New build - 5E7 - Bias Question

[lowell]

Not against them bias probe ( ok, I confess, I am against them) but you are way beyond them, just solder a 1 ohm resistor from cathode to ground (instead of the wire which is there now) and measure voltage across it.
Please post results.

Bias probes don't do any better and add unreliable connections , who needs them?

-JM Fahey

I agree...mainly because I have a meter and dont need an extra unnecessary tool or expense. G1 said to measure DC volts across OT primaries and calculate current via that number and the OT primary resistance. Whats the difference between touching probes to measure DCV vs Current? I dont see one. To each his own. Ive been using the transformer shunt method for many years. I'm EXTREMELY CAREFUL and often choose to clip the leads on first. I didn't say the danger disclaimer because I knew if he was unfamiliar with this approach he would respond with HOW to do it. At which point I would make the danger clear and suggest the clip-lead method. Really if one is concerned use the 1ohm cathode method. Just be sure they are within tolerance and be sure to compensate for the reading when simply shorting probes together.

[DRH1958]

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Just be sure they are within tolerance and be sure to compensate for the reading when simply shorting probes together.

Good point about shorting leads. When "zeroing" your DMM, most meters will be at about .2 or.3 ohms, so find your "zero" and subtract that from your reading. This is critical when measuring low resistances like these 1 ohm resistors. If you measure 1.2 ohms, you really have 1 ohm. I vote for the 1 ohm resistor method. You will be measuring the lowest voltage in the amp. Safest method available.

[g1]

G1 said to measure DC volts across OT primaries and calculate current via that number and the OT primary resistance. Whats the difference between touching probes to measure DCV vs Current? I dont see one. To each his own. Ive been using the transformer shunt method for many years. I'm EXTREMELY CAREFUL and often choose to clip the leads on first. I didn't say the danger disclaimer because I knew if he was unfamiliar with this approach he would respond with HOW to do it. At which point I would make the danger clear and suggest the clip-lead method.

I understand your position, and as I said I sometimes use the method myself. As far as the difference between touching the probes to measure voltage or current? On voltage range you have something like 10 meg between the probes. On current range, you have a dead short between the probes! Any slip up can be catastrophic. And if you forget you have your meter set for current, and go to check voltage, WHAM! If you've been using the shunt method for years, I'm surprised you haven't had that rude awakening, I certainly have.
But as you say, if they need to ask, then you can mention the dangers.

Now back to the burden voltage issue: out of curiosity, what make & model of meter do you use, and on which current range for shunt bias?

[lowell]

Ive definitely had the probe touch something and had the old spark and meter-fuse blow. Not a biggie but yah teaches you to respect what your working with!

[g1]

And your meter of choice & range?

[lowell]

Always have had cheap rat shack meters

[g1]

I'm not a fan of depending on 1ohm resistors for bias measurement. It's too important. Please make sure those 1ohms are spot on. It's hard to do with a basic meter. Better yet ditch em and measure bias current via the transformer shunt method.

Ok, I brought up the burden voltage issue so I might as well try to explain it as well as I can. I found out the shunt method can be a lot less accurate than I thought it would be, dependent on your meter. The spec in question is "burden voltage" which is not available for the rat shack meters. So I'm not knocking the meter, just that it's an unknown. On current range, your meter inserts a resistor in series and measures across it to come up with a current number. So it's not the ideal dead short with think it would be. It's often a 1 ohm resistor in series with the fuse. So the spec given as "burden voltage" is the voltage drop across the resistor as the current flows through it. A typical example for many fluke meters is 1.8mV per ma. So that's a 1.8 ohm resistor in series with the circuit. Not very substantial unless it's a low voltage circuit, then it can mess things up. But it can also skew the accuracy of shunt type bias measurements.
Let's round it up and call it a 2 ohm resistor. Many OT primaries are 100ohm or more on each half, so the meter's 2ohms in parallel with the OT's 100ohms gives a fairly accurate reading, most of the current is "shunted" by the meter but a little is still flowing through the OT primary. So it's saying the idle current is a tiny bit lower than it really is. Now consider an OT with much lower primary resistance, like some Marshalls or others, where each half of the OT primary can be around 50 ohms. So the meters 2 ohm resistance is in parallel with 50ohms, and the meter is only showing about 96% of idle current. Still not bad, but we end up biased hotter than our number tells us, and we were wanting to be more accurate than 1% or 2% cathode resistors.
Now comes the real kicker, some meters have much worse burden voltage specs on their lower mA current ranges. One of my meters in the $200 range is 10mV/mA on the 200mA range. That's a 10ohm resistor. There goes the accuracy, 10ohms in parallel with 50ohms is not a great shunt. No where near the ideal "dead short", and only shows me around 84% of the actual idle current. So for that meter, I get way more accurate numbers using the 10A range instead of the 200mA range when doing shunt bias.
Anyway, not likely you have this problem, just wanted to share how the shunt method proved to be less accurate than I thought. You could experiment by actually doing a series current measurement breaking the circuit at the plate and comparing to the shunt method.
But at least I would try to measure the resistance of the meter on current range: remove the battery and measure resistance between the probes with another meter. Try on various current ranges. It may turn out to be fine, or it could be a surprise.

[lowell]

Good stuff G1. Wasn't aware of this. I need to get a REAL meter. I just don't have the budget right now. Curious....could one measure the resistance of another meter? I mean, could i use a meter to measure the resistance of another meter, which is set on ma? Wonder if that would work. The ma meter would be reading ma of the meter set to ohms and vice versa. Finally, can you suggest a solid meter for $100 ish?

[g1]

You should be able to measure the resistance with another meter. I would remove the battery of the meter set to mA just so it doesn't do anything funny to the reading. Also check what the resistance is on the various current ranges, or if it has different probe connections for the bigger Amp range.
Not sure about any particular meter, I would see what Fluke has in your price range. They seem to be very reliable and everyone swears by them.