slightly larger than 5V, e.g., from a 5V1 zener.

You might try hanging an LED+1k resistor off each of the 4017 outputs, so you can check that it is counting properly.

[bnwitt]

Not to hijack this thread, but Merlin, I received and started reading your new book.  Nice job on it.

[Merlin]

Thanks bnwitt 

Tubesornothing, how did you get on with the curve tracer? I can help out if you are having any problems?
(My laterst curve tracer has been a right pain in the neck, the PCB is covered in mods now, and I think the leads from the screen-grid transformer have broken inside due to being bent around too often, dammit!)

[tubesornothing]

Hi Merlin,  I got the decade counter "sort of working", but not complete - I needed to get a couple of amps done.  That being said, thanks for the kick in the butt, I will tear into it this weekend.  I would love to get this done, as a lot of the data sheet I have dont have pentodes for higher voltages which would really help with some of my designs.

It would also be nice to use this as a full on tube tester, which would allow for me to consistently hand select tubes to go into my amps.

[Merlin]

I saw your other thread Tubesornothing, shame you don't have the time for this one!

I've managed to get all the triggering on my MkII tracer working, but in the process the screen transformer burnt out (Chinese rubbish pulled from an amp). Typical, it'll have to wait around for ages now until I find another 30V transformer.

Anyway, hope you find someone to build you a tracer- I am happy to provide help or clarify the circuit.
Once I have the MkII working I intend to start work on a MkIII using a random-phase opto-triac to trigger the anode and screen voltage. Hopefully this will allow me to use full-wave rectification while keeping anode dissipation to a minimum. This in turn allows the use of a smaller HV transformer and a more rapid refresh rate of the displayed curves. If it works, I will try and make it into a workable project for others.

[DummyLoad]

I intend to start work on a MkIII using a random-phase opto-triac to trigger the anode and screen voltage.

perhaps what you're looking for is available packaged already...

http://www.crydom.com/en/Products/Catalog/l_pcv.pdf

[PRR]

> P.S.  Don't ask why

Yeahbut.... WHY?

You started this over 2 months back. You are hung-up, not on the measurement, but on a gimmick to "save time measuring" (a doo-dad to turn the voltage knob for you).

In this time, you could have HAND-measured dozens of tubes.

You want 2 Variacs (100VA is plenty), a big (Bassman) PT and B+, a small (Champ) ditto, another PT for some 6VAC, some 9V batts, a pot, five $9.95 DMMs, and a spreadsheet (if you can't find spreadsheet paper, I guess you can use a PC).

Really, this goes faster than you seem to think. A lot faster than fumbling with CMOS and MOSFETs and a 'scope. And a 'scope doesn't record.... the spreadsheet gives a permanent record of your data. (Adding A/D conversion and a PC to data-acquisition automagically is "possible" but a lot of work and debugging.)

This IS how all the data-books were made. Junior engineers with white shirts and pocket protectors locked in a lab with meters and notebooks.

> hand select tubes

Put the tube in a mock-up amp, target Plate and G2 supply voltages, with a 1K or 500r 50W plate resistor in place of an OT. Set nominal G1 bias. Note cathode current. PUSH-button short G1 to ground. Note cathode current ASAP. These are the two essential points for a power tube. ""Match"" by selecting two tubes with similar idle current and peak current.

[tubesornothing]

Thanks PRR - I love how you put it very plainly for me.

You are right, I want a do-dad to do the work for me, so I can make my life easier.  I have the ingredients you have listed and will give it a go, just for the learning experience - and to get me by until I get an automatic unit in my hands.

I will still go for the automatic unit, because it make it a little easier for me.   I spend most of my "shop time" on building amps, so in what spare time I have, I would rather be building amps, rather than testing tubes to see if they are within my specs. BTW, I have a PC based scope, so recording screen shots is not much of an issue.

...I am just one of those guys who likes toys...  things that go beep and spin, and flash, and whir...  (and plots curve characteristic charts)

[Merlin]

This IS how all the data-books were made. Junior engineers with white shirts and pocket protectors locked in a lab with meters and notebooks.

I'm not sure I believe that. Since they had to test hundreds of tubes to get a bogey value, a curve tracer just sounds cheaper to me than a bunch of white-collars. But I could be wrong...

In any case, having a tracer is like having a hole punch. You don't realise how you can't live without it until you have one! Now that I can view the curves of a valve in under a minute there's no way I'm going back to a stack of DMMs and a note pad. Plus you can do interesting things like watch the curves change during the warm-up period, or when varying the screen voltage, for example.

And where I come from, two variacs can cost as much as building a whole tracer!

[jjasilli]

http://www.tech-diy.com/MCTracer_no_pix_files/MCTracer_opto_ver_feb6_04.htm

Does this help? These guys are into POOGE (Progessive Optimization Of Generic Equipment).  The idea is to use a standalone HV bench Power Supply; and a standalone 'Scope.  Then you build only the "interface" between them. 

[tubesornothing]

Hey thanks for the link.  I recall seeing that one.  It actually does not drive a scope directly, but uses a basic stamp to send out data on a serial port to a computer. However, it might have some good ideas that might help. Merlin and I are working on a high voltage tracer together (IOW he is doing the hard work, and I am offering meager support).  Hopefully we'll have something early next year.