Some people send me an email asking about how to measure cathode current and resistor rating. I will show you how to do it and actually this way is not only applicable for cathode resistor (but commonly used for that purpose).
Basically, there is 2 common way to do it. First, put you Ampere meter in series with the resistor. Second, is to measure the voltage on the resistor, then calculate the current based on the Voltage, Current, and Resistance formula (which V=I*R).
For the sake of simplicity, we can opt to use the second method. We cut the shrink from the resistor pins and put the probes there.
More detail of the process, we can use hook type probe and put there for longer time to observe more.
Value read on the Ampere meter. Since this is only small Pre-Amp, the reading is around 3.7 Volt. Slightly up and down, but we can take the average of the value.
We put them on the simple formula (you can always use Excel to simplify your life). Basic formula is V=I*R and P=V*I. V is for Voltage, I for Current, R for Resistance, and P for Power.
As you can see from our sample case. With 330R cathode resistor and 3.718V reading, the current should be around 11mA. Now we know how much current flows on this cathode resistor. Then, we can find the suitable power rating for the resistor. Off course you can always use higher rating for this resistor, but how if you don’t have such option? Example, high end resistor like Shinkoh or AudioNote probably hard to find in higher rating, as they are getting rare. So, what is the safe level?
Base on our calculation, with 3.718V and 11mA current, we can use 0.04W resistor. Well, 0.5W should be common value, 1W should be more than enought, while 2W will be overkill. Some people prefer to spare rating for 3-4x more. So if your calculation produces 0.04W, it’s always comfortable to use 0.5W. If your calculation produces 0.5W, better to go with 2W. Always measure your cathode resistor temperature if you are not sure. More than warm mostly means you have power rating problem. Fix it before it’s too late.
Gary
May 2, 2013 05:22Hi Jimmy,
Been reading thru your pages… some interesting posts!!
As for this one on measuring cathode current, the SIMPLEST way is to use a 1 ohm precision (1-2%) resistor… that way no calculations are necessary.
Measuring voltage across the resistor gives you the exact current reading (ie 33v = 33ma)
The best way is to o it just make custom a set of 2-4 custom probe sockets – (8 & 9 pin sockets) with a 1 ohm resistor between the socket & plug cayhode pins, with a piece of zip wire soldered to the resistor legs, coming thru the socket, & terminating in a dual-banana plug (or whatever your meter uses) (I have Sencore DVM56’s on my bench, so the dual banana plugs right in…)
(I service high end home audio & pro audio tube gear for bands/studios etc… & have been using these probes for 25-30 yrs now)
Alternately, for your own custom amps, or building a test amp, mount a 1 ohm resistor between the socket’s cathode pin & ground, & run the zip wire to a pair of test jacks on the back of the amp (for each tube… so they can be monitored simultaneously.)
Even so-called “matched” output tubes are often NOT current matched… as I’ve seen anywhere from 10-30ma+ DIFFERENCE between tubes (amps were fully serviced & working properly… so not the circuit, & swapping tubes between the sockets showed the same readings in the other socket.)
Anyway… home this helps!!
Enjoyed your post!!
Selemat Malam!!
Gary
(Saya bisa mengerti dan bicara Bahasa Indonesia sidikit-sidikit.. 🙂
P.S. lived/traveled in Indonesia for 4 months in 2002 – Bali, Lombok, & Sulawesi… learned a bit of Bahasa Indonesia, but very rusty, for lack of anyone to speak with locally.
Nice to know there are some electronics (& Sencore) enthusiasts there… will look forward to meeting some next time I can get down there!!
If you need any info on Sencore, I’ve owned 35-40 models & have a LOT of info on most all their products (bought my first Sencore FE20 FET Multimeter when I was ~16 (1974) & still have it!
Auw Jimmy
May 3, 2013 18:17Hi Gerry,
Yes I believe so. Use 1 ohm precision resistor is the best approach.
Your Bahasa is good 🙂
I love vintage equipment (two precious are Hickok 539B and off course, Sencore LC-102). Try to find the original probe, though. Meanwhile I use a ‘DIY’ one. Not bad to recharge my old capacitor before giving full power.
Thanks.