Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: basschops1528 on April 25, 2016, 04:30:06 pm
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Hey guys,
I drew up a quick schematic for a voltage reducer so that I can take measurements without blowing up my scope. It can measure up to 300VAC max so I'd rather use cheap resistors to reduce voltage and raise impedance rather than buy expensive leads. Could someone verify my drawing and give me some feedback on both setups .
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can't say on your probe, I built one awhile back, just googled diy hv scope probe, it was built in an ink-pen housing, good to 1000VAC, NOT DC, I use my meter for that. can't find the notes, but it was 10 equal R's with 1 or 2 caps
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> It can measure up to 300VAC
AND down to less than a Volt.
I'm in pain and unable to jabber. But pencil 1Meg against 10K. 100:1 division. (101:1 but who cares?) A 600V amp over-over-driven to 1,500V peaks will give 15V peaks, safe yet plenty large; a 150V flea-amp will give 1V-3V output to 'scope.
The 'scope's ~~30pFd input C loads the divider. 30pFd on 10K is 500KHz (check me) which is more than we really need in a transformer/tube stage. (With your 1Meg+1Meg divider this point is 10KHz, enuff for basic guitar-amp verification but not really enough for squirreliness checks.)
The 1Meg top resistor can be 1/2-Watt up to 700V DC or 500V with a lot of AC/audio, eXcept resistors have voltage ratings and often only 300V. Over-volt may be a minor shift of value or may be a minor blow-up. At today's resistor prices, ten 100K 1/4W in series may be the most economic way to get over 1500V rating. The 10K never sees real voltage and is uncritical.
If you can truly work to 1% precision (few can), the "10K" should be 2% high. 1% to correct the 101:1 ratio, and 1% to correct the 10K||1Meg loading of 'scope across 10K. So 10,200 Ohms.
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PRR, I do appreciate the response but I have no idea what any of it meant :dontknow:
I just want to be able to read up to B+ voltages and used 500V as an example for my diagram.
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what any of it meant
your scope has an input impedance, the R's you use have a tolerance/voltage rating, together they will affect your probes accuracy? when you make voltage measurements.
Note: hopefully you find relief PRR, my back took me a year, knock on wood so far no surgery
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An equally as important factor when probing high voltage circuitry (i.e. besides the voltage reduction for scopes) is the insulation capability of the probe(s).
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Can somebody draw-up my jabber for Johnny?
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By the time you are measuring 500 volts or so, a Meg across such a voltage is a very small load. But you could get even smaller (meaning a smaller load) if you want to shoot for greater accuracy. I occasionally have to develop a meter shunt to re-purpose a meter I have that's sitting around. It's approximately the same task.
The "easy" way to do this is to go buy 10 qty 1 MEG resistors, try for the best ones you can. Metal film Rs today are pretty remarkably accurate, very often 1.5%. The ones Doug sells are great. Put ten of them in series, between your 500 volts and ground. Now you would have a 10 Megohm load, essentially invisible to a tube circuit. 1/50th of 1 mil @ 500 volts, nothing. Take your scope output 9 resistors down from the top. 500 volts will read as 50 volts. That's it. No need to deeply overthink it.
You really do not need to use 1 MEG resistors. *Any* highish value would work, I just use that for illustration. You put ten in series and measure across one of them. That's all there is to it.
For best accuracy, you would very carefully measure the string of 9 resistors in series and note their series resistance. The task then becomes "tuning" the lowest resistor in the chain so that the long chain is exactly 9 times the resistance of the lowest resistor. Could you use a 1 MEG pot? Sure, although pots often are +/- 10 or more percent, so if you get a 930K pot labeled 1 MEG, not so good. So you put yet another resistor under the pot, between the lowest leg and ground.
You probably want to put such an appendage in a plastic project-box with 2 qty BNC connectors on it so you can plug your probe into the box, connect a BNC-BNC cable to your scope, and measure away. If you are really clever, you could use a larger-size pill bottle of the type prescriptions come in. But a fuzz-box sized plastic box would be a lot easier, though bigger, sitting on your bench.
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I thought increasing scope input just required new Higher voltage probes no? Case in point, this probe:
http://smile.amazon.com/SainSmart-P4100-Universal-100-Oscilloscopes/dp/B00D8TROP4/ref=sr_1_1?ie=UTF8&qid=1461701860&sr=8-1&keywords=high+voltage+oscilloscope+probe (http://smile.amazon.com/SainSmart-P4100-Universal-100-Oscilloscopes/dp/B00D8TROP4/ref=sr_1_1?ie=UTF8&qid=1461701860&sr=8-1&keywords=high+voltage+oscilloscope+probe)
its 30 bucks and can handle up to 1000V. Or am I misunderstanding what's being asked? Or can you build this addon for cheaper than that?
~Phil
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> Could you use a 1 MEG pot?
Pots are not generally rated over 500V, and may be 250V.
You also put your hand on them. (Unless you propose set and seal in box, which seems silly.)
I would not run the Plate-OT nodes direct into any standard pot.
I'm not sure how much accuracy is warranted. 'Scope is mostly about shape, not exact size. Use a meter to read a steady voltage such as B+. See that on the 'scope, and how it relates to signal swings, you know the size of swing for all practical purpose.
We have folks here who, in past lives, used high accuracy 'scopes to calibrate aircraft radar and military comm/weapon systems...... but this here is rock/jazz/funk/punk Guitar Amps, not the AWOS RCDC for the NNIIRT OTH (Mk 2).
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> Or can you build this addon for cheaper than that?
If you accept non-MHz bandwidth and 5% error, you can build a 100:1 probe with a few resistors and clips (and high-volt insulation; BIC pen shell works). That's fine for car ignitions and most tube audio amps.
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Set & seal in a box is exactly what you'd want to do. The pot would never see more than 1/10 measured volts; at a 250 volt rating it could handle a 2500 volt point under test. It's at the bottom of a large pile of resistors
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Can somebody draw-up my jabber for Johnny?
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I just want to be able to read up to B+ voltages and used 500V as an example for my diagram.
So do you mean you want to read an AC signal that is 500Vp or read AC signals of various sizes when the B+ voltage is 500V or both?
What 'scope do you have?
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> Or can you build this addon for cheaper than that?
If you accept non-MHz bandwidth and 5% error, you can build a 100:1 probe with a few resistors and clips (and high-volt insulation; BIC pen shell works). That's fine for car ignitions and most tube audio amps.
gotcha which would be super inexpensive at a few dollars I guess. (most of the cost would be the bnc connector probably)
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Heh. I only have thousands of those sitting around. Another case for scrounging dead gear.
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Maybe I can clear up any confusion as to why I need a means to measure high voltages. I'm trying to figure out why there is a 120Hz hum in my amp and I want to be able to measure any level of voltage without damaging my only scope by going beyond its input rating. In other words, have the scope read a lower voltage and an external circuit take the brunt.
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2deaf it's a Hitachi 100MHz I think. 10x/1x probes. Up to 300VAC and VDC I believe. Both scope and probes I mean
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or just buy one. the one in the link below has:
a 2KV rating,
is 100:1,
has 100MHz BW,
has input Z of 100Mohm/6pF,
costs 40bux.
https://www.testpath.com/Items/High-Voltage-Probe-2kV-100MHz-1001-127-704.htm (https://www.testpath.com/Items/High-Voltage-Probe-2kV-100MHz-1001-127-704.htm)
--pete
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I was hoping to save some money. Just dumped 175 on parts for a revive and am buying a slew of speakers off of CL
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If you have a probe that will do X10, try the attached voltage divider. With the probe set to X10, the bandwidth with a 100K in parallel is much better than when set to X1. The components used are frequently found in the possession of those who harass tube amplifiers.
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Thanks 2deaf. I will save a copy of that and experiment with it sometime. I actually decided to just buy 2 pairs of probes from amazon. What do you guys think? http://www.amazon.com/Hantek-Oscilloscope-Voltage-Passive-2000VDC/dp/B0030L0X58?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00 (http://www.amazon.com/Hantek-Oscilloscope-Voltage-Passive-2000VDC/dp/B0030L0X58?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00)
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I thought increasing scope input just required new Higher voltage probes no? Case in point, this probe:
http://smile.amazon.com/SainSmart-P4100-Universal-100-Oscilloscopes/dp/B00D8TROP4/ref=sr_1_1?ie=UTF8&qid=1461701860&sr=8-1&keywords=high+voltage+oscilloscope+probe (http://smile.amazon.com/SainSmart-P4100-Universal-100-Oscilloscopes/dp/B00D8TROP4/ref=sr_1_1?ie=UTF8&qid=1461701860&sr=8-1&keywords=high+voltage+oscilloscope+probe)
its 30 bucks and can handle up to 1000V. Or am I misunderstanding what's being asked? Or can you build this addon for cheaper than that?
~Phil
or just buy one. the one in the link below has:
a 2KV rating,
is 100:1,
has 100MHz BW,
has input Z of 100Mohm/6pF,
costs 40bux.
https://www.testpath.com/Items/High-Voltage-Probe-2kV-100MHz-1001-127-704.htm (https://www.testpath.com/Items/High-Voltage-Probe-2kV-100MHz-1001-127-704.htm)
--pete
You did it again Pete lol
~ Phil
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So does anyone still have there old CRT probe from way-back? Mine was lost somewhere on Chicago's south side :BangHead:
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> their old CRT probe
Yes.
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There are zillions of good X100 2KV probes for sale on ebay for 15 to 20 dollars. Probes are pretty generic and a probe designed for a 100Mhz scope will work well on most 100Mhz scopes. I don't know what kind of scope you have but it should not be hard to buy a high voltage probe for cheep that will work just fine.
I have two or three I got off ebay that work fine and have not killed me yet..lol
Cheers,
Billy
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I managed to buy a pair of Hantek high voltage probes 100x rated peak ac/dc about 2kv or 1kv I forget. Either way, we're dealing with AF here so waveforms being distorted shouldn't be an issue. Things like hum and HF squeals should be pretty obvious. I'm just hoping they're safe enough to just tap wherever I want. I would like to know one important thing: when I probe any part of the amp, should I keep the ground on the PT at all times for accuracy and safety? Or do you ground to respective grounds such as probe preamp-preamp ground, probe power amp- power amp ground?
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I'd rather have someone else confirm, but I'm almost 100% confident any ground is good, (the chassis ground is the final way to ensure safety and so long as ground is also tied there, you're good). I've probed a ton in the high voltage areas of a Vox AC100 at 500+ V with ground to any available source that was accessible, and had no issues.
~phil
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Well I know that you can run into problems where placing the ground lead in an area can sink lots of current and blow something up. I think it usually occurs in situations where people are looking for digital signals and they think they can probe between two points when they shouldn't. Check out this video. It's long but it makes sense.
https://www.youtube.com/watch?v=xaELqAo4kkQ (https://www.youtube.com/watch?v=xaELqAo4kkQ)