Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 6v6Pin1 on November 15, 2015, 10:43:31 pm

Title: Bias with 1.3 ohm cathode resistors
Post by: 6v6Pin1 on November 15, 2015, 10:43:31 pm: My project has "1 ohm" cathode resistors to set power tube bias. My meter says they are 1.3 ohm. Using 1.3 instead of 1.0 to calculate current makes a big difference.
How likely is it that my handheld meter is accurate in this range? How would you bias the amp in this situation?
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: sluckey on November 15, 2015, 11:07:55 pm: I always use 1% tolerance resistors and trust that the value is correct. IOW, I blindly trust that a 1Ω 1% resistor is really 1Ω regardless of what my meter says, unless I have some reason to believe the resistor value may be compromised. If that's the case (never has been so far) I would just grab another 1Ω 1% resistor and put the blinders on.

So, if you short your meter probes together, what does your meter read? Do you have a relative button to 'zero' the reading when the probes are shorted?
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: DummyLoad on November 16, 2015, 01:17:48 am: errr....
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: TIMBO on November 16, 2015, 01:53:51 am: Will using a 1.5r resistor going the give you a much different reading than a 1r????
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: HotBluePlates on November 16, 2015, 05:08:20 am: Quote from: TIMBO on November 16, 2015, 01:53:51 am
Will using a 1.5r resistor going the give you a much different reading than a 1r????

50% different reading, IF the resistor is actually 1.5Ω.

Except the original post probably doesn't have "1.3Ω" resistors, but "1-to-1.1Ω" resistors, and 0.2-0.3Ω of meter lead resistance. Unless you can remove the lead resistance from the measurement with the meter's Relative Mode, Sluckey's advice is the way to go. Assume the tight-tolerance resistors are more trustworthy than your meter...
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: TIMBO on November 16, 2015, 01:05:29 pm: Its just that I ran out of 1r resistors and had 1.5r, these are metal oxide that are as close to "1r" as I can get and if the reading across them is only a mA or 2 is that a problem.
I have ask in the past "what bias setting should I use" and I most response is that it is better to bias a little "cold"

Is it that CRITICAL???
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: VMS on November 16, 2015, 01:23:42 pm: 1 ohm resistor is used so that you can convert the measured mV straight to mA without doing the math.

U = RI => I = U/R or I = U/1ohm

If you have 1,5 ohm resistor then the bias current (mA) is not same as the measured voltage (mV).

Some amps have 10 ohm resistors and then your bias current is measured voltage divided by 10. Which is also easy to do without calculator.
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: jjasilli on November 16, 2015, 03:24:14 pm: +1 to Sluckey & VMS

Re 10 ohm bias sense resistor: a 1/4W 10 ohm resistor ALSO serves as a "fuse" to protect a power tube from a high over-current condition - a "trick" sometimes used in vintage hi-fi and PA amps. Just remember where the decimal point belongs!
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: PRR on November 16, 2015, 03:56:09 pm: > How likely is it that my handheld meter is accurate in this range?

+/- a couple of Ohms.

With great luck, +/- a few tenths of an Ohm.

As Sluckey says, many meters will show a few-tenths across a dead-short.

For extra fun, a few I have seen will "round down" anything under 0.4 Ohms to "zero". (Avoids dumb/smart questions from owners who expect to see 0.0.) But if there is really 0.3 Ohms parasitic in the meter, a 1.0000 Ohm will read 1.3 Ohms. Pure deception.

If you got good resistors, trust them a LOT more than you trust any meter with unknown low-Ohms errors. The resistor makers got meters which read right-- that's their job. And they have savvy customers who will call them out if their resistors are off the mark. So we little guys don't have to worry, we just KNOW the resistors are what they say they are.

> accurate in this range?

There should be hints in the specs.

Unless you paid a lot, it may be 2% (or worse) of FULL-scale reading.

If you can display "1.3", then this is probably a 19.9 Ohm scale.

2% of 19.9 is 0.398 Ohms (say 0.4 Ohms).

Most meters also exclude lead (and fuse) resistance, which is typically several tenths on top of that 0.4 Ohm error.
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: PRR on November 16, 2015, 04:06:58 pm: > I ran out of 1r resistors and had 1.5r

V=I/R

If you want "I" to be 0.1 Amps (100mA), and have 1.0 Ohms, then you look for 1.0r*0.1A and aim for 0.100V (100mV) across the 1.0 Ohm resistor.

If you want "I" to be 0.1 Amps (100mA), and have 1.5 Ohms, then you look for 1.5r*0.1A and aim for 0.150V (150mV) across the 1.5 Ohm resistor.

1.0 Ohms makes the math simpler. 1mV measured is 1mA current.

1.5 Ohms makes brain-pain. 1mV measured is 1mV/1.5r or 0.667mA. Life is too short to struggle with divide by one and a half. Multiply by two-thirds is hardly any better.

But you can use 12.3456 Ohms if you love math. Dynaco used some "odd" values because it made the cathode current they wanted come-out to a "nice" voltage such as 1.56V (can be checked against a fresh D-cell) or 1.23V (mark on a VU meter).

10 Ohms was very popular in days before meters read milliVolts well. A 40mA Champ in 10 Ohms is 0.4V, which can be read on a 0-1.5V scale, while 0.04V is practically "none" on a 1.5V scale.
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: 6v6Pin1 on November 16, 2015, 06:52:33 pm: With the leads clipped together, my meter reads 0.2 ohms. So, that plus some error percentage on both measurements is why I'm getting 1.3 ohms. That makes sense. Thanks.
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: TIMBO on November 17, 2015, 01:19:12 am: I'm off to buy some 1ohm resistors. :lipsrsealed:
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: Platefire on November 17, 2015, 02:14:32 am: Boy! Boy! Send you to school and send you to school and you still eat the resistors :dontknow:
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: TIMBO on November 17, 2015, 03:30:00 am: What are the specs on these resistors. Thanks
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: EL34 on November 17, 2015, 05:28:27 am: Your meter leads have resistance
So you are reading 1 ohm plus your meter leads

If you bought them from me there is this note in the description
Note: Make sure you zero out your meter or take into account your test leads resistance when reading a 1 ohm resistor on your meter
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: jjasilli on November 19, 2015, 11:03:39 pm: See: the Tone Lounge Lizard extol the virtues of the VTVM: http://tone-lizard.com/vtvms/ (http://tone-lizard.com/vtvms/)

"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. Want proof? Take that Fluke, and set it to measure ohms. Short the two leads together, and note the reading. What did you get; 1.2-ohms? Tsk, tsk, tsk. My $10 Heathkit VTVM can find a 1-ohm resistor that is exactly 1-ohm. If we remember that low-wattage carbon composition resistors drift, and avoid them, we can put resistors that are ‘precision matched’ in our amplifier that will accurately measure what is purported to be plate current. Read ‘Biased Opinions’ to refresh your memory on my feelings about using 1-ohm resistors in the cathode to try and set the bias."
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: HotBluePlates on November 20, 2015, 12:55:12 am: Quote from: jjasilli on November 19, 2015, 11:03:39 pm
See: the Tone Lounge Lizard extol the virtues of the VTVM: http://tone-lizard.com/vtvms/ (http://tone-lizard.com/vtvms/)

"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. Want proof? Take that Fluke, and set it to measure ohms. Short the two leads together, and note the reading. What did you get; 1.2-ohms? Tsk, tsk, tsk.

:icon_biggrin: Except the $350 Fluke also has a ∆ (relative mode) button, allowing you to zero the meter lead resistance just like the old VTVM's. Still, a 60-year-old VTVM is much cheaper...
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: Ed_Chambley on November 20, 2015, 08:17:14 am: Seen the Fluke 17B made in China for international sales. $100 to your door. Reviews on the net are positive. I have an old 77 and a newer 87, but I still have some analog meters. I found a Triplett new in box at an antique store. Haven't had a chance to see if it works. The old "c" battery is corroded and I tried a new one and felt it start getting hot. Needs repair, but I am having fun trying to learn all the features on my new O-scope. :icon_biggrin:
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: jjasilli on November 20, 2015, 11:53:05 am: @ Doug & Hotblue: Yes, a good DMM can zero out its low ohms readings. I don't think a bargain DMM, say under $20, can do that.

@Ed: cools test eq! Here's a battery eliminator circuit. It uses a voltage regulator, & taps off the VTVM's filament supply to feed 1.6VDC to the Ohm's reading circuit. Note that a new battery 1.5V actually yields 1.6VDC. There's posts online if you google it.
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: Ed_Chambley on November 20, 2015, 12:08:07 pm: Quote from: jjasilli on November 20, 2015, 11:53:05 am
@ Doug & Hotblue: Yes, a good DMM can zero out its low ohms readings. I don't think a bargain DMM, say under $20, can do that.

@Ed: cools test eq! Here's a battery eliminator circuit. It uses a voltage regulator, & taps off the VTVM's filament supply to feed 1.6VDC to the Ohm's reading circuit. Note that a new battery 1.5V actually yields 1.6VDC. There's posts online if you google it.
Nice, I am just putting together an order of opamp and transistors. I will get the parts as it sure beats the battery.
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: PRR on November 20, 2015, 01:57:09 pm: > cools test eq! Here's a battery eliminator circuit.

If his C-cell hots-up just sitting, then he has a short in the ohms circuit. An odd fault, but a batt-elim won't fix that. (Unless he builds a 100-Amp supply, which will burn out the short, tho not necessarily at the fault point.)

> new battery 1.5V actually yields 1.6VDC

I've used 1.56 and often been real close.

However it does not matter. The VTVM is really scaled for about a 1.2V batt. The Range calibration pot (set with open probe) adjusts that so your battery and condition just hits the infinity mark. I think I have used a naked LM317 (1.25V), it calibrated exactly, and held its calibration pretty-near for decades. (I guess I'm saying make your trimpot a jumper, it may be fine, or may want 50 or 100 Ohms added there.)

1N914 should possibly be a 1N4007. First: probably cheaper. Next: it does nothing when all is right but protects when things go wrong. When wrong they can be more-wrong than a 0.1A '914 can stand. 1N400x can stand 1A steady and dozens of A surge. 1N4007 is hopefully total overkill for voltage but in DIY size bags the '4007 is the same price as the '4001.
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: eleventeen on November 20, 2015, 02:16:39 pm: Ed, there are a few VTVM refurb videos on youtube. One guy "ask Mr. Carlsen" is quite instructive. FYI.
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: EL34 on November 20, 2015, 02:49:32 pm: Quote from: jjasilli on November 20, 2015, 11:53:05 am
@ Doug & Hotblue: Yes, a good DMM can zero out its low ohms readings. I don't think a bargain DMM, say under $20, can do that.

you don't have to zero out the meter
Just put the leads together and read them and remember that reading

In this case the leads would read .3 ohms

So 1 ohm + .3 = 1.3 ohms
Or 1.3 ohms - .3 ohms = 1 ohm
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: Ed_Chambley on November 20, 2015, 05:34:51 pm: Thanks for the heads up 11teen. It is one of those back burner things. This one is the Triplett 666-HH. A portable unit. I found it new in the box and even the probes don't have a scratch.

I am in the process of building a new bench/workstation and I am convinced this time I will get everything organized correctly. :l2:

Seriously tho, I have gathered up some really nice equipment. As PRR mentioned there is something amiss with the volt meter, but it needs nothing other than whatever is shorting the battery. I really like it tho. It has that art deco look and is made from Bakelite.
Title: Re: Bias with 1.3 ohm cathode resistors
Post by: alerich on November 21, 2015, 08:46:19 am: Quote from: HotBluePlates on November 20, 2015, 12:55:12 am
Still, a 60-year-old VTVM is much cheaper...

We used those in the shop when I worked for a record company to set bias and adjustment on tape duplicator machines. Had to leave them powered up 24/7 to keep them warm. Cannot say that I recall them with much fondness. I'll take my Greenlee DVM.