If the tube tester was designed for a lower voltage i.e. 110v to 115v and your voltage is 125V, when not install a bucking transformer?
A 12v may work, but will depend on current and the I^2R losses of the transformer. Size the transformer small enough so it doesn't output
much more than 12V when under load, but with the capacity that it doesn't overheat. You may have to look* at lower voltage transformer if the
current drawn by the Tube tester is low.

*As you know a transformer will output 17v at a low load and then 12V when loaded to max specified current.

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Eric,
I'm not to fond of using a Sola transformer for various loads on my test bench, as I understand the output waveform? distorts significantly with different loads. I prefer to use a? a manual Variac.

However, related to your comment on easing the stress off of your tube testers, I learned a good tip from Paul Hart (the tube tester guru). Note: My incoming line voltage stays around 125VAC. Most tube testers were built during the days when typical line voltages were closer to 110-115VAC.

Paul advises to connect my TV-7U tube tester to my Variac before powering. I then set the "line control" pot to maximum, then turn the tester power switch on, with the Variac off. I then bring the line adjust meter set point to where it belongs with the Variac on, (not with the set pot on the tester). This takes most of the unnecessary heat off of the tester power transformer (at least in most Hickoc models, (which includes the TV-7 series)) and tends to preserves the tester, by reducing the accumulated heat in it. I do the same on my old Eico 667.

I hope this makes some sense for you.
73,
Ray, W4BYG

I have a new to me Sola constant voltage transformer. I am curious as this is a VERY vintage model it has an output at 115 Vac. I am hoping to use this to soften the power line in for some of my tube testers so that i can take some of the stress off the line adjust pots in the testers as they are becoming unobtainable. However being new to this gear I am not sure what is "normal operation" and should I change the cap. It has an extremely large non polar 16uF cap at 660 Volts. This is a bathtub oil filled capacitor. Also the input characteristics are HORRINDUS under no load it is burning about 84 Watts of power. But it is happily supplying 118.5Vac to a dmm and nothing else. It also has an appauling power factor of about .224

Some Images can be found here:
/g/Test-Equipment-Design-Construction/album?id=284406

Eric

--
"If you want to build a strong house, I'll give you my engineer's number.
 If you want to build a strong life, I'll introduce you to my carpenter."
  Lebron and Heather Lackey

Hello,

Here is the link to the current probe that I built to be used from 50 KHz to 50 MHz:



I also have an external Current? Probe for the MFJ 854? that covers 100 KHz to 50 MHz using an Irwin Clamp:



Jacques Audet
VE2AZX
ve2azx.net

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Interesting how they operate, gapped transformer, one side saturates, etc.

The site also has a waveform showing the clipped waveform, and to me it looks
like the rise and fall time are faster than a pure sinewave.

I have a new to me Sola constant voltage transformer. I am curious as this is a VERY vintage model it has an output at 115 Vac. I am hoping to use this to soften the power line in for some of my tube testers so that i can take some of the stress off the line adjust pots in the testers as they are becoming unobtainable. However being new to this gear I am not sure what is "normal operation" and should I change the cap. It has an extremely large non polar 16uF cap at 660 Volts. This is a bathtub oil filled capacitor. Also the input characteristics are HORRINDUS under no load it is burning about 84 Watts of power. But it is happily supplying 118.5Vac to a dmm and nothing else. It also has an appauling power factor of about .224

Some Images can be found here:
/g/Test-Equipment-Design-Construction/album?id=284406

Eric

Very nice work, Ed. Looking forward to future installments of "Ed's transformer adventures"!

--Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070

The two-stage transformer works beautifully up to just over 100 MHz. The low frequency cutoff is around 5 kHz at 2.5 R driving source. I also tried direct drive from 50 R, with a 50 R load in parallel. The lower cutoff went up to around 100 kHz, as expected, and the output was flat within 3 dB or so. It was very flat up to around 50 MHz, then gradually sloped upward. This all was using the primary in series hookup (it's 20 T bifilar so can be 20 or 40 T), with maximum magnetizing inductance (about 120 uH) for lowest cutoff. I realized during final assembly that the winding phase of T2's primary is reversed by the termination to the post, so it subtracts, making the ratio somewhere around 4.5:1 rather than 5.5 as I thought before. The overall ratio then is either about 90 or 180 to 1, depending on hookup. As it is now, the Z ratio is quite large, so feeding direct from the TG 50 R parallel the termination 50 R, gives less than 1 milliohm output impedance magnitude, apparently at all frequencies right up to 100 MHz. I did a test, shorting the output post to the bushing (ground) with a small piece of the copper mesh material, that is some unknown but very small R and L. I could get the output to drop about 10-15 dB, and flat as a pancake across the board.

I'm almost tempted to not even bother with a driver amplifier, since it is almost usable as it is. The drawback is limited dynamic range - the small level leaves only about 30 dB dynamic range at the SA. The high-Z amplifier gain can make up for a lot, but it depends on the noise situation in each part of the system. One thing I should explain is that my Q-meter plan is quite different from the HP4342A or any other conventional one. It's actually a swept-frequency system that will display log Q (dBQ) over a wide dynamic range, rather than spot frequencies and and range and capacitor settings read on a meter. I expect that I can use a (properly chosen) fixed amplitude source, so there's no Q-ranging - it's all out there on the log display. The idea is that the source level is first traced on the SA and stored, then subtracted from the DUT voltage level, to correct for variations in the actual test level. As long as the output Z remains very low at all frequencies, a single reference trace should be good for lots of tests. The down side is that it's not as accurate as individual meter measurements (and possibly including manual corrections). The upside is that it puts up a lot of info, fast, which is what I want to quickly evaluate unknown chokes and cores for suitability in various applications. I don't need extreme precision (but wouldn't turn it down) for this.

Anyway, it's looking very good, and meets my original goal to reach 100 MHz. I'll explain more later, and try to cover the amplifier too.

Ed

I put up two photos of the two-stage construction.

"Parts" is the parts. The winding is a ribbon type, five turns, which is just right to wrap nicely on the core with very good fill. Note that the inner layer ribbon is formed to go inside the? output post, where it gets clamped to the rim along with the outer mesh of T2. The outer ribbon is dressed to pass through the mesh and connect with T1. T1 will be the same type of deal, with its output post simply a continuation on the same axis, terminated on the back of T2's post and mesh. T1's mesh will start on a tabbed ring facing T2, insulated from everything, and soldered to T2's ribbon with a very short (about 1/4") path. T1's mesh will wrap around the core assembly and terminate/clamp on its post back end, as with T2. The axis will continue with a 10-32 stud and mounting features for a terminal/compensation circuit board, and an outer shield. I have to be careful to not let the assembly get too long to fit the cabinet height. BTW the present plan is to build it into the carcass of an HP331 (or 332?, I forget which) distortion analyzer. It's not that great for the purpose - the only reason is to utilize the very nice 4-gang variable capacitor (and lashless gear reduction train and big dial) included, without doing a major mechanical project to use it separately in something else. I still might end up doing that, but haven't decided.

"Test setup" shows the temporary rigging for experimenting, with T2's ribbon extended to T1's post, completing the loop, but of course with a lot of leakage inductance to consider. Also note the flimsy temporary RF signal cabling. When everything is finally built and properly connected, it should work very well.

Ed

The following photos have been uploaded to the Ed's Q meter transformer album of the [email protected] group.

• test setup.jpg
• parts.jpg

By: Ed Breya <edbreya@...>

Preliminary experiments so far indicate the two-stage unit should be able to do 100 kHz to 500 MHz, or 5 kHz to well over 200 MHz, depending on the hookup of the bifilar primary. It's not flat - I'm trying to get it within about a 10-15 dB p-p amplitude range over all frequencies for now, and assuming closed-loop leveling will be used. I can't make meaningful measurements anymore, with the ground loops and RF squirting out everywhere from the loose wiring. I believe it will do much better, like maybe a few dB ripple, once the RF system is properly built. The output transformer (now T2) will remain at 5:1 for all following experiments - I don't want to change it since it's a work of art. The first stage transformer T1 will be experimental for a while yet. It is mounted on the tail of T2, on-axis, using a tricky low-Z interconnect structure that makes T2 partly an autotransformer, with a ratio somewhere around 5.5:1, since the core winding turns are superimposed on the back end of the output post, near the midpoint of the output winding (mesh).

I'll be putting up some photos later or tomorrow.

Ed

I found in the magnetic parts dept a few more cores of the right mechanical size, with around 60-80 nH A sub L range. This gives some more options. I'm also looking at a two-stage type setup, going 20:1 then 5:1 for 100:1 ratio overall. If all goes well I expect it might go from about 5-10 kHz to 200 MHz. Not flat though. From what I've seen so far, it's probably better to just regulate to output level closed-loop.

Ed

I have done some experiments on the transformer, with the initial design, and several different winding arrangements. I didn't have any suitable copper foil in stock, but found some fine expanded-mesh screening that actually worked better for construction, because of its see-through nature. A 110 W soldering iron was just barely enough to bond it to the bushing, by soldering through the mesh to the rim. The turns count was 69, as I thought before, but I managed to squeeze on two more, so got 71 T and about 45 uH. I changed the winding lead access to be parallel to the main axis instead of radial, so assembly was easier. I added some layers of 1 mil Kapton film, in sloppy fashion, just in case I needed to solder on the outside of the mesh wrapping - the film would keep solder from flowing through and sticking to everything. I'll be putting up some photos later showing the results.

I set up a low Z divider with the TG feeding 47 ohms going into four or five 10 ohms in parallel to get around 2 for the primary. A 49.9 ohm on the output, and another on the primary, are used for monitoring those levels with the SA. The transformer exhibited SRF around 50 MHz, so as-was, so could be workable for a 30-40 MHz top end. The problem is that I'd like to get to at least 100, so the inductance was too much. I'd give up some at the low end to get a higher top limit. So, I had to trash the nice looking transformer by having to unwrap and redo it a number of times, for experimenting. Remember, winding this thing was a PITA, so I tried to get as much use as possible. I first took off a bunch of turns to get down to 50, and spread them out and re-wrapped. The SRF went to around 90 MHz, not bad. At an earlier point I tried to make about 33 bifilar turns from the whole winding, but they did not balance. So,after the 50 turn round, I ended up stripping the winding and doing a true bifilar, and managed to get 2x36 turns and a full core. I'm getting better at winding this thing, so it wasn't as bad as the first time. The SRF was right about 100 MHz, still not quite there. I also tried various schemes to cancel displacement currents and such, since there are two "identical" windings, but still didn't move it up much. Finally, I deleted one of the windings, leaving the other nicely spaced out on the core. This time it was around 110 MHz, as long as the wrapping isn't too tight.

During all the experimenting I could see how significantly the "foil wrapping" that I've been advocating for low secondary leakage inductance, affects the primary winding capacitance and SRF. So, the foil tends to foil the high frequency performance that I want - always another trade-off. I have some more experimenting to do yet, but I think I'm close to the right turns count, and will figure out a way to not wrap it too tightly, so I can get a good margin above 100 MHz operation. Since the now lower turns count has less impedance reduction, I plan to make a very low Z driver amplifier, somewhere around a half ohm, if things work out. More about that later.

So anyway, I still like the foil wrap, if the goal is in the medium frequency range, but it gets tricky when you want to push higher. My problem here is that I have only this one core type and size, that happens to fit just right on the post as-is, so the winding on the inside layer is very tight, adding quite a bit of C to begin with, then aggravated by being quite a large core, and the nice tight foil wrap that would keep the secondary leakage down. I've seen a number of designs others have done, in the discussions - there are plenty of ways to go, and use whatever works.

Next time, transformer pictures and amplifier stuff.

Ed

Thanks Ed,

Your thinking echoes mine exactly. I agree that iron dust is not the way to go so will try to find a bigger ferrite core and maybe that will help extend the frequency range. The one I tried was about 2.5 cm in diameter and using a 10:1 winding it performed well from 50 kHz to about 2 MHz but then gave out.

73, Morris

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