Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Dec Sun 05, 2021 6:14 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 73 posts ]  Go to page Previous  1, 2, 3, 4  Next
Author Message
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Thu 04, 2021 5:57 am 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
i did buy a triplett 850 VTVM guess its not 1000 ohms per volt either anyway to make it one?


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Thu 04, 2021 6:14 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 5412
Location: Lincoln City, OR 97367
Greetings to tysman and the Forum:

Any VTVM (Vacuum Tube Volt Meter) as opposed to a VOM (Volt-Ohm Meter) is a high input impedance device. Typically around 11 megohms. If you read John's (KX5JSC) posts, he tells you specifically what sort of resistance to shunt the meter probes with to make the meter into a 1K Ohm per Volt meter.

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Thu 04, 2021 8:21 am 
Member

Joined: Jun Tue 02, 2020 11:47 am
Posts: 223
KX5JSC wrote:
Jthorusen wrote:
The shunt resistance is based on the SCALE of the METER that you use... not the voltage you are measuring. Since the voltages were measured with analog meters, these had discrete scales. A 50K ohm resistor turns a high-Z meter into a 50 volts full scale analog meter. You would not measure 56 or 68 volts with a 50 volt full scale analog meter because the meter would be pinned and might even be damaged. Common meters in use in the 1K ohm/volt era had either 100 volts full scale ranges or 250 volts full scale ranges, although there were some meters with odd scales like 150 volts.

^This.

Ranges for a TS-352 or PSM-37 are 2.5, 10, 50, 250, 500, and 1000 volts at either 1 Kohm/volt or 20 Kohm/volt. That's why calibration instructions say to use a 1 Kohm/volt meter. So, in the case of a TS-352, you use the pin jacks on the right side of the case. You use a 50 Kohm shunt for your VTVM or DMM to measure the bias voltage (0 - 42 volts or so) and a 250 Kohm shunt to measure all the other DC voltages.

John



TS-352 and PSM-37 look like VTVM's. So if you use a VTVM, you still need to use shunt resistors? I thought only DMM's need shunt resistors.

My DMM has DC scales of 2V, 20V, 200V and 1000V, so I should get 2K, 20K, 200K, 1000K ohm resistors to shunt the corresponding scales, am I correct? :)


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Thu 04, 2021 8:57 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 2907
It's strange that a better meter has to be compromised to measure old stuff. A VTVM has a very high impedance so if you want to duplicate the old insensitive VOMs you have to shunt it.

Personally, I'd figure out what the unloaded voltage should be and go with that.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Thu 04, 2021 11:14 am 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 5176
Location: Melbourne, Florida
tysman wrote:
i did buy a triplett 850 VTVM guess its not 1000 ohms per volt either anyway to make it one?


Instead of just using whatever resistor others recommend, I suggest you look up "Ohms per volt" on the web. There are lots of explanations about what it means and why it's important.

RRM


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Thu 04, 2021 1:18 pm 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 5412
Location: Lincoln City, OR 97367
Greetings to TubeFan and the Forum:

Let's try this one more time.

An ideal voltmeter would not load or interfere with the circuit it is measuring in any way. Unfortunately, there is no such thing as an ideal voltmeter. ANY meter will affect the circuit that it is connected to. However, some meters will affect the circuit less than others. A meter that has a power supply built in (be it batteries or the AC line) uses an amplifier to generate the meter reading. Since the amplifier has gain, the influence of the meter on the circuit is reduced roughly by the gain of the amplifier. Any meter that is powered from the circuit being measured imposes a greater influence (load) on that circuit which affects its operation.

Let us further take cognizance of Thevinin's Theorem, which states that any circuit with a voltage present can be reduced to a black box containing an ideal voltage source in series with a resistance. It is this internal resistance that prevents the current from becoming infinite when a power supply or battery is short-circuited.

Let us take the Hickok voltage that was discussed earlier. I think it was 68 volts or so, but I am going to round it up to 100 volts for ease of computation. Let us further suppose that this is a fairly high impedance source... that is, according to Thevinin's Theorem, it has a fairly high internal resistance. Let's suppose that that resistance limits the short circuit current to 5 mA. That gives an internal resistance of 20,000 ohms. Now, let us suppose that the meter used by Hickok was a 1,000 ohms per volt meter. According to John, the scale that would be used to measure 100 volts would be the 250 volt scale. Now, a 1,000 ohms per volt meter happens to have a 1 mA full scale meter movement. You can do the arithmetic for yourself if you wish a proof of this.

Therefore, a 250 volt full scale meter will have a resistance of 250,000 ohms between the probes. Now let us insert this meter into our circuit to measure its voltage and see what happens. The total resistance in the circuit is the 250,000 ohms of the meter PLUS the 20,000 ohms internal resistance of the circuit we are measuring. Thus, the total current flow is 100 volts divided by 270,000 ohms or .37 mA. 37/100 of a milliampere would be 37% of a full scale reading on the 1mA 1000 ohms per volt meter or 92.5 volts.

If a 100 volt full scale meter were used by the people who wrote up the procedure, the meter would read 83 volts. Now, if they wanted 100 volts at the measuring point, they would tell you to adjust for 92.5 volts with the first meter or 83 volts with the second meter.

Therefore, I hope it becomes apparent that the shunt resistance that you use to make your modern high Z meter look like an old-fashioned 1,000 ohms per volt meter depends ENTIRELY on what the resistance of the meter was that was used to develop the procedure back when Hickok or whoever wrote it. The scales available on your modern meter are IRRELEVANT to this procedure. You must make an educated GUESS as to what the meter Z was back when the procedure was written and make your meter look like the historical meter.

Fortunately, that is not too hard to do.... the specification for the ohms per volt rating of the meter used is usually given in the procedure and a bit of historical research will tell you the most popular scales employed by 1000 ohms per volt meters at the time. This is what John so kindly supplied us with. These two pieces of information will allow you to calculate the shunt resistance to put across your meter in order to get the same READING that the factory did. The results are usually close enough to the original to allow the calibration to be successful. If you want it to be perfect, you will have to jump into your time machine, go back to the Hickok factory and arrange to borrow the same meter used by the engineer that wrote the calibration procedure.

It's late and I am tired of beating a dead horse.

Good Luck,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Thu 04, 2021 6:35 pm 
Member

Joined: Dec Sat 28, 2019 4:18 pm
Posts: 1124
Location: Corinth, TX
TubeFan wrote:

TS-352 and PSM-37 look like VTVM's. So if you use a VTVM, you still need to use shunt resistors? I thought only DMM's need shunt resistors.

My DMM has DC scales of 2V, 20V, 200V and 1000V, so I should get 2K, 20K, 200K, 1000K ohm resistors to shunt the corresponding scales, am I correct? :)

No, TS-352s and PSM-37s are old style VOMs, so you need to mimic their 100 and 250 volt ranges in their 1K ohm/volt readings.

John


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Fri 05, 2021 3:18 am 
Member

Joined: Jun Tue 02, 2020 11:47 am
Posts: 223
Jthorusen wrote:
Greetings to TubeFan and the Forum:

Let's try this one more time.

An ideal voltmeter would not load or interfere with the circuit it is measuring in any way. Unfortunately, there is no such thing as an ideal voltmeter.

......

The results are usually close enough to the original to allow the calibration to be successful. If you want it to be perfect, you will have to jump into your time machine, go back to the Hickok factory and arrange to borrow the same meter used by the engineer that wrote the calibration procedure.

It's late and I am tired of beating a dead horse.

Good Luck,



Great, thank you so much for taking the time to explain it in great detail, Jim! Definitely not beating a dead horse, as I've never seen a better explanation on the topic than yours. :)

I kinda got the idea now. I think the key thing is that Hickok told us to use 1000 ohms/volt in their calibration docs, that defines the resistance per volt characteristic of the meters they were using. From there we can figure out the resistance of their meter and choose a shunt resistor to simulate their meter. :)


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Fri 05, 2021 4:33 am 
Member

Joined: Jun Tue 02, 2020 11:47 am
Posts: 223
KX5JSC wrote:
TubeFan wrote:

TS-352 and PSM-37 look like VTVM's. So if you use a VTVM, you still need to use shunt resistors? I thought only DMM's need shunt resistors.

My DMM has DC scales of 2V, 20V, 200V and 1000V, so I should get 2K, 20K, 200K, 1000K ohm resistors to shunt the corresponding scales, am I correct? :)

No, TS-352s and PSM-37s are old style VOMs, so you need to mimic their 100 and 250 volt ranges in their 1K ohm/volt readings.

John


Thanks. Just checked some photos on the 2 VOM's and didn't find any tube inside them, the initial guess was obviously incorrect. :oops:

If we can find a similar VOM or VTVM from the era when Hickok engineers were doing their calibration job, then we don't have to use shunt resistors to mimic them, right? We just dial the settings to 1000 ohms/volt, plug the probe into the 100V or 250V jack and start to measure?


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Fri 05, 2021 5:31 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 2907
The common VOM of olden days was 1000 Ohms per Volt. It was made with a 0-1 mA meter and for Voltage, used multiplier resistors which were, amazingly, 1000 Ohms for each Volt the range was to be, allowing just a bit less to accommodate the meter's resistance.

You can build one if you have the meter and some resistors. By the time I got into this stuff, a meter that insensitive was looked down upon by anyone in a lab and most had graduated to 20,000 Ohms per Volt. That was the beginning of the Simpson 260 era. Triplett jumped on the bandwagon with the 630. Soon everyone was making meters like that so the next popular iteration was the VTVM. Most of them ended up at 11 Megohms input resistance, constant for all ranges, so Ohms per Volt became an obsolete term.

In fact the first Heathkit intended for electronics use was, I believe, a VTVM.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Fri 05, 2021 6:01 am 
Member

Joined: Jun Tue 02, 2020 11:47 am
Posts: 223
Cool, learned something new every day. :)

Thought VTVM's were before the VOM's, as they still employed the old school tube technologies. So they came after the 20,000 ohms/volt VOM's!

The high Z from VTVM's definitely helps to improve measurement accuracy.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Fri 05, 2021 6:44 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 2907
Well your last statement isn't quite right. It's not accuracy that is improved, but the lighter loading of the circuit gives readings closer to the values without the meter. They often sold lab quality meters rated at perhaps 0.5% FS but only a few hundred Ohms per Volt so they imposed a heavy burden.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Feb Fri 05, 2021 7:10 am 
Member

Joined: Jun Tue 02, 2020 11:47 am
Posts: 223
bob91343 wrote:
but the lighter loading of the circuit gives readings closer to the values without the meter. They often sold lab quality meters rated at perhaps 0.5% FS but only a few hundred Ohms per Volt so they imposed a heavy burden.


That's right, and it's what I meant. :) I think I should have avoided using the confusing Z letter :D

When we put a 11 megaohm VTVM in parallel with the original circuit, the load is very light, barely affecting the original circuit.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Aug Tue 31, 2021 1:49 am 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
So I might be using my SS83 i made since my original 83 tests weaker on one side. it also looks like its not stable on one side. i have added a fuse in case diodes where to short. I would need to know what should i get for a proper fuse in ma and voltage so i know for sure if diode was to short fuse would blow.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Aug Tue 31, 2021 11:02 pm 
Member
User avatar

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11319
Location: Sunbury, Ohio 43074
Or you could buy a bright shiny new 83 ;-) and not have to guess about anything. It will probably outlast you.

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Aug Tue 31, 2021 11:36 pm 
Member

Joined: Aug Wed 31, 2011 11:23 pm
Posts: 547
Location: Tucson, AZ
If you are really worried you can use two diodes in series and forget about the fuse. I replaced my 83 tube about a decade ago and used a two 3A diode with a 5W Zener, I can't imagine that the diode is going to give up the ghost on a 100-200 mA. I do not need to sit around and wait for the tube to warm up and stabilize, and the plate to plate balance/drift which was a constant problem. I have a bunch of NOS JAN military 83 tubes, they continue to appreciate in value thanks to those that insist on using tubes. Ironically after the 539 series Hickok switched to diodes in the 580 series.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Wed 01, 2021 4:13 am 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
ok currently there 2 1n4007 and 2 zener diodes and 2 10ohm 3w resistors the fuse is between the resistors and diodes to protect filament winding. Right now i have a 315ma 250v fuse. in The 539 update calibration post on this forum they use a 200ma fast blow but they dont mention the voltage rating so wanna make sure what im using will be fine. And yes i agree i dont wanna wait either for it to warm up and with the ss83 dont have to worry about 1 side ever going weaker then the other


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Thu 02, 2021 7:11 pm 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
I plugged in the ss83 and checked voltages. If i set plate voltage to 150v the other plate voltage is to low its 63v should be 68v so might be cause zener diodes are wrong value i use 1N5339. My screen and bias voltages are still within spec. My kit did come with 1N5341 which is a 6v zener. If i was to leave it the way it is my screen and bias voltages are correct. Just my plate voltage will be 144v and other plate voltage will be 61v


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Thu 02, 2021 11:17 pm 
Member

Joined: Oct Thu 29, 2020 10:17 pm
Posts: 188
Location: Centennial, Colorado
I think far to much effort is in trying to get the plate and screen voltages spot on. Using the plate voltage as the reference point to get the line test set was likely a result of the meters available back in the day. However we now have modest priced digital multimeters that are accurate on AC. The Hickok tube testers have a dedicated 5 VAC signal winding on the transformer that may be divided down depending on the model. The signal and the bias voltages are the most critical, so why not start with the best reference voltage which is the AC signal. For the Model 752 & 752A a string of resistors divides the voltage down. First across R31 a 530 ohm resistor, then the remain resistors which add up to 530 ohms. So we should find 5 VAC on transformer side of R31 and 2.5 VAC on the other side. So how about we make the 5 VAC our reference point to get the Line Test set. Now a word of caution your digital multimeter needs to be reasonably accurate on the 20 VAC range and those inexpensive like HF type meters are not. Bill


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Thu 02, 2021 11:30 pm 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
I tested the same 6L6 tube on my ss83 as was on the regular 83 tube. Results where very close a difference of 1 tick lower on meter so maybe the lower voltages on plate doesnt seem to effect the results


Top
 Profile  
 
Post New Topic Post Reply  [ 73 posts ]  Go to page Previous  1, 2, 3, 4  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Chris108, Lance Hughes, PhilF, Robbdoe1, stevebyan and 15 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB