Simple cathode-resistor biasing question

[Tubetwang]

The Kiwames I get are from partsconnexion.com in Canada. There's also a US vendor, but I can't remember the name. The Japanese company limits their sales to only one vendor per country, in order to keep the prices up. However, Partsconnexion does have several good sales each year. Great folks to deal with.

Thanks Rich,

they don't seem to have anything lower than 2R2 so i got Mills 1R from them...

[fperron_kt88]

Hello Guys,

Here is an experiment that you can try that will definitely put things into perspective as regards to the trust you CAN have on your tools.

Pick any two 1 Ohm resistors labeled with 1% tolerance. Connect them in series to a 200mA current source (*more on the current source below...). Now measure the voltage at the top of this 2 resistors string against ground and let's call this Vtot_1. Then measure the voltage in the middle of this two resistor string and call this Vmid_1. Lather. Rince.

Now _swap_ the two resistors. Repeat the measurements. Let's call these new Vtot_2 and Vmid_2 for good measure.

I bet that you will find interesting results! Hint: Vmid_2 and Vmid_1 should be very similar. Also, Vmid_x should be very very close to the half of Vtot_x...

Do this with, say, a Fluke21, on the 300mV scale. Mine is a 15 year old unit, and is still worthy of this experiment... You will find that 1% resistors are usually called 1% resistors for a specific reason . You should also have a hint as to why a lot of folks use a _voltmeter_ to _compute_ the total cathode current passing through 1%, 1 Ohm resistors in an otherwise very fine tube output stage circuit!

Google for "kelvin measurement" or look at these: http://www.cirris.com/testing/resistance/fourwire.html
http://www.allaboutcircuits.com/vol_1/chpt_8/9.html

Have fun !


Some important details:

*Note on the "current source": pick a good power supply you have and select a resistor R that will safely pass 200mA between Vsupply and ground (power wise, I mean, and you guys know how to deal with 300Vdc circuits, so I won't comment on the lethal voltage thing). Verify that Vsupply/R is about 200mA and you should be fine. Now connect the supply across the series combination of R and the 2 other 1 Ohms resistors. That's right: one supply, R, the first one Ohm resistor, The second One Ohm resistor, then ground. Make sure that the 2 one Ohm resistors are connected on a very short path (twist them half a turn and solder them together) and make all the measurements directly on their legs.

**Note2: BTW, sticking a 1K resistor from a 200Vdc preamp supply is NOT a good idea since it could draw up to 40Watt!!! So start with something smaller supply wise. Batteries are fine, as long as you know how to safely fuse the thing and measure quickly before the batteries die out... Remember, we want 0.2A in the dual 1 Ohm series resistor string to get the scale going on the DVM... You can use less current, but we want to play on the top of the 300mV scale to get good numbers to play with. We also want to stick to the same scale for all measurements... Turn that autoscale _off_ !!!

[davent]

Thanks for all the great replies guys; you all more than answered my question.

Guess I'll be putting the money I've been saving for my first 'Wreck build toward a Fluke instead , which is gonna be rough but ultimately worth if they are that much more accurate. Good to hear that they're durable as well.

A little under two years ago I picked up a recently calibrated (at the time) Fluke 8050A DMM (four and half digit) on ebay for $31 plus shipping. Took a while to get one at that price but finally scored and it's served me well and no reason it won't continue for a good long time. Not sleek, sexy and yellow like the new stuff but looks retro cool on the workbench.

You can get the manuals as a download from Fluke.
http://us.fluke.com/usen/support/manuals/default.htm

Also there's two version, an AC version and one with batteries and AC. The rechargeable batteries are obsolete so to replace them can be a challenge and the battery version won't work properly if the batteries are shot. An article on replacing the batteries with some current production ones was in AudioXpress Apr. 2005.

Take care
dave

[Lonely Raven]

With a so low value resistor, many errors came out of the basket.
The first two tips coming in my mind are: test resistence value as near as possible to resistor's chassis, and test resistence value of one pin of the resistor. A little bit of superficial Oxide can change values. Pay attention on it!

I keep 0000 Synthetic Steel wool at my work bench. I buff the oxide off each lead of every part I use or test. I don't know if it makes a difference, but it's attention to detail that I feel helps me be a better building in the long run.

[Structo]

Thanks for all the great replies guys; you all more than answered my question.

Guess I'll be putting the money I've been saving for my first 'Wreck build toward a Fluke instead , which is gonna be rough but ultimately worth if they are that much more accurate. Good to hear that they're durable as well.

A little under two years ago I picked up a recently calibrated (at the time) Fluke 8050A DMM (four and half digit) on ebay for $31 plus shipping. Took a while to get one at that price but finally scored and it's served me well and no reason it won't continue for a good long time. Not sleek, sexy and yellow like the new stuff but looks retro cool on the workbench.

You can get the manuals as a download from Fluke.
http://us.fluke.com/usen/support/manuals/default.htm

Also there's two version, an AC version and one with batteries and AC. The rechargeable batteries are obsolete so to replace them can be a challenge and the battery version won't work properly if the batteries are shot. An article on replacing the batteries with some current production ones was in AudioXpress Apr. 2005.

Take care
dave

Nice find Dave.
I have an old HP DMM from the 80's that served me well.
It had the rechargeable battery pack (I think it was more like a storage cap) but of course as the years went by it wouldn't hold a charge anymore.
I haven't even looked at it in years but your post reminded me of it.
It had a red LED display, that really dates it huh.

A few months back I was going to buy a Fluke handheld meter at a local electronics supply house.
They were out of the one I wanted so I bought a Wavetek Meterman 37XR.
True RMS, auto ranging, blah blah blah.
I paid $125 for it and I like it a lot.
I heard a rumor that they are made by the same company that makes Flukes.
The 37XR has been discontinued and one should be able to pick one up for a bargain, if so inclined.
Data Sheet:
http://support.metermantesttools.com/me ... _ENG_W.PDF

[Phil_S]

Some time ago, I concluded that the 1-ohm method has problems that mean it isn't right for some amps. One problem is that you are measuring both plate and grid voltage. Another is what's being discussed here, that it is hard to either get accurate 1-ohm resistors or get an accurate reading on them with anything but a Fluke.

For those like me who don't like to measure amps, you can very easily check bias if you know B+, plate voltage, and the resistance of the winding. With the voltage drop across the winding and the number of ohms, you have everything you need to calc mA draw by the plates. It almost as fast and easy, especially if you build in test points.

Just my 2 cents.

[d2camero]

As others have said measuring low resistances with a cheap meter is a waste of time. Even a lot of very good meters are not all that great at measuring low resistances.

But why are you even bothering to measure the resistance anyways? All you really care about is the voltage drop. Your cheapo meter will work fine for this.

Don't bother with buying the Fluke, and spend your money on 1ohm 1% resistors (and YES you can trust the manufacturer - purchase 10 1ohm 1% and use the test as previously mentioned to find the two most consistent values). The 10% resistors are garbage for this application.

If you still feel compelled to accurately measure low resistances, then get a kelvin bridge or wheatstone bridge.

d2