High voltage Oscilloscope Probes - Page 1
Forum section: electronics / testgear

author: retrotubeguy
author_rank: Contributor
timestamp: July 26, 2015, 06:37:27 am
content: Hi Guys & Gals, I have a question about High voltage probes. I am just getting back into repair of vintage tube guitar amplifiers and was interested to see if anyone had any comments about using High voltage oscilloscope probes for this purpose. I worked in a small repair shop for many years and we did not use them but it always scared the joules out of me. Most of the time your only looking at the speaker output or the control grid so no real problem there, but it would not be difficult to probe the plate by accident which can be any where from say 180Vdc to 500Vdc. Never had a disaster but maybe we were just lucky. Also any thoughts on weather or not I should be using isolation. I don't like the idea of an isolation TX and no earth bond. I know that's done allot with tube amp repair but it seems a little dangerous to me. any Help is appreciated. There are a lot of smarter people her than I.Thanks.

author: owiecc
author_rank: Frequent Contributor
timestamp: July 26, 2015, 06:41:39 am
content: Buy a differential voltage probe. They come in many voltage ratings. From single volts to few kilovolts with bandwidths up to hundredths of kilohertz.http://teledynelecroy.com/probes/probeseries.aspx?mseries=430http://www.tek.com/oscilloscope-probes-and-accessories/differential-probe-high-voltagehttp://www.newark.com/fluke/dp120/differential-voltage-probe-1kv/dp/91B5086http://www.pmk.de/en/products/bumblebee

author: retrotubeguy
author_rank: Contributor
timestamp: July 26, 2015, 06:54:05 am
content: Thanks. They are expensive. I will shop around. I assume that this will take care of Isolation to. Any opinion on weather or not an isolation TX offer any safety advantages. It seems to be more dangerous.

author: tautech
author_rank: Super Contributor
timestamp: July 26, 2015, 07:37:14 am
content: Your normal 10x probe is usually rated to ~600V atlow frequencies.Voltage de-rating with frequency is often overlooked, placing the user and equipment at risk.100:1 probes much reduce risk but care is needed like any other probe where the reference (GND) is connected.100x probes are the cheapest method to probe a HV DUT and even though they are still subject to de-rating you start with a much higher safety margin.IMHO you would be wise to get at least one 100x probe, ~USD25 and up.When restricted as to where you can reference from, Differential probes and isolated input scopes are the safe options.Many, many inexperienced scope users are unaware to fully equip a scope with a good range of probes WILL cost more than the scope did initially.Member joeqsmith is presently experimenting with homemade HV probes.....not recommended for the average hobbyist.https://www.eevblog.com/forum/testgear/teledyne-lecroy-waverunner-64xi/msg715402/#msg715402

author: HighVoltage
author_rank: Super Contributor
timestamp: July 26, 2015, 08:50:37 am
content: The company TESTEC makes some really nice but still low cost HV probeshttp://www.testec.de/32.0.html?L=1The one I really like is the TT-HV 250 100:1Attenuation Ratio: x100Input Impedance: 100M?, 4 pFBandwidth: 300 MHzRise Time: 1,2 nsCable Length: 1,2 mCompensation Range: 10..50 pFMax. Input Voltage: 2500Vp CAT I, DC incl. Peak AC (derating with frequency)

author: owiecc
author_rank: Frequent Contributor
timestamp: July 26, 2015, 08:59:13 am
content: Quote from: retrotubeguy on July 26, 2015, 06:54:05 amYou typically put an isolation transformer before the DUT so that you can easily probe your circuit with a non-isolated passive probe. If you don't have the solution transformer you can destroy the scope by connecting its ground to some potential causing a short. Differential probes isolate your scope from the DUT.However it depends on what you are probing as most thins are galvanically isolated from the mains (on the PSU's secondary side) and you can use a normal passive probe like tautech mentioned. You can use 1:100 and even 1:1000 probes if necessary. Differential probes are, on the other hand, very easy to use. If you don't apply voltage higher than rated you can probe everything. Passive probes need a common ground connection in the DUT. It is not always easy to have that.

author: German_EE
author_rank: Super Contributor
timestamp: July 26, 2015, 09:00:23 am
content: At 500V you will need things like wooden benches and some high quality tools that won't flash over if you touch the wrong spot. An isolation transformer may not protect you because your scope is referenced to mains earth, for starters please watch this for some ideas:http://www.eevblog.com/2012/05/18/eevblog-279-how-not-to-blow-up-your-oscilloscope/Above all, please do not work on gear like this on your own and don't rush. Mistakes can be costly.

author: Unknown
author_rank: Unknown
timestamp: July 26, 2015, 09:14:41 am
content: Hello it may pay you to watch this,Alan's on here, and very good at explaining things, I trust him totally, well he is a fellow radio ham.https://youtu.be/-_LJXKLgIYc

author: retrotubeguy
author_rank: Contributor
timestamp: July 26, 2015, 11:03:08 am
content: Wow, Overwhelmed by the support on this forum. Thanks. I think I know what to do now. Differential probes look like the way to go although i like the testech probes. Measurements are always taken on the secondary side so I think I am ok in regards to isolation in respect to personal safety.Thanks for all the help.PS. I always use the fluke 87 for anode voltage measurements. The scope is just used to look at the signal for symmetry & distortion. But I wanted to be prepared for that accidental contact or brain fart that can occur.

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 26, 2015, 12:19:57 pm
content: I can highly recommend differential probes from Pintek.These differential probes are the best value for money, and the quality is excellent!Note that Pintek is the OEM manufacturer for several test equipment manufacturers (e.g. Rigol, etc.), who simply rebadge the differential probes from Pintek with their own brand. The design is identical, just the labeling is different.http://www.pintek.com.tw/product_classify/landersound-tail/index.php?Product_Site_Classify_SN=17072&Company_SN=6002I ordered two differential probes from them. Built quality is excellent and they come in a very nice case.

author: commongrounder
author_rank: Frequent Contributor
timestamp: July 26, 2015, 03:05:01 pm
content: I repair tube equipment on a regular basis in my audio business. I feel the need to comment on the use of differential probes in tube gear, in particular tube audio amplifiers. I own two differential probes, but only rarely use them on tube gear for two reasons. The first is, they typically have relatively low input impedance. My Sapphire S-9000 series probes have a total eight megohm input impedance. This is low enough to potentially affect a tube circuit being probed. The second, related, problem is differential probes are not isolation probes. They use resistive/capacitive voltage dividers at the positive and negative leads, and have a common ground reference. In my Sapphires case this means only four megohms between either lead and the ground reference. Most of these probes state very clearly in the documentation that the BNC output connector must be connected to a grounded scope input for stable operation. My units even have a ground pigtail banana plug at the BNC to attach to a ground post on the scope to be sure. So, you must be aware of the chassis ground status of your device under test. If the chassis has an earth ground connection (as in a three prong AC plug), then an electrical connection (hopefully) exists between the DUT and the scope ground. Using a differential probe between the chassis and a test point will mean one of the probe leads will be shorted to ground and you lose all the benefits of the differential probe (and you have lower input impedance to boot).I use good quality 100x probes with my scope. Having 100 megohm input impedance is a great help in probing a tube circuit without disturbing its operation. Most 100x probes have a healthy maximum voltage rating margin for tube work. Tube audio amplifiers are not likely to test the upper bandwidth limits of a decent probe (even in unstable oscillation).There. My two cents worth.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 27, 2015, 08:25:05 am
content: Quote from: retrotubeguy on July 26, 2015, 06:37:27 amThere seems to be a tendency these days to portray our activities as somewhere between Snake Charming & Crocodile Wrestling in the scale of perilous things to do!Back in the day,all equipment was tube type,& we used normal 'scope probes on it.Hell,even old Mantel radios from the 1930s used HT lines of around 300v!The big,old clunky,Tektronix probes that came with things like 524ADs & 545B's were a bit limited in frequency response,but made up for it in ruggedness.Larger size meant more clearance between internal components & less likelihood of breakdown.If I recall correctly,they were rated at 600v ,obviously derated for higher frequencies.From DC to the maximum audio frequencies you will find in a Guitar amp ,the derating was virtually nil.The later,smaller,Tektronix X10 probes may not stand up as well,but they were commonly used in troubleshooting line output stages in CRT type TVs,where transient peaks in the primary side of the EHT transformer can reach 500v or so.El Cheapos from China may or may not be able to handle the voltages you are dealing with.I don't quite understand your comment about Isolation transformers.Unless modern tube amplifier designs use solid state switchmode supplies,you,should not need one of these devices.Traditional Tube amplifiers use large power transformers,followed by rectifiers,so that all the internal circuitry of the amplifier is isolated from the Mains.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 27, 2015, 09:00:45 am
content: If you are looking for a low cost but still ok quality differential probe, take a look at this one:TesTec- TT-SI51- 50 MHz- 100:1- 1400VI just scored one for Euro 90 in like new condition but without the small power supply.

author: McBryce
author_rank: Super Contributor
timestamp: July 27, 2015, 09:14:37 am
content: If it's only momentary "accidental" probing you are worried about then you don't really need anything new. I assume you normally use a 10x probe, so even at 500V your scope will only be getting 50V and both the probes and the scope should be fine. When fixing things with 500VDC in them, I tend to be very aware of where not to touch/probe, so accidents should be extremely rare anyway.@Highvoltage: €90! Why don't I ever find any of these deals!McBryce.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 27, 2015, 09:24:23 am
content: Quote from: McBryce on July 27, 2015, 09:14:37 amDon't be afraid to use the "Preis-Vorschlag" (Make Offer) button on ebay Germany.Some sellers are desperate to sell and take the first offer they get.Others give you a slap in the face kind of email back.But many times I have been very lucky and getting some real good equipment for a huge discount.

author: McBryce
author_rank: Super Contributor
timestamp: July 27, 2015, 09:43:00 am
content: I often make offers on ebay, but obviously not the right onesDespite scanning ebay regularly I haven't seen many diff rpobes on offer lately.McBryce.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 27, 2015, 10:33:30 am
content: Quote from: McBryce on July 27, 2015, 09:43:00 amThis is not a fast turn around project as it might sounded in my previous post. Every once in a while I am getting lucky. And it takes a lot of time. In this particular case of the differential probe, the seller did not know how to spell "differential" correctly in the German language and I think this was the main reason why it was not sold.

author: retrotubeguy
author_rank: Contributor
timestamp: July 27, 2015, 10:37:14 am
content: I would have to agree with vk6zgo on this I think. I have never used differential probes so was not aware of the impedance issue and yes it would effect the operation of the amp. People keep saying I should use an isolation transformer on the DUT but the mains TX of the DUT should be isolation enough. May be I should take some comfort in the fact that I have never killed a scope or obviously myself on one of these amps. My old boss used 100x probes for 35 years and he never blew up is old CRO or himself or anyone else for that mater. Also as was mentioned in one comment I agree that accidental contact with the HT would not be anything to worry about, I would not make a habit of it though. I think good quality probes in good condition should be just fine. Maybe I will get some differential probes for other work. I don't know. Looks like I stirred up a hornets nest. Sorry that was not my intention. I just figured I would get some opinions. As my old man always said- somebody always knows more. Cant hurt to ask. Thanks for all the help guys. Gives me something to think about.

author: McBryce
author_rank: Super Contributor
timestamp: July 27, 2015, 10:38:36 am
content: I do mispelt searches too. That's how I got my Keithley 2000 on ebay.fr: Keithley spelt wrong. Item in the wrong section, crap photos and seller didn't really know what he was selling (was sold by liquidation company) = Good price. To top it all, it turned out to be a (relatively new) 2005 model and had recently been calibrated.McBryce.

author: owiecc
author_rank: Frequent Contributor
timestamp: July 27, 2015, 01:47:27 pm
content: Quote from: HighVoltage on July 27, 2015, 10:33:30 amOut of curiosity, how do you spell differential in german?Can you write "differential probe with high input impedance" as one word?

author: HighVoltage
author_rank: Super Contributor
timestamp: July 27, 2015, 02:34:50 pm
content: @owieccQuote from: owiecc on July 27, 2015, 01:47:27 pmLOL,Well, some German words are long and look funny to people with an other language background. But in this case no.Usually you would call it a "Differenzial Tastkopf"Some sellers make it a mixture out of German and English, like this guy:Yokogawa Differential Tastkopf 50 MHz 10:1 Oszilloskop Probehttp://www.ebay.de/itm/Yokogawa-Differential-Tastkopf-50-MHz-10-1-Oszilloskop-Probe-/191640164434?pt=LH_DefaultDomain_77&hash=item2c9ea4d852This one was just reduced by 100 Euro, I think and is not a bad deal.McBryceGreat deal, congratulations!Especially on ebay France you can find amazing test equipment from time to time because of totally wrong spelling.This way I got an almost brand new Agilent 33509B about a year ago for so little money that I will not say, how much I paid.This seller has misspelled everything!The only risk was, that the seller accepted no paypal and I had to make a money transfer with bank to southern France.But the item arrived a few days later in pristine condition.

author: McBryce
author_rank: Super Contributor
timestamp: July 27, 2015, 02:46:23 pm
content: Yes, I had to do a money transfer too, but the seller had a 100% rating and had sold quite a bit. I also have the feeling that the market for such electronic measurement devices in France is much smaller than in Germany (I may be wrong)?McBryce.

author: zaphod_beeblebrox
author_rank: Newbie
timestamp: July 27, 2015, 04:58:49 pm
content: Owiecc: Sure that's easy :-DHochimpedanzdifferenzialtaskopfwould't shock an ham radio operator or tube enthusiast like meBut I have to admit that it is definitely not a common word

author: McBryce
author_rank: Super Contributor
timestamp: July 27, 2015, 06:57:23 pm
content: You forgot a "t" : HochimpedanzdifferenzialtastkopfMcBryce.

author: owiecc
author_rank: Frequent Contributor
timestamp: July 27, 2015, 07:39:17 pm
content: Obviously, you shouldn't be selling hochimpedanzdifferenzialtaskopfs on eBay because you will be loosing money all the time

author: McBryce
author_rank: Super Contributor
timestamp: July 27, 2015, 07:48:36 pm
content: Ah the finer points of German - The plural would be: HochimpedanzdifferenzialtastköpfeMcBryce.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 27, 2015, 07:52:18 pm
content: And what do you call the user manual?"Hochimpedanzdifferenzialtastkopfbenutzerhandbuch"And yes, I would not recommend using that word on an ebay listing.Actually the longest official German word used to be:"Rindfleischetikettierungsueberwachungsaufgabenuebertragungsgesetz"But the EU made some changes and it is no longer an official word.

author: McBryce
author_rank: Super Contributor
timestamp: July 27, 2015, 08:15:49 pm
content: It's German, it can go on forever...HochimpedanzdifferenzialtastkopfwiderstandshalterungMcBryce.

author: bitwelder
author_rank: Super Contributor
timestamp: July 28, 2015, 05:41:05 am
content: That makes it for a good Mailbag item to send to Dave with the note: "Must readaloudbefore open!"EDIT: added important detail

author: Deathwish
author_rank: Unknown
timestamp: July 28, 2015, 05:49:12 am
content: Quote from: bitwelder on July 28, 2015, 05:41:05 amAloud of course