NOSValves Posted August 5, 2003 Share Posted August 5, 2003 I'm sure that this will do nothing but upset Lee but do what you see fit. You had absolutely no part in changing the Site you were the only person against it is what Lee told me ! He said your world were. " Its fine just like it is " Quote Link to comment Share on other sites More sharing options...
mike stehr Posted August 5, 2003 Share Posted August 5, 2003 Okay, so you hook up the milliammeter to measure current in parallel, which shunts the cathode resistor. How could you tell if the resistor value has drifted? I would geuss that it would cause problems with proper biasing adjustment. Wouldn't one still have to check the resistor value for tolerance? Wouldn't one want to check the resistance values on the cathode resistors, before taking bias measurement and setting bias? Maybe in the circumstance that the unit is one's own and the owner knew and was confident with the cathode resistors values. Hell, I don't know.... Quote Link to comment Share on other sites More sharing options...
mace Posted August 5, 2003 Share Posted August 5, 2003 At the risk of butting in on this thread.... I'll ask (and no, I haven't read all 3 pages of this beastly thread) about setting AC bias on my Scott 299A rev2: 1) What type of 8 ohm, 40 W resistor should I buy and where should I get it so I can set AC bias per Scott service manual (manual says don't use wire wound resitors because of high inductance and that's all they had at rat shack). 2) Can I just use a CD with a 1 kHz sin wave (made on my computer) as the source or do I really need a variable amplitude source (audio signal generator). Thanks in advance, Mace Edit Note: I skimmed through some of the posts in this thread. Ya know, I sometimes think Craig and Ryan are the same guy, just putting up post after post having a conversation with himself. I mean c'mon... they disagree on everything... absolutely *every* thing. It's truly astounding. On the other hand, there is a certain blessing in each having a nemesis here... it keeps them both on their toes! Quote Link to comment Share on other sites More sharing options...
Deang Posted August 6, 2003 Share Posted August 6, 2003 So, if you measure the resistance of the resistor, and use the simple calculation -- you can use the VOM you already have. You also said the newer meters allow one to do this. Correct me if I'm wrong here -- but isn't this what Craig said from the very beginning? Quote Link to comment Share on other sites More sharing options...
Jimbo Bob Posted August 6, 2003 Share Posted August 6, 2003 ---------------- Correct, if you know the actual resistance of *each* resistor, you can then use Ohm's Law to determine the voltage drop. Using vintage meters with a lower input impedance will yield decent results, using a modern meter with a much higher input impedance will yield more accurate results. This discussion was about which method is the most accurate, measuring cathode (as per Scott spec) or calculating voltage drop. Either method will work, but most technicians back then wouldn't go through the trouble of measuring the actual resistance of the resistors, therefore, the cathode current method was a better choice, since the resistance had no affect on the current reading. The correct meter (non-switchable milliammeter) is not expensive, and can be purchased for $20 or less from any major distributor, and will produce the most accurate bias adjustments. Voltmeters do have a tolerance range, and whatever the tolerance may be, that will also be the tolerance of your bias adjustment, plus the resistor tolerance. The non-switchable milliammeter has a tolerance rating of 1/10 of a percent (that's close enough for me). The tolerance rating of your voltmeter can be found in the owners manual for the meter. The tolerance rating of the resistors can be viewed by directly looking at the resistors. A silver band means 10% tolerance, gold band means 5% tolerance. Best Regards, Ryan ---------------- Dear Mr. Expert! You are surely the king when it comes to running around in circle. Read again Mdeneen's posts please and try to think for yourself instead of pulling out again this "H H Scott said..." over and over again. An engineer like you is suppose to think, not to worship. Quote Link to comment Share on other sites More sharing options...
Jimbo Bob Posted August 6, 2003 Share Posted August 6, 2003 Dear Expert, Even your magical non-switcheable ammeter have a resistance, would it be in the coil of the d'Arsonval meter itself. The fact that you don't see this is "food for thoughts" to say the least. The multimeters of today render useless your method from "back then". Measuring the voltage drop will be as much accurate. Why can't you see it!!! Sometimes reading your posts I feel like you have some knowledge but when you come up with ridiculous stuff like your biasing accuracy measurement I really wonder what cross your mind. Quote Link to comment Share on other sites More sharing options...
Deang Posted August 6, 2003 Share Posted August 6, 2003 Quote Link to comment Share on other sites More sharing options...
Jimbo Bob Posted August 6, 2003 Share Posted August 6, 2003 This might interest you. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/ammet.html#c1 I'd be really surprised that even on your non-switchable ammeter there wouldn't be any shunting resistor (would it be only for protection purpose). Anyway... Have fun with your quest for the most accurate way way to bias a Scott. Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted August 8, 2003 Author Share Posted August 8, 2003 I can't believe I just spent 20 minutes typing and the freaking server sent an error message when I sent it, loosing all info! That SUCKS!!! Anyway, here's how I see the math of this. Let's assume a resistor value of 10 Ohms and a Voltage of .23. Using Ohm's law and a meter with a .3 Ohm internal resistance, here is what you get: Through the resistor: (E/R) .23/10 = .023 A Through the meter: .23/.3 = .767A Not quite a shunt, but with the meter getting 97% of the current, it's close. Let's see what happens with a meter with a 1.5 Ohm int res: The resistor still gets the same amount of current, .23/10 = .023A while this meter gets .23/1.5 = .153A ...a bit fuirther away from being a shunt at 85% of the current. the moral of the story, know your meter! Unfortunately I lost my documentation for my multi meter, so the safest bet for me is to use Ohm's law, especially considering that I just bought a buch of 10 Ohm resistors and picked out the ones that measured exactly (as possible) 10 Ohms. Both Craig and Ryan have been of tremendous help to me in my quest for better audio. I have exchanged many emails with Craig and Ryan, each explaining things to me as well as a few hours of phone conversations with Craig making me feel like I know him pretty well! Ryan has also corrected some wiring errors on my amp, sent me resistor and helped me select which resistors to replace when I wanted to replace possible out of tolerance resistors. He also sent me a couple 6678s to try in place of my 6U8s, all gratis! So Ryan and Craig, if you're ever out this way, you've got dinner and beer on me! Ryan can explain circuits well, but at times gives information that has not been thought through, such as the bias info here. Save for the name calling and insults, it was a very interesting thread and has made me look at parallel resistance and Ohm's Law again. It was interesting...thanks all! Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted August 11, 2003 Author Share Posted August 11, 2003 I tore that out when I did the bias mod...it utilizes the rca jacks and the switch...but we have to assume, if we use the biult in method, that the cap and resistors in that circuit did not drif, change, die, etc... Quote Link to comment Share on other sites More sharing options...
Marvel Posted August 11, 2003 Share Posted August 11, 2003 I think you all have a biased opinion of what is the best way! Ga'day mate, Marvel Quote Link to comment Share on other sites More sharing options...
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