4th Feb 2020, 2:36 pm | #21 |
Heptode
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Re: Making an RF probe for VTVM
Yes the heater needs to be heated! Otherwise nothing will work, so you need to find a source of that voltage.
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4th Feb 2020, 2:44 pm | #22 |
Dekatron
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Re: Making an RF probe for VTVM
Normally the heater supply for the probe's diode would come from the VTVM, a common arrangement was to use a double screened cable to connect the probe to the VTVM, center conductor for the DC+ve output, inner screen being common for DC-ve output and one of the heater feeds, the outer screen as the other heater feed.
Lawrence. |
4th Feb 2020, 2:48 pm | #23 |
Octode
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Re: Making an RF probe for VTVM
Okay according to the spec sheet the cathode crosses paths with the cathode but not externally connected.
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4th Feb 2020, 2:48 pm | #24 |
Octode
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Re: Making an RF probe for VTVM
Ooops heaters crosses paths with cathode.
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4th Feb 2020, 7:29 pm | #25 |
Octode
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Re: Making an RF probe for VTVM
I see what your saying Lawrence so one heater is on the ground DC-? which I thought would need to be but not sure how you would connect another outer screen to the second heater well I know how to attach but its where that feeds too and from in the circuit? Plus what type of bnc would you use? The unit I have just got is a kyroitsu K-142. the original DC input socket was similar to Simpson 303/311/312 mic socket which has been removed and normal twist lock bnc connector installed. I have plenty of good quality bnc connectors but as said only room for center conductor and sheild. EM maybe not possible to do on this unit or I need to build a separate circuit in Ali can then feed out from there the prob and earth.
Actually I'm thinking you need an external ground via croc clip or probe as well. |
4th Feb 2020, 7:38 pm | #26 |
Octode
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Re: Making an RF probe for VTVM
couple pics of new bnc and one of the original.
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4th Feb 2020, 8:42 pm | #27 |
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Re: Making an RF probe for VTVM
There’s a chap on YouTube who used a largish plastic syringe body to make a probe. And very effective it was too.
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4th Feb 2020, 8:45 pm | #28 |
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Re: Making an RF probe for VTVM
If it were mine I would hard wire the RF probe cable to the inside of the unit, I would remove the existing RF double diode and connect the DC+ve output from the probe to the existing DC input via a small S.P.S.T. switch mounted on the case somewhere, the probe heaters being connected in place of the original double diode heaters.
Lawrence. |
4th Feb 2020, 9:46 pm | #29 |
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Re: Making an RF probe for VTVM
There’s a chap on YouTube who used a largish plastic syringe body to make a probe. And very effective it was too.
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4th Feb 2020, 10:09 pm | #30 |
Octode
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Re: Making an RF probe for VTVM
That's a good idea I have plenty of switches. look into that
I left the meter on for couple of hours as I don't think it's been used for a long time and seems to have had a few alterations. Anyhow I cannot adjust ohms needle only zeroing needle and going through the righthand side switches meter supposed to stay on zero but it's not so further investigation needed. I know the 20uf electrolytic cap is over double 42uf. Changed valves for NOS which already had no difference. Oh well rf probe on hold. |
4th Feb 2020, 10:18 pm | #31 |
Nonode
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Re: Making an RF probe for VTVM
It was certainly OK before I packed it up.
Al
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4th Feb 2020, 10:30 pm | #32 |
Octode
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Re: Making an RF probe for VTVM
I'm sure it's something simple like anything old when not used for a while and then left on for a while it can put things under stress and electrolytics are notorious for failure in any equipment at any time.
Al don't fret not worried about it sure I'll sort it. |
5th Feb 2020, 12:14 am | #33 |
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Re: Making an RF probe for VTVM
Good morning Chris,
To answer your question: Yes the valve ( EA50 ) must be heated. I have a Hewlett Packard 410B, and I show how they overcame the idea. I think the pics are self explanatory except: The little grey connection box screws to the bottom of the meter case. There is a large square hole in the bottom of the case through which a series of pins plug into the 5 sockets shown on the "bottom" picture. If you download the 410B manual you will see how this works. Your problem will be to find some co-ax with a centre conductor, PLUS two additional wires that supply the 6.3 heater volts. Im sure you can use a piece of co-ax, and carefully heatshrink a couple of flexible wires onto the co-ax. The screen of the co-ax connects to the outside of the probe casing. Hope this helps some Joe |
5th Feb 2020, 12:21 am | #34 |
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Re: Making an RF probe for VTVM
It's actually as Lawrence describes in post #22. HP, of course made a beautiful bit of cable with moulded connections.
Joe |
5th Feb 2020, 9:16 am | #35 |
Heptode
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Re: Making an RF probe for VTVM
Since the output from the probe is DC (or possibly the AF modulation) and not RF, then the cable from the probe to the meter may need to be screened, but doesn’t need to be RF coax.
You could use fig8 audio twin screened cable of the type used for stereo phono cables. The two leads can be pulled apart to go to separate sockets. One screened lead would carry the signal and earth from the probe, terminated with whatever plug type is needed to connect to the meter. The other lead would carry the power for the heater, terminated in whatever was needed to connect to a power source, possible a separate power supply. Stuart |
5th Feb 2020, 3:27 pm | #36 |
Octode
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Re: Making an RF probe for VTVM
Cheers again guys quite alot of variations to choose from. I think I'll need to take down notes and work from there.
I've ordered new caps so should have them tomorrow and get them in. Hopefully that will fix the problem as I noticed the electrolytic is connected to the 12au7 pins 1&6 plus the other adjusters are linked around the valve. Anyhow off the subject. One thing these soldered fuses do they use a different type of solder? Purely based on salvage for a spare. Thanks again all. Cheers Chris |
5th Feb 2020, 4:39 pm | #37 |
Octode
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Re: Making an RF probe for VTVM
managed to get it settled down made up quick dc probe with earth croc clip, connected that and it's settled down a bit. Think the dc prob obviously needs to always be connected? Just a thought! or maybe it should make no difference if connected or not.
I'll get there. Better play tomorrow with new cap installed. |
5th Feb 2020, 6:16 pm | #38 |
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Re: Making an RF probe for VTVM
Back to the RF probe, Chris seems to not understand what's required in terms of connections, to get the EA50 diode to function.
The attached sketch might help a bit in relating the circuit to how the valve needs to be wired. The valve needs a 6V3 .01A heater supply, which the VTVM is capable of supplying, but that would need say a pair of sockets fitting to the meter, wired in parallel to the heater supply to the two valves in the meter. The anode of the valve would need to be connected to the probe tip. What I'm a bit puzzled by is why an EA50 valve has been chosen rather than to simply use a germanium diode such as a iN34A, (albeit depending on what RF frequency it's desired to measure, a more suited diode might be called for. Just because it's a VTVM (Vacuum Tube Volt Meter), doesn't mean that it has to have a valved RF probe, especially when the VTVM wasn't designed with a valve RF probe in mind. For example, why not something like this: https://www.robkalmeijer.nl/techniek...e22/index.html Or this: http://n5ese.com/rfprobe1.htm Just my point of view - sorry if I'm missing something.
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5th Feb 2020, 7:00 pm | #39 |
Dekatron
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Re: Making an RF probe for VTVM
The heater current for the EA50 is 0.15 Amp not 0.1 Amp:
https://frank.pocnet.net/sheets/030/e/EA50.pdf You only need three separate conductors. Double or twin screened cable would be ok. The meter was designed with a thermionic diode incorporated for AC/RF measurements (6AL5): https://nvhrbiblio.nl/schema/Kyoritsu_K142.pdf Lawrence. |
5th Feb 2020, 7:05 pm | #40 |
Octode
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Re: Making an RF probe for VTVM
Ah that was interesting David as I thought just a few minutes ago the transformer has 6.2v taps. Now as you say you can use jumpers to tap into that and connect to two banana Jack's plus add a toggle switch and hey presto we have power as and when required. Then it's just a case of connecting up as and when power required. The whole circuit including the EA50 will fit into the tubes you gave me.
There's also plenty of room to instal additional components on the meter. Why go for a valve RF probe? Think it makes it a bit more challenging and also from what I've gathered it's good for/more stable for picking up low as well as high frequencies as opposed to a general probe where you prob need to make a few for picking up high as well as low frequcencies according the the capacitance of the diodes used. I think I'm on the right path here? Again I know nothing about building probes for meters. But yes it would be much easier just to whack a diode in the circuit. This is intetesting: RF energy may be challenging to measure for one or more reasons, depending on the nature of the circuit to be measured and the measuring equipment at hand. The first kind of difficulty arises when the RF energy to be measured is at a frequency too high for available test equipment, such as a low-bandwidth oscilloscope, to process directly. In that case, the RF has to be converted to a DC or near-DC signal. In this situation, a simple probe type sometimes called an RF detector can be used to convert the RF signal to DC. Such device will work as a RF rectifier and give a pulsed DC voltage.[2] The second kind of difficulty arises when RF energy has to be measured in a circuit which is sensitive to small changes in its electrical environment. For example, with some oscillator circuits, the presence of an ordinary wire within a few centimeters of the active components may change the amplitude or frequency of oscillations, or even prevent the circuit from oscillating at all. In that case, the signal has to be acquired by a measurement probe which extracts very little energy from the circuit. This can be achieved by employing very thin conductors, or tiny coils kept at some minimum separation from the active elements of the circuit. In a situation, where circuit loading rather than high frequency is the real problem, a variety of small-geometry, high impedance probes can be used, sometimes including an amplifier to boost the tiny amount of energy extracted from the circuit to a level that allows it to be measured by available high-frequency test equipment. So I'm sure the EA50 may overcome these problems just a thought. Cheers Chris |