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Installing a current meter..

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Posted on June 18, 2017 at 13:24:13
DAK
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 I wanted to monitor the current of the power tubes and wonder if i just hookup one lead from the cathode to the meter and from the other screw post on the meter to ground? It is a used meter so no instructions. thanks for the help, Dak

 

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RE: Installing a current meter.., posted on June 18, 2017 at 13:54:34
vinnie2
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Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Not unless you want to fry things. Take a look at the video below. I would also do a little more searching on the web to find some other examples. I found this one in about 30 seconds.

 

RE: Depends on what kind of meter you have, posted on June 18, 2017 at 14:47:25
deafbykhorns
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Posts: 1067
Location: Florida
Joined: October 17, 2003
 Any good Simpson can be wired just like you say.
I like to include a fuse for insurance.

 

RE: Installing a current meter.., posted on June 18, 2017 at 16:25:30
Uncle Mike
 Please read for a starter.......What you want to do is an excellent idea but you need to be sure you understand what you want to accomplish so that no meters, tubes or yourself are harmed in the process.

 

RE: Installing a current meter.., posted on June 18, 2017 at 16:38:53
DAK
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 Yeah, that looks like a cheap chinese made amp meter. I don't know if that procedure really applies to my meter.

 

RE:I have a 200ma Fluke or something like that., posted on June 18, 2017 at 16:55:51
DAK
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 I have a pair of Japanese ones, they are mini meters, also 200 ma. Since the cathode current should be less than that. It should be ok to just wire it inline or series?
About the fuse If i size it at the same current as the meter it should be what you would want to do?
Thanx for the tips.

 

RE: Installing a current meter.., posted on June 18, 2017 at 17:34:18
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
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 I don't think that's an amp meter at all.

I think it's a volt meter being used to calculate amps by ohms law.

voltage divided by resistance is current.

A true amp meter would be connected in series.

Connecting your meter inline, from the bottom of the cathode resistor to ground, should work just fine it what you have is a true amp meter.


Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Installing a current meter.., posted on June 18, 2017 at 17:55:20
Johno
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Posts: 388
Joined: June 9, 2002
 As already mentioned a good ole fashioned Simpson ie not china made meter should be fine. I still reckon any form of fusing is ideally suited in the HT side.

The alternative which is more expensive but bullet proof is to use a Volt meter across the kathode resistor. Jacmusic has some good info at there website under panel meters ie Yamamoto Panel meters

Verbatim below from above website
Yamamoto Shunt designed current meter, for tube autobias only
Please take a moment time, to understand those intelligent meters, because these are not like any other. When measuring the plate current of a tube, it is generally not desired that the signal path will go through the ampere meter. Reason is, that Ampere meter has a coil inside, and any Audio signals would be disturbed by this coil. The intention of these meters is to prevent frequency dependent effects, and other kind of interference. Besides not all low cost meters will indicate precise.
Yamamoto has found a nice solution for this. The meter is connected in parallel to the cathode resistor, and the meter is high impedance. This method is really genius. The meters gets a series resistor, which you can conveniently calculate.
What is also good to know, is by this method the meter has excellent short circuit protection. If by any mistake you make a short circuit, the cathode resistor will absorb the short circuit energy, not the meter"

Johno

 

disregard this post bad info.........., posted on June 18, 2017 at 18:34:54
vinnie2
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Posts: 4481
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 Since I am not able to delete it......

 

Most meters go full scale at 0.2 Volts, posted on June 18, 2017 at 19:48:47
Chip647
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 Using one as an amp meter would mean that there would be or should be a shunt resistor across the meter of an ohm or so. Using a a meter as a volt meter that goes full scale at 0.2 volts means that you have to determine the correct resistor network to measure across.

Measure the resistance of the meter. It is likely that your volt meter will drive the meter to full scale or it has an internal shunt

 

RE: Installing a protection fuse., posted on June 18, 2017 at 21:52:37
Johno
Audiophile

Posts: 388
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 G'day as per attached link the reason why fusing the Kathode is not such a great idea.
Verbatim from link
It took me quite a while before I could reconstruct the chain of events which led to this result. Mr. Jadis decided to protect the power valves with a fuse in series with the combined cathodes of each output channel. This seems sensible but it now appears it is not. Think of what happens if a valve looses its vacuum. The valve will develop full conductivity. Now the fuse blows. The cathode will be lifted to a voltage near the +500 Volt of the HV supply present at the anode.

 

RE: My meter is scaled to 200 ma., posted on June 19, 2017 at 02:51:02
DAK
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 Do you think it also needs a shunt resistor

 

RE: My meter is scaled to 200 ma., posted on June 19, 2017 at 05:58:48
Chip647
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 When you measured it, did the needle deflect? A volt meter will put 0.1 to 0.2 volts or so out when measuring resistance. Be careful with a meter as you can easily fry it by putting too much voltage or current on it if there is no internal or external shunt or voltage divider.

If your meter is set up as an ammeter, it will have a shunt resistor that the volt meter measures across. So, if the scale is 0.2A or 200mA, a 1 ohm shunt (internal or external) will make the meter see 0.2v at full scale. Good ol' Ohms Law.

 

RE: My meter is scaled to 200 ma., posted on June 19, 2017 at 07:08:19
Triode_Kingdom
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 Is this a panel meter you're discussing? If so, I would expect a 200mA meter not to need an external shunt. The only way to be certain is to rig up a variable power supply and fixed resistor for a test. For example, connect a 100 ohm resistor in series with the meter, then slowly begin applying voltage. If the needle goes to full scale quickly, it needs a shunt. Otherwise, it should read half scale (100mA) when you reach 10V.

 

RE:I have a 200ma Fluke or something like that., posted on June 19, 2017 at 07:23:40
SteveBrown
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Posts: 2454
Location: Portland, OR
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 Using a ma meter as a ma meter should be straight forward, but not all ma meters are created equal, some expect you to know to put in a shunt resistor but others have this built in. Use a battery and a resistor, figure out current and put the meter in series, if it reads correctly (you can verify with a digital meter), then fine, just insert it. That being said, I'm always nervous of wires and screw terminals coming lose so I put a shunt to ground, like 100 ohms. The small resistance of the ma meter will swamp that resistor in normal practice, but if the meter connection opens, the output tubes will not red-plate.

 

What about the audio aspect of having the meter in the signal chain?, posted on June 19, 2017 at 09:03:14
DAK
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 In my case, i am using a 10 0hm resistor to calculate the bias voltage on the power tube of my fixed bias amp. I would then dispense with the resistor and have the meter take its place? Also i have 2 tubes to measure so i can put a switch before the meter to be able to measure both tubes. Dak

 

That seems like a good idea, posted on June 19, 2017 at 09:18:11
DAK
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Location: PACIFIC
Joined: August 8, 2010
 Ok, if i understand you i will have the meter to ground and from the same poiint i would also install a 100 ohm resistor to ground to act as a fail safe in case the meter fails?

 

RE: What about the audio aspect of having the meter in the signal chain?, posted on June 19, 2017 at 12:18:06
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 Make the switching so the meter is out of circuit when not in use.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Good idea, posted on June 19, 2017 at 12:34:37
DAK
Audiophile

Posts: 2712
Location: PACIFIC
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 So, at first i was thinking of using a dpdt switch with the meter on the center terminal to ground. So, to make the meter offline, i would have to install another switch or what other way can you think of? thanks for the help.

 

RE: Good idea, posted on June 19, 2017 at 15:52:46
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
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 The only thing I can think of right now is 2 SPDT switches. (cathode going straight ground or through the meter to ground)

If both are switches are engaged at the same time the meter would show the total current for both tubes.

Otherwise, just remember to only engage the meter switches one at a time.


I keep thinking there's a way to do this with one switch but I haven't thought of it yet.


Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: That seems like a good idea, posted on June 19, 2017 at 17:59:27
SteveBrown
Audiophile

Posts: 2454
Location: Portland, OR
Joined: November 14, 2002
 Yes

 

RE: Depends on how it's biased, posted on June 19, 2017 at 18:09:35
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
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 If standard cathode bias, an open cathode shouldn't red plate the tube?

 

What about a rotary switch., posted on June 19, 2017 at 18:40:36
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Is it possible to configure a rotary switch so that
Off to ground and bypasses the meter
1 to tube 1 current measurement
2 to tube 2 " "
I don't know how rotary switches are configured but i have some.

 

RE: What about a rotary switch., posted on June 19, 2017 at 19:31:44
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 Yes, it's just a matter of being able to make and break what needs to be.

Break the cathode from ground and connect it to the top of the grounded meter while doing nothing to the other side.

Then re-make that connection from that cathode to ground while breaking the other cathode from ground and connect it to the top of the grounded meter.

Then back to neither cathode being connected to the meter but both are connected to ground.

Doable with the right switch.

I'm liking the 2 switch option but only because it's easy.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

You guys are making this way too hard!, posted on June 19, 2017 at 22:14:37
Triode_Kingdom
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 First things first, there's a good chance the meter will have no impact on sonics. Measure it's resistance before going to the trouble of adding a switch. It might be only a few ohms. If it turns out that's not the case, there's no reason to switch it out of circuit when it's not needed. Just leave it in series with the cathode and add a switch in parallel to bypass it to ground. Nothing more is needed.








 

Doh (nt), posted on June 20, 2017 at 07:35:34
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
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 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: My meter is scaled to 200 ma., posted on June 20, 2017 at 08:06:10
Chip647
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Posts: 2649
Location: The South
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 I have a ton of old panel meters. Most labeled "mA" that have a low scale (500mA or less) are 200mV DC meters without an internal shunt.
Amp meters that are above that always have an external shunt. Far cheaper to test than to assume. :-)

 

RE: Depends on how it's biased, posted on June 20, 2017 at 08:12:02
SteveBrown
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Location: Portland, OR
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 I think the OP mentioned going from cathode to ground, though I could be wrong, and that implied fixed bias to me but yes, if it is cathode biased, not an issue.

 

RE: My meter is scaled to 200 ma., posted on June 20, 2017 at 10:05:17
Triode_Kingdom
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Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 "Most labeled "mA" that have a low scale (500mA or less) are 200mV DC meters without an internal shunt."

If I understand, you're saying the meters are mostly 0.2V FS, and it's the difference in coil resistance that provides different FS current readings. That would mean a 200mA DC panel meter is likely to have a coil resistance of one ohm. Is that correct?





 

how do i measure both channels, posted on June 20, 2017 at 10:32:25
DAK
Audiophile

Posts: 2712
Location: PACIFIC
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 power tube current without at least 2 switches, 1 switch for bypass?

 

RE: how do i measure both channels, posted on June 20, 2017 at 12:00:18
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 For that you'll need a two pole, three position rotary ("L - Off - R"). Contacts should be non-shorting, make-before-break.














 

RE: My meter is scaled to 200 ma., posted on June 20, 2017 at 12:47:19
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 Most old panel meters need an external resistor network to function correctly. My advice is to test it carefully before putting it in a circuit and destroying it. But yes, if the coil resistance is 1 ohm then it could be used directly as a low current ammeter.

 

RE: That seems like a good idea, posted on June 28, 2017 at 22:27:22
SteveBrown
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Posts: 2454
Location: Portland, OR
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 Yes

 

RE: Installing a current meter.., posted on July 2, 2017 at 07:16:08
Stuben
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 I would think placing a measuring device in the loop might be a bad idea. Just thinking about it's potential for adding color.. but a calc off a know voltage drop would be less intrusive. ..might be over stating the influence of the added device...

Stuben

 

RE: Installing a current meter.., posted on July 4, 2017 at 08:34:49
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 "but a calc off a know voltage drop would be less intrusive. ."

That's all a current meter is. A volt meter connected across a known, small value resistor. Something like 1 ohm.

That shouldn't change much of anything as far as the circuit is concerned.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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