help sourcing cathode bias probe resistors please

[Sponger D]

I installed a long time ago 1 Ohm high watt resistors in series on cathodes for my 1965 Fender Super Reverb so I could measure bias current with my DVOM in mV mode.  I recently got 2 bias probes that adapt between the EL34s and the tube sockets- these alow to measure actual milliamps.  This is interesting method but more problematic because mA meters that resolve to 0.001A are less available.  My Fluke 88 was at work, so I bought a Fluke 15B+ for $130.  (BTW, so far, I really like this meter- huge readouts and I can configure the backlight to stay on rather than auto time-out)

So I decide to cross check the mA readings I'm getting by also checking using the cathode resistor method- and I get large variations. I am also going to cross check the Fluke 15B mA readings with my Fluke 88 when I replace the blown fuse in it.  It never occured that resistors could be so far off (assuming that's the problem).  Nor do I know if I can rely on Ohm readings in the 0.8 to 1.3 Ohm range using my various meters- this is a new issue for me.  I AM seeing values around 1.2 to 1.3 Ohms though- which corresponds to a degree with the discrepancies.

So, I want to replace the 1 Ohm cathode resistors because it's possible that I didn't get good enough quality or tolerance resistors when I originally installed them.

I'm not sure it matters, but what type is preferable? Metal oxide, flameproof, etc.?  Easy source?  Best brands? I know that I want at least 1 Watt and 1% accuracy. 

Thanks, Kip......

[jjasilli]

https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Resistors&ORDER_ID=698793569
Doug sells the above 1% 1/2W bias sense resistors which are good enough for me.


If you want, you can use 1% 3W or more resistors.


Vintage amps often used 5% 10R 1/4W resistors for this purpose; they also served as "fuses" for the power tubes.  The need for utmost precision is more a perception than an absolute requirement. 


Note Doug's instructions on the page I quoted.  If you don't zero out your meter, your readings may be skewed.  Also, if you use different meters; or the same meter: set for volts for one reading; current for another reading, you may find that the meter disagrees with itself. 


You should also control the voltage into the amp under test with a variac to always put, say, 120VAC into your amp.  If your wall voltage is 120VAC now, but 123VAC later, this can throw off your readings.  E.g., if your PT is 480 - 0- 480 with 120 VAC in; then @ 123 VAC in, that's 492 VAC output.  This could skew your voltage & current readings.

[shooter]

JJ pointed out the high points and why;

I want at least 1 Watt and 1% accuracy.

If this is a tube amp, under dynamic conditions I have accepted the fact that 10% variations is a GREAT place to put down meters and pick up guitars.

If you really can't sleep well, look into some of the arduino kits for current/voltage measuring.
something else I've done, once I've "zero'd" out my fluke, I put it on min/max and play for a 1/2 hour, then put the data into excel, repeat enough to "see" what is that amps "normal" operating specs

[Willabe]

So I decide to cross check the mA readings I'm getting by also checking using the cathode resistor method- and I get large variations. .......... Nor do I know if I can rely on Ohm readings in the 0.8 to 1.3 Ohm range using my various meters- this is a new issue for me.  I AM seeing values around 1.2 to 1.3 Ohms though- which corresponds to a degree with the discrepancies.

Like JJ wrote;
 

Note Doug's instructions on the page I quoted.  If you don't zero out your meter, your readings may be skewed.

If you can't zero out your meter, clip the meters leads together and take an ohms reading, then subtract that from your 1 ohm bias current sense reading.

[Sponger D]

"Zeroing" out the Flukes.... I assume you mean Ohms and difficulties at 0 to 1 Ohm accuracies.  I will research this and see if there are any options on my meters.  I've had my Fluke 88 for 30+ years- was looking in the manual- amazing how much I don't know about it.  My Fluke 88 never reads less that 0.2 Ohms- need to get to the bottom of this.  Looks like Willabe is addressing this.

Thanks so much for your answers.  Just getting into this forum.  I want to tune up/modify my Deluxe Reverb RI- or buy an original one and go thru it.  Looks like EL34 World is great for this....   

[sluckey]

Connect your leads together. When the display settles push the relative button.

[Sponger D]

Connect your leads together. When the display settles push the relative button.

OK.  I see now.  My Fluke 88 is so old that the current terminology was not used for the buttons at the time it was made.  Upper left button is called "Zero/Delta"- same thing.

[Willabe]

Note Doug's instructions on the page I quoted.  If you don't zero out your meter, your readings may be skewed.

Upper left button is called "Zero/Delta"- same thing.

[sluckey]

Most meters will have a difficult time accurately measuring 1Ω. I don't even check the value of my 1Ω resistors. I just use 1Ω 1% tolerance resistors and trust that they really are 1Ω.

[Sponger D]

Most meters will have a difficult time accurately measuring 1Ω. I don't even check the value of my 1Ω resistors. I just use 1Ω 1% tolerance resistors and trust that they really are 1Ω.

That was my gut feeling- and that is going to be my approach.  I felt like I needed to use the amp measurements to determine the actual resistance of 1 Ohm resistor - silly.  Still glad to know how to improve my attempts to use a meter to see what's going on though.

[jjasilli]

How to measure 1 ohm, per the Tone Lizard:  "How can you be sure your 1-ohm resistors are exactly 1-ohm? With your $350 Fluke DMM, you cannot ever be certain. With a $10 Heathkit IM-11 you can.  http://tone-lizard.com/vtvms/  "  About 1/2way down the page.

With regard to measurements you're talking about both precision AND repeatability.  For that you need to eliminate variables, such as inherent meter discrepancies; different wall voltages into the amp.  And there's time & heat.  As a tube warms up & stays warm its readings will vary.  If you measure a tube amp's voltages & currents at turn on, readings will usually be different after 20 minutes, and after two hours.  One reason is the tubes.  Another reason is that as resistors and coils become warmer, their resistance rises, thereby changing voltages and currents.  So if you really want .001 precision AND repeatability you need to perform at the level of a NASA grade laboratory, not a build or repair tech.

[PRR]

There is about a 99.9% chance your "1 Ohm" resistor is more exact than you can measure with everyday tools.

Yes, some passive and VTVM needle-meters will put 10 Ohms half-scale and 1 Ohm where you can read it, but you are at the mercy of battery voltage (and zero-trim) and the 2% internal reference resistor in ordinary meters. One BIG advantage is that as you try to get GOOD contact on the resistor leads you will SEE the needle jump around because it is very hard to get a <<1r contact through tarnished leads and probes. Also you normally zero-set these meters WITH the probes, while the modern DMMs tell you exact zero at the *internal* node, neglecting the lead resistance. (This is where the Rel/Delta function is needed.)