My New Fluke 115 on testing capacitors

[Platefire]

My first Fluke ever, I got in the other day as a e-bay buy. Never had a MM that would test capacitance before but apparently it only test Electrolytics. It would read those correctly but never could get it to read a regular coupling capacitor and get the correct reading. Couldn't see anything in the manual that explained this? Maybe I'm doing something wrong! Platefire

[stingray_65]

From what I can see on the www. it should test from 1nf to1000uf

[Platefire]

yeah thats what I read in the specs too. I must not be setting the range right or something?

[TubeGeek]

I have 3 fluke's on my bench, I love 'em. I use two 85's for metering bias current and the other one I have is a 117 that I use for general probing.

All you need to do when measuring µF is select the capacitance and press the yellow function button. You'll see it noted on the lcd screen.

[Platefire]

TubeGeek

When you measure bias current of an amp without the 1 Ohm cathode resistors or external test points, do you use the shunt method from OT center tap to plate?? I personally thought this was a great way to measure bias current on older amps but I get the feeling from a lot of other tube amp builders that this is way to riskey due to opprotunity to create shorts but I think if you are extreamly carefull using insulated alligator clips, take your time observing all connections, it can be done rather safely.

Some said it would be better to temporarily solder in some 1 Ohm resistors using the Mv setting for testing, test and removed them when done.

That's the reason I bought the Fluke is to accuratly perform the shunt method when needed on vintage amps.

What's your take on this?  Thanks, Platefire

[TubeGeek]

TubeGeek

When you measure bias current of an amp without the 1 Ohm cathode resistors or external test points, do you use the shunt method from OT center tap to plate?? I personally thought this was a great way to measure bias current on older amps but I get the feeling from a lot of other tube amp builders that this is way to riskey due to opprotunity to create shorts but I think if you are extreamly carefull using insulated alligator clips, take your time observing all connections, it can be done rather safely.

Some said it would be better to temporarily solder in some 1 Ohm resistors using the Mv setting for testing, test and removed them when done.

That's the reason I bought the Fluke is to accuratly perform the shunt method when needed on vintage amps.

What's your take on this?  Thanks, Platefire

When I measure current without 1 ohm resistors I am using bias adapters that I have built specifically for setting bias.  I see 5-10 amps per month come across my bench and I need to be efficient with my time otherwise I will get behind.  

I bought the parts to make the adapter from Doug.  http://www.el34world.com/charts/BiasChecker3.htm

They are inexpensive, easy to assemble and make the process super quick to measure bias current using my flukes.  That is why I have two fluke 85's...one meter for each side of the OT plate leads.  I simply install the adapter into the sockets and install the tubes into the adapter socket, turn the amp on and read mA on each meter.  I then take my other fluke (117) and connect that to the plate and monitor B+ vdc.  I go even further sometimes and connect more meters to a hot amp and then play the amp and monitor all the voltages while I'm jamming out.  It makes me feel like I can see how the amp behaves to the way I play.  It's just interesting to me as I'm trying to teach myself how these amps really work....long process!

The transformer shunt method can be done safely.  I've done it a few times but it kind of freaks me out.  I always use tight meter leads.  Installing a 1 ohm resistor is pretty quick too.

Either way works... just don't let the lead slip in the transformer shunt method!

[jjasilli]

Me too - except I restored a quad of Heathkit VTVM's to monitor voltage in a few places at once (compared to a digital display, the analog meters give a quick visual reference); and Fluke 8000A digital multi-meters to monitor current.  To test caps I use an old Eico bridge comparator which goes to 500V

@Platefire:  Congrats on the new Fluke meter!

[Platefire]

TubeGeek-I read the Hoffman link you provided and understood everything pretty well except one thing---where Doug shows his MM with the banna plug, I can't tell in the picture if the banna plug is plugged in to some seperate sockets than the regular probe sockets on his MM or the banna plug is un-plugged and outside the regular probe sockets. All three of my MM's only have three sockets for the regular two probes and I'm assuming that's what Doug has too and he's showing the banna plug un-plugged adjacent to the probe sockets--it's just hard for me to tell in that picture?

My Fluke manual directs me to use the 10Amp fused socket for all Amp measurements. So if I built a checker the red would have to go in the 10A fused and black center common.

I have a Ruby BiasMaster I had been using but after I leared the shunt method I found that my MM readings and the BiasMaster were way off from each other, so now I don't trust the BiasMaster anymore.

So I may be interested in Dougs Bias checker. I'll study it a little more. Thanks, Platefire   

[sluckey]

So if I built a checker the red would have to go in the 10A fused and black center common.

That's correct for your meter. Don't worry about Doug's meter. And set your meter to check DC amps. Your meter reads 1mA to 10A DC amps. (up to 20A with some precautions)

IMO, an improvement to Doug's bias checker would be to simply permanently install a 1Ω/1%/1W resistor across the dual banana jacks. Then just set your meter to measure DC millivolts.

[Platefire]

OK, Thanks! sluckey  I'll put the checker on the list of possible future Projects

Well by reading Dougs info about the Checker I learned something new about checking the bias on cathode resistor biased amps---that you could measure the voltage accross the cathode resistor and dividing that voltage reading by the Ohms of the cathode resistor get mA's(2 tubes, mA divided by 2). And I read somehwere else you could further refind this reading by subtracting the 3 or 4 mA screen current from that reading. So I checked my Champ and it was 21.9 VDC accross cathode and 21.9 divided by 470 Ohms=.04659 or 46.6 mA minus 4 mA screen current=42.6 mA
Seems kind of high for a 6V6 but I understand that normal for a champ. A little high, probably should be more like 37 mA. The plate voltage is 390 though--so that is kind of high for a 6V6--but it is a JJ .

Another good thing is I took my Rudy BiasMaster apart tonight to see if I could see any problems--didn't find anything and put it back together and then it started reading more normal and not high as before. In fact it was reading 63mA before on the 6V6 before and after I worked on it is read 46.9, right in line with my cathode R check. I know I'll get to the bottom of all this bias checking sooner or latter. Thanks, Platefire

[FYL]

My first Fluke ever, I got in the other day as a e-bay buy. Never had a MM that would test capacitance before but apparently it only test Electrolytics. It would read those correctly but never could get it to read a regular coupling capacitor and get the correct reading. Couldn't see anything in the manual that explained this?

Your 115 is able to measure from 1000 nF to 9999 μF full scale, typical coupling caps - say in the 4.7 to 100 nF range - should be OK, lower value caps could be a problem as it's resolution is 1 nF with an accuracy of 1.9 % + 2 counts.

Reset the DMM. Use a known good cap, say a 100n (.1 µF). What is the reading?

[Platefire]

Yeah the Fluke is testing most all capacitors very well. The first time I tried it I couldn't get it to read all caps but tried it again latter and it worked fine. Platefire