NOSValves Posted June 28, 2003 Share Posted June 28, 2003 I think I will read this thread once a day just to keep from Flibbering !! Its also dang funny Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 29, 2003 Author Share Posted June 29, 2003 I must admit that I have suffered from a severe reluctance to open up the case and get to work, though Craig hinted some time ago that I should do a bias mod to be sure it's biased and balanced correctly, and I wanted to be sure of that myself before I resoldered connections or looked at wire routing. But now that it's done, that will be the next step. Craig, the phase reversal switch has been disconnected when I got the amp, so that's not an issue. Lynn, swapping tubes does not affect the hum and I think it is either a poor connection somewhere or induction as you have stated, I just need to sit down and trace! But I've been having too much fun listening...to vinyl. 8. Because transconductance is a variable on the tubes, grid voltage (called bias voltage) will often be different than specified for any given Plate Current. This is ok. When both tubes are at "50.0 ma" you might have one grid at -22.5V and the other at -23.5V. This is not a problem. DO NOT TRY TO ADJUST THESE TO BE EQUAL. It is the current you want equal, not the voltage. Same goes for the plate voltages. Mark, this is what best answers my original question, thanks to you it's now cristal clear! Quote Link to comment Share on other sites More sharing options...
Jimbo Bob Posted June 29, 2003 Share Posted June 29, 2003 "Comments?" Yes. Since you claim to be an engineer, you should never take anything for granted, be the source H.H. Scott or even God himself! So do this little experiment. Take your measurements with your "current across the resistor method", by measuring the current after breaking open the circuit and by measuring the voltage drop across the 10 ohm resistor and by applying Ohm's law with the actual resistor value and report back on the 3 current values measured/infered. While at the same time, try to be humble enough to learn how a ammeter works. Go back to the EXCELLENT post by Mark. Check his suggested reading it might help you a bit. Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 29, 2003 Author Share Posted June 29, 2003 I have a copy of the service bulletin that Ryan referred to and this is correct. But what conclusion can be drawn from this? I think Mark has very well established the fact that the ammeter does not act as a total shunt, and even suggested a book that details this. Those who have more than one meter can probably set one meter to measure current and the other to measure resistance and see what you get. It's probably a little safer than Ryan's previous suggestion of measuring line current. But this would bring up the question, why did the Scott engineers write these instructions? could it be that they were seeking a higher actual bias; that the reading done in this manner would produce the true desired bias? If so, why would they not have also stated what the true bias is that they were trying to achieve? Or were the VOM of the day not as accurate as todays? It is indisputable that there is a difference in both readings. (20.8 mA measuring across the resistor vs 23 mA using volt meter and Ohm's Law to calculate. I strongly suspect that if I were to break the connection and measure through the circuit it would be 23 mA), so now, what I beleive is in question, is how did the Scott people intend this to be? Incidentally, the same bulletin referrs to setting balance first by connecting a 16 ohm resistor to the output and adjusting the pots for minimum reading. Looking at the circuit, I thought the idea was to get both tubes of each push pull set balanced...for now, I have set the balance to be equal in both tubes of each chanell first, then brought up the bias to where the V drop is .23. She sounds great! Seems to be stable too. Quote Link to comment Share on other sites More sharing options...
Deang Posted June 29, 2003 Share Posted June 29, 2003 Well, you just proved you're not an engineer. No engineer could do all that in 30 minutes -- it takes them that damn long just to get the plate off the bottom of the amp. Quote Link to comment Share on other sites More sharing options...
Jimbo Bob Posted June 29, 2003 Share Posted June 29, 2003 Geez this was funny!!! The two resident Scott experts here are no engineers. Entertaining they are for sure. But I'll give Craig the real will to learn as opiniated he may be! Quote Link to comment Share on other sites More sharing options...
Flux Posted June 29, 2003 Share Posted June 29, 2003 If I were you I'd cut my losses. BTW, you have the charisma of a gym sock. Quote Link to comment Share on other sites More sharing options...
Deang Posted June 29, 2003 Share Posted June 29, 2003 What do I know? I know you've about worn out your welcome here. I know you are more interested in discrediting Craig than you are interested in directly dealing with many of the facts presented. Basically, you keep saying the same thing over and over again, in spite of the fact that it has been been shown to be incorrect. You have yet to respond to Mark Deneen, or any of the information in his posts. You seem content to stay with the "because H.H. Scott and H.H. Scott Expert said so" approach. I'm not a mental giant in the amp department, but even I could understand what Mark and Craig were saying. I read what you are saying and it doesn't really make much sense in light of the details Mark provided. As far as Scott and the 299 go -- it's a freaking push-pull amp for cryin out loud. Are you trying to say that all fixed biased push-pull are biased one way -- but the Scott 299D needs to be done this way? Here's the way I see it. If Craig is wrong than the tubes would either be biased cold, and closer to cutoff during large signal swings -- and there would be so much crossover distortion they would sound like sh!t. If they are biased too hot -- the damn tubes would only last a year. I don't see, or hear any evidence that shows any of these things are happening. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 29, 2003 Share Posted June 29, 2003 Ryan, I think one simple thing here. You refuse to learn and are to stubborn to admit your wrong no matter if Herman Hosner Scott himself rose from the dead to tell you !! Oh and I truely believe your stretching the truth on the measuremnts you took. I'm sure you would never do that Also a interesting part of the instruction about not using a VOM is that it has nothing to do with the setting of the bias voltage !! They never say what procedure to adjust the bias on the 222D in those instructions by measuring the mA or the voltage drop. The statement about not using the VOM was in reference to the balance procedure measuring the AC across the 16 Ohm load resistor. The Facts have not changed here measuring the mA can not be done across a resistor properly in any amp !! In fact I have never seen instruction printed by HH Scott that say to do this procedure the way HH SCOTT EXPERT recommends ! ALL they ever say is to set it to 44mA this in no way implies how you should arrive at 44 mA . Heck very good portion of the meters available back then didn't even have a mA feature at all !! Craig Quote Link to comment Share on other sites More sharing options...
BEC Posted June 29, 2003 Share Posted June 29, 2003 Well I guess I can chime in. My experience with vintage amps is from way back in the vintage days of the 60s. However my experience with current measurements has continued and is recent. There are two correct ways to measure current. Preferred way is to break the circuit and place the ammeter in series. No way to get fooled with this one. All current must flow through the meter. Accuracy will be the tolerance of the meter. That is the usual way we do that for current loops at nuclear power plants. Easier to back up your test results if the test is done using only one instrument and read directly without calculation. The other way is to measure voltage drop across a resistor in the circuit and with the voltage drop known and the resistance known, current is calculated by ohms law. This can be done with great accuracy if the resistor value is known. Here you must take into account the tolerance of your measurement of the resistor and add to that the tolerance for your measurement of the voltage to come up with accuracy. I can think of no case where it would be good to shunt a resistor in a circuit with an ammeter. In every case the result will be a parallel circuit with some current flow through the resistor and some through the meter. I believe in the case of the 10 ohm resistor in the Scott being shunted by .5 ohms (meter and leads) this would introduce an error in the measurement of about 5%. The main reason though not to do this is that although this resistor in the cathode circuit of the Scott may not be critical, many resistors are critical in value. If you try that trick on a resistor whose job is current limiting, you may find out the true meaning of "shock and awe". Quote Link to comment Share on other sites More sharing options...
Deang Posted June 29, 2003 Share Posted June 29, 2003 Mdeneen's long lost twin brother. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 29, 2003 Share Posted June 29, 2003 AND THE SCORE IS !!!!!! 42 for everyone 0 for Ryan !! Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 30, 2003 Author Share Posted June 30, 2003 This is kind of like Fox News Channel...we report, you decide! I've been looking all over the internet for the formulas, but it seems that with 2 resistors in parallel (which would be the case of an ammeter with say, .5 ohms resistance placed across the 10 ohm resistor) that it would indeed produce a higher tolerance level... Quote Link to comment Share on other sites More sharing options...
BEC Posted June 30, 2003 Share Posted June 30, 2003 Andy, here is a link. http://webhome.idirect.com/~jadams/electronics/parallel.htm Bob bcrites@cswnet.com Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 30, 2003 Share Posted June 30, 2003 Bec, I visited that site way back when ! He is coming along pretty good with it since I was there last. I bet his CD will be a wonderfull tool for those just starting out playing with electronics ! Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 30, 2003 Share Posted June 30, 2003 Mark, Once again absolutely stunning posts both of them and the second almost made me fall off the chair laughing thinking of you trying to reason yourself out of a very dangerous but comical situation !! I agree on absolutely every point you made if that means anything from your favorite Jedi apprentice at least I hope I am ! Craig Quote Link to comment Share on other sites More sharing options...
Parrot Posted June 30, 2003 Share Posted June 30, 2003 This thread has been an enjoyable read. And what an excellent example of the dangers of flibbering. Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 30, 2003 Author Share Posted June 30, 2003 I just made the mistake of reading this as I was eating lunch...I almost recycled it from laughter! You do have a way with words LOL! Mark, if you were to look at the service bulletin, it has two sets of instructions for setting balance and bias, one for the end user with no equipment (utilizes a circuit which through a switch and cap and resistors would cause an audible pop which would get louder or softer as you turn the bias pot. It had the other set of instructions for those with test equipment. Thanks Mark! Bob, thanks for the link...I was getting frustrated at not finding that and I knew it would be out there somewhere...I'll look at it closer tonight. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted August 3, 2003 Share Posted August 3, 2003 Ryan, Made a A$$ out of myself on this one too !! Quote Link to comment Share on other sites More sharing options...
NOSValves Posted August 4, 2003 Share Posted August 4, 2003 Hate to tell you this Ryan but it IS NOT 1958 anymore !! What you don't seem to realize is the member of this forum do noit have simpson 260 most of them use $29 radio multi meters that are inaccurate if used with your prefered and lazy method. Craig PS its pretty funny that Lee sent out a number of emails to his most trusted hobbyists and techs and they all aggreed with me except you and hence why Lee updated his bias page to make it clear the MA meters can be very far off. Quote Link to comment Share on other sites More sharing options...
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