1 ohm bias resistors. Needed?

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RockinRocket
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1 ohm bias resistors. Needed?

Post by RockinRocket »

How important are the 1 ohm bias resistors off pins 1 and 8 to ground on EL34??

How many mA differance will threr be?
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Re: 1 ohm bias resistors. Needed?

Post by pompeiisneaks »

The only reason to have them, is for easy bias adjustment, 1 ohm isn't going to impact the current in any significant way, and you can just adjust the bias to where you want it anyway. They make bias measurements super easy. Just make sure you buy 1% or better. Then your mV == mA current.

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Phil_S
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Re: 1 ohm bias resistors. Needed?

Post by Phil_S »

I'd add, get 1W or 2W rated. You'd probably be fine with 1/4W rated, but the higher wattage ones have thicker leads, which makes things a whole lot better if you are taking measurements on an open chassis. If you run tip jacks into the chassis, then it probably doesn't matter at all.
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Re: 1 ohm bias resistors. Needed?

Post by pompeiisneaks »

Phil_S wrote: Thu Feb 08, 2018 6:41 pm I'd add, get 1W or 2W rated. You'd probably be fine with 1/4W rated, but the higher wattage ones have thicker leads, which makes things a whole lot better if you are taking measurements on an open chassis. If you run tip jacks into the chassis, then it probably doesn't matter at all.
I have heard, though, that they sometimes work great as a 'save your ass' tool as they can fail open if they're 1/4 watt and you're cranking that much current through there, it can save tubes and OT's if they work like a fuse. They can, on the other hand, fail short sometimes which basically leaves you like they weren't there in the first place.

But yes I get that is a pretty good way to make them easier to probe, I hate the thin leads on 1/4 watters
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romberg
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Re: 1 ohm bias resistors. Needed?

Post by romberg »

RockinRocket wrote: Thu Feb 08, 2018 4:38 pm How many mA differance will threr be?
If by this you mean what is the difference between having the 1 ohm bias resistors installed or not then the answer is "Yes there is a difference. But you probably can't hear it". The one ohm resistors will cathode bias the tube by a small amount. Typically an el34 is running say 32mA of current at idle. So, there will be a 32mV drop across the 1 ohm resistor (ohm's law and all). So, the tube will be biased 32mV off from ground and the total plate voltage will also be that amount less. With typical plate voltages in the 400 to 500 V range 32mV is nothing (one tenth of one percent). The bias difference is not important as the amp will be adjustable fixed bias anyway.

However they do make biasing the amp both easier and safer. Without them you need to use some other method to measure the idle current of the tubes such as shunting the OT with a DMM. Putting 400-500 volts across a DMM made to meet a low price point by the lowest bidder is asking for trouble. It is safer for both you and your DMM to be measuring 30-40mV than 400-500 volts :).

Mike
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Re: 1 ohm bias resistors. Needed?

Post by Phil_S »

pompeiisneaks wrote: Thu Feb 08, 2018 9:06 pm
Phil_S wrote: Thu Feb 08, 2018 6:41 pm I'd add, get 1W or 2W rated. You'd probably be fine with 1/4W rated, but the higher wattage ones have thicker leads, which makes things a whole lot better if you are taking measurements on an open chassis. If you run tip jacks into the chassis, then it probably doesn't matter at all.
I have heard, though, that they sometimes work great as a 'save your ass' tool as they can fail open if they're 1/4 watt and you're cranking that much current through there, it can save tubes and OT's if they work like a fuse. They can, on the other hand, fail short sometimes which basically leaves you like they weren't there in the first place.

But yes I get that is a pretty good way to make them easier to probe, I hate the thin leads on 1/4 watters
I would be very hesitant to suggest that you might depend on a resistor as a fuse. I'm a big believer in using the thing that was invented to do the job. Use a fuse if you think that there's real danger of drawing too much current. I'd suggest a B+ fuse on the PT CT, 500mA fast blow should do the job.
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Re: 1 ohm bias resistors. Needed?

Post by RockinRocket »

I think romburg might have answered some of my question (that I poorly asked).

I want to bias my amp but I do not have a 1 ohm resistors installed to ground from pin 1/8.

The measurement I read now in mA is not anywhere acceptable to bias my amp??
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Re: 1 ohm bias resistors. Needed?

Post by romberg »

RockinRocket wrote: Fri Feb 09, 2018 2:38 am I think romburg might have answered some of my question (that I poorly asked).

I want to bias my amp but I do not have a 1 ohm resistors installed to ground from pin 1/8.

The measurement I read now in mA is not anywhere acceptable to bias my amp??
How/where exactly did you get this current measurement? You don't need to read the idle current by using the 1 ohm resistor. It is just the easiest. In the end you just need to know the amount of current flowing from the plate to ground. There are several ways to measure this.

For example you could disconnect the connection between 1/8 and ground and replace it with your DMM. This would directly measure the current flowing in one tube. Replace the connection with your DMM. Measure/set the bias and then put the original connection back. Since there are two (or more) tubes you may need to repeat this procedure (alot).

Some companies make things sometimes called bias probes. They are an adapter that plugs into the power tube sockets which temporarily adds a 1 ohm resistor to measure the current while biasing an amp.

Or there is the OT shunt method (search how to safely do this). The method you used may also find the correct current to use for a bias calculation. But it is not clear exactly what you have measured.

Mike

EDIT: This page describes several methods to measure the bias current: http://www.aikenamps.com/the-last-word-on-biasing

In the end, I'd get a handful of 1 ohm resistors. They are not expensive or difficult to install.
Last edited by romberg on Fri Feb 09, 2018 4:58 am, edited 1 time in total.
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Re: 1 ohm bias resistors. Needed?

Post by pompeiisneaks »

Phil_S wrote: Fri Feb 09, 2018 1:23 am
pompeiisneaks wrote: Thu Feb 08, 2018 9:06 pm
Phil_S wrote: Thu Feb 08, 2018 6:41 pm I'd add, get 1W or 2W rated. You'd probably be fine with 1/4W rated, but the higher wattage ones have thicker leads, which makes things a whole lot better if you are taking measurements on an open chassis. If you run tip jacks into the chassis, then it probably doesn't matter at all.
I have heard, though, that they sometimes work great as a 'save your ass' tool as they can fail open if they're 1/4 watt and you're cranking that much current through there, it can save tubes and OT's if they work like a fuse. They can, on the other hand, fail short sometimes which basically leaves you like they weren't there in the first place.

But yes I get that is a pretty good way to make them easier to probe, I hate the thin leads on 1/4 watters
I would be very hesitant to suggest that you might depend on a resistor as a fuse. I'm a big believer in using the thing that was invented to do the job. Use a fuse if you think that there's real danger of drawing too much current. I'd suggest a B+ fuse on the PT CT, 500mA fast blow should do the job.
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.

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Re: 1 ohm bias resistors. Needed?

Post by R.G. »

I am in the camp that says that trying to make a resistor be a fuse is not a good idea. However, knowing the current and deciding that some catastrophe is happening is a good idea. Back in the 1990s, I put together a prototype of what I called the Tube Saver. It used some opamps to watch the current in tube anodes, and compare that to some reference. When the reference is exceeded, it turns off the current to all the output tubes until it's reset. Worked fine. I used a big MOSFET as a switch to turn off the high voltage to the output stage.

Later, in the early 2000s, I designed a biasing circuit that went into the Workhorse line of amps. It used the 1-ohm resistor for sensing bias, but included a comparator circuit that sensed current and turned on a green LED when it was at the desired current, a red LED for too much current, and turned both off if the bias was too cold.This had the advantage of letting us apply the reference voltage for "just right" by a switch, so we could include target currents for different tubes. We used a four-position switch for 6L6, a hotter 6L6, EL34, and hotter EL34. Flip the switch to the target bias, then turn the bias knob for the tube til you get a green light. Biasing became something that could be done in literally 10-15 seconds. You could replace your output tubes between sets and go right back on stage.

A few years ago, I generalized that. I did a microcontroller that watched the voltage on 1-ohm cathode resistors for up to six tubes individually. The controller would run red/green/blue LEDs and let you individually bias tubes, and add to this the shutdown on disaster-too-high currents.

Knowing what currents are flowing lets you use some circuits that are these days very simple and easy to do. This can make biasing in general easy, and shut things down if the reactor needs scrammed.
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erwin_ve
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Re: 1 ohm bias resistors. Needed?

Post by erwin_ve »

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
RockinRocket
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Re: 1 ohm bias resistors. Needed?

Post by RockinRocket »

Maybe my question isn't making very much sense :lol:

I 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.


With my MM grounded to chasis and set to mA. I will get a mA reading here, yes, at pins 1+8 at the ground lug.
Is this mA reading anywhere near the measurement I would get if I had a 1 Ohm resistor there instead??
Does this make any sence??
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Re: 1 ohm bias resistors. Needed?

Post by martin manning »

RockinRocket wrote: Fri Feb 09, 2018 5:08 pmIs this mA reading anywhere near the measurement I would get if I had a 1 Ohm resistor there instead??
No, if I understand you correctly you would be measuring the voltage developed over a short length of wire, of some unknown resistance. In your situation I would measure the DC resistance of each side of the OT primary (plate to CT), and use that resistance to calculate idle current, using the voltage on each side (plate to CT) with the amp powered up. I = V/R.
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Re: 1 ohm bias resistors. Needed?

Post by pompeiisneaks »

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

Also, you can and should measure the exact ohms reading, and get it the calculation right.

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Re: 1 ohm bias resistors. Needed?

Post by pompeiisneaks »

martin manning wrote: Fri Feb 09, 2018 5:31 pm
RockinRocket wrote: Fri Feb 09, 2018 5:08 pmIs this mA reading anywhere near the measurement I would get if I had a 1 Ohm resistor there instead??
No, if I understand you correctly you would be measuring the voltage developed over a short length of wire, of some unknown resistance. In your situation I would measure the DC resistance of each side of the OT primary (plate to CT), and use that resistance to calculate idle current, using the voltage on each side (plate to CT) with the amp powered up. I = V/R.
Yup this exactly. This is the way to get the most accurate result, and with most good meters having a very high impedance DC Volts reading, (10M in most cases) you're not going to impact the actual operation, and get a great reading.

~Phil
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