1 ohm bias resistors. Needed?

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Phil_S
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Re: 1 ohm bias resistors. Needed?

Post by Phil_S »

pompeiisneaks wrote: Fri Feb 09, 2018 4:48 am [I'm not suggesting "It's a good fuse" I'm suggesting, it has the added benefit of being able to protect in some cases. I completely understand that a fuse is the right tool, that or some diodes on the OT etc.
I must have been feeling rather cranky when I wrote that thing about not using a resistor as a fuse. Reading again, i think you were more than clear enough in what you wrote the first time. I apologize. I was out of place.
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romberg
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Re: 1 ohm bias resistors. Needed?

Post by romberg »

pompeiisneaks wrote: Fri Feb 09, 2018 7:05 pm
erwin_ve wrote: Fri Feb 09, 2018 4:20 pm 1 Ohm resistors are handy but not accurate. In a pentode there is also screen current which is also measured with the 1 Ohm resistors.
That said: I put the 1 Ohm resistors in my builts :D
Yes but most people don't worry about that, because it is often around 5mA, and if you're assuming it in your calculation that means you'll always be 5mA under max current setting you're shooting for, and in a safe range, it basically increases the room for 'safety' from over biasing a bit.

OTOH there's a general rule of thumb taht since it's 5mA pretty consistently, some just read the mV and subtract 5 and call that the active mA
It is not all that difficult to measure the screen current if you want to since there is almost always a screen resistor attached right to it. Just measure the voltage drop (be careful these are all high voltages) across this resistor. Every time I've done this, I get something close enough to 5mA that I don't bother to measure it anymore unless the screen resistors have unusual values.

Mike
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Phil_S
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Re: 1 ohm bias resistors. Needed?

Post by Phil_S »

RockinRocket wrote: Fri Feb 09, 2018 5:08 pmI have misplaced my bias probe and I need new tubes for my 72 JMP. The amp is about 98% untouched and that's how its going to stay. So no 1 ohm resistors are going to be installed or temporarily.
If you don't have 1Ω resistors, try this. Measure the DCR of the output transformer primary, each leg to center tap. You must pull the tubes for this. Measure from the plate/anode pin to the center tap on each half. I have found sometimes the result is a false reading. To keep it accurate, disconnect the CT from the circuit when you measure the DCR. Also, write it on the transformer with a Sharpie. These will not be equal. The halves have the same number of turns, but the outer half of the winding requires a longer length of wire. Put the tubes back in their sockets. Now, determine the voltage drop across each half of the winding. IOW clip your meter to the CT feed and the plate pin. Do the math. Vdrop/Rwinding = Ia. Check each side. That's your answer. No fooling with screen current necessary. I like this because it is considerably safer than using the shunt method with the meter set for amps, but it really does the same thing, just indirectly.

Edited on 1/10/23 to correct an error, changed "secondary" to "primary."
Last edited by Phil_S on Tue Jan 10, 2023 3:04 pm, edited 1 time in total.
tubeswell
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Re: 1 ohm bias resistors. Needed?

Post by tubeswell »

1 Ohm won't make any difference to the bias (or to the operation) of the tube. Electrically, 1 Ohm is akin to having an ordinary piece of wire instead, except it's a known measurable resistance. If you imagine cathode voltage as a 500-sheet reem of photocopier paper, (for a cathode biased 6V6 for example) then using 1R is like putting 1 sheet of paper between the cathode and the ground return. What's important if you want accuracy, is to get 1% tolerance resistors in there.

In terms of the resistor power rating, 1R hardly dissipates any power in this application, because the current and voltage drop are both in the 'milli' range, and multiplying these together keep you well within the milliwatt power range e.g. for a typical EL34 idling at 35mA across 1R, 0.035A x 0.035V = 0.00123W (i.e. 1.23 milliwatts), so even a 1/8th watt resistor (if you can even get an axial-mount one) is going to be ample.
Last edited by tubeswell on Sat Feb 10, 2018 12:16 am, edited 1 time in total.
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romberg
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Re: 1 ohm bias resistors. Needed?

Post by romberg »

pompeiisneaks wrote: Fri Feb 09, 2018 7:05 pm Also, you can and should measure the exact ohms reading, and get it the calculation right.
Measuring really low resistances is more difficult than one would first guess. Most all DMMs are not good under 1ohm. And the numbers it may display may
not reflect the accuracy it is capable of. I remember an electronics lab back in college where we built a special circuit to measure low resistances. Something like this:

http://www.robotroom.com/Measuring-Low-Resistances.html

A 1% 1 ohm resistor would be in the range: 0.99 to 1.01 ohms and still be in spec. I don't own anything that can measure 0.01 ohms right out of the box. So, I just use my DMM as a sanity check to see if the resistor is burnt out or way out of spec. If it reads anything close to 1 ohm I trust the manufacturer and the 1% tolerance they provide. I can't easily test it.

In addition, the total error in the measured idle power is just the errors of the voltage and current added in quadrature:

dP = sqrt (dV * dV + dR * dR)

So, it will not buy you anything to be more accurate than the error in the voltage measurement :).
Last edited by romberg on Sat Feb 10, 2018 12:52 am, edited 1 time in total.
tubeswell
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Re: 1 ohm bias resistors. Needed?

Post by tubeswell »

romberg wrote: Fri Feb 09, 2018 10:40 pmMeasuring really low resistances is more difficult than one would first guess. Most all DMMs are not good under 1ohm.
So what you do for 1R is measure the resistance with the meter leads connected directly together, then measure the resistance across the 1R resistor, and the 1R will usually be the difference. e.g. the leads may measure 0.7R directly connected and 1.7R across the 1R resistor
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martin manning
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Re: 1 ohm bias resistors. Needed?

Post by martin manning »

I would never trust such a measurement.
R.G.
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Re: 1 ohm bias resistors. Needed?

Post by R.G. »

And you're right not to do so.
tubeswell
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Re: 1 ohm bias resistors. Needed?

Post by tubeswell »

Well its worked for me for many years. If you can't trust the 1R measurement, then use 10R bias-measuring resistors, or buy a commercial bias checker and trust that.
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RockinRocket
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Re: 1 ohm bias resistors. Needed?

Post by RockinRocket »

Well I'm not surprised :oops: you guys are way to smart for me here!

Ill just do some early pre spring cleaning.
Now where are you bias probes!
pdf64
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Re: 1 ohm bias resistors. Needed?

Post by pdf64 »

My experience is that 1/2 watt MF 1 ohm cathode resistors tend to pop pretty quick when its associated 6L6 / EL34 shorts.
Whereas in the I've come across amps where their 500mA HT fuse hangs on for dear life until the OT overheats and develops shorted windings, in the face of a similar scenario.
Of course 1 ohm cathode resistors won't blow if the tube shorts internally plate to heater etc, and obviously not a substitute for actual fuses on the PT secondaries.
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angelodp
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Re: 1 ohm bias resistors. Needed?

Post by angelodp »

So is Metal Oxide preferable for this job? I just bought a packet of of MF 1 ohm 1 watt for a build.
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martin manning
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Re: 1 ohm bias resistors. Needed?

Post by martin manning »

1W MF is fine.
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xtian
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Re: 1 ohm bias resistors. Needed?

Post by xtian »

But you do want as much accuracy as you can afford, so 1% if possible.
I build and repair tube amps. http://amps.monkeymatic.com
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angelodp
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Re: 1 ohm bias resistors. Needed?

Post by angelodp »

Yes I got it as a 1%. Thanks
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