NOSValves Posted June 27, 2003 Share Posted June 27, 2003 Ryan, You are unbelieveable when you get backed into a corner you just start making up what ever might save your posistion. Why don't you just admit your wrong then maybe some people here will begin to have some respect for you. Its really funny that for all these years that you have supposedly worked on this gear and are the "HH Scott Expert" and you don't even know how to measure the Bias properly or use a Multi Meter. The reason your method is not accurate is that some current is still flowing thru the 10 Ohm resistor because there is some resistance in the meter so when you use Ma to do this and SET the Bias to the recommended setting your biasing the amp to hot. If what you said was true then if you were to tag 2 resistors together say a 1 Ohm and a 10 Ohm it would become a 1 Ohm resistor which you know is not true. For the Meter to completely shunt the resistor it would have to be absolutely ZERO Ohms leads , meter and all !! Multi Meter 101 for Ryan Inman the HH Scott Expert This information was scanned out of "Multi Meter How to use Them " Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 27, 2003 Share Posted June 27, 2003 Andy, If I was you I would completely desregard Ryan's statements about using 5 Ohm's instead of 10 Ohms this doesn't change anything crucial the resistor is just a measuring device ! In some scott amps you shunt the resistor completely after you set the bias ! The resistor doesn't change the way the circuit works at all. It will change the negative voltage applied to Pin 2 to achieve the bias by a very small amount to achieve the same current draw but this doesn't change the sound or proper operation of your amp. It is however crucial that you set the bias and balance accurately which this Mod does a fine job at !! Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 27, 2003 Share Posted June 27, 2003 Bla Bla Bla !! The resistors in Andy's amp is exactly 10 Ohms !! Your the one that should read !! No meter is Zero Ohms that is impossible !! That is why almost all meters have a adjustment to zero the reading and compensate for the meter , leads and there inherent resistance. Wake up Ryan your wrong ! The fact that you have been in the electronics field for years and don't know these things and refuse to even openly discuss or learn is the Ironic part of all this ! Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 27, 2003 Share Posted June 27, 2003 Here we go backed into a corner and start swearing and name calling. This sh!t never ends with you !! If you truely were a Engineer I might heed your advise ! Craig Quote Link to comment Share on other sites More sharing options...
Marvel Posted June 27, 2003 Share Posted June 27, 2003 The point is, if you place the meter across the resistor, it also isn't accurate as it takes it out of its current limiting role as well. The only real way is to do the following: <> Marvel Quote Link to comment Share on other sites More sharing options...
sunnysal Posted June 27, 2003 Share Posted June 27, 2003 I really wish these discussions could be kept civil since all partcipants have obviously thought out their positions quite well and have done an excellent job in presenting here. I must admit I now remain unsure about which approach is "better" though both approaches appear to result in more or less the right results (assuming resistor accuracy, lol). as mark pointed out many vintage designers opted for the easiest method to assure fairly accurate results for layman users (I, for one, have benefited from this truely simple biasing method for my dynaco equipment). could both approaches be correct but different? cold we be plitting hairs, could one be easier and somehwta prone to minor inaccuracy and the other harder but surer. I am just asking for the simple reason that I would like to know if there is one "right" answer to this issue or two viable methods for bias setting. let's not let this look too much like the "propellor head plaza" over at AA! regards, tony Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 27, 2003 Share Posted June 27, 2003 Sunny, If you read this thread and look at the results Andy posted after his test you will see that he used very accurate resistors and the results using the Ma feature are off by a Very large margin. Maybe you should reread that. His findings mimic mine and if you desolder the resistor like Marvel posted and insert the meter in the circuit it confrims that without desoldering the voltage drop across the resistor as long as its value is reasonably close is more accurate. Ryan theory is wrong there is no questioning it. The fact that he believes the meter can somehow eliminate the resistance that is a absolute within the meter and its leads is black magic !! Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 27, 2003 Share Posted June 27, 2003 Mark, No I think your posts say's it all ! Thanks Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 27, 2003 Share Posted June 27, 2003 Mark, I agree this entire thread is silly ! But from my stand point I have to make sure the information is correct because my customers are reading it. If they use the Ma feature of there meters to bias amps I have worked on they will be biasing them to hot ! This will come back to haunt me not Ryan !! So I have to set the story straight the best way I know how. Thanks to you the Story is now complete ! Craig Quote Link to comment Share on other sites More sharing options...
sunnysal Posted June 27, 2003 Share Posted June 27, 2003 I was not a successful brain surgeon either! I am jsut rrying to understand if there can be two vaild approaches to this, I am a total layman. the thread does seem to be closing in on an answer though...the hhscott site seems to suggest both methods mentioned here to set bias work, no?...tony Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 27, 2003 Share Posted June 27, 2003 No the HH Scott site is absolutely incorrect they are just reprinted what was in the service bulletins which were sent to service departments ! These bulletins took for granted that competent techs would translate the mA to volts for the proper setting these service bulletins have been leaked to the public and hence the confusion. Sunny Mark Deneen is a engineer and Knows what he is talking about the story is over !! Craig Quote Link to comment Share on other sites More sharing options...
sunnysal Posted June 27, 2003 Share Posted June 27, 2003 thanks, tony Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 27, 2003 Share Posted June 27, 2003 Mark, Absolutley brilliant post !! Craig Quote Link to comment Share on other sites More sharing options...
sunnysal Posted June 27, 2003 Share Posted June 27, 2003 thank you for taking the time to explain that to me mark, I will print that out and save it. that is why I love this klipsch community! regards, tony Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 27, 2003 Author Share Posted June 27, 2003 Thank you all for your contributions. I have been struggling with this question for some time now and hence my motivation for doing the mod and asking the question and I'm now getting a much better idea of how this works. Just for the record, the resistors I used were NTE 2%, I believe they're metal and I wanted a closer tolerance since I was going to measure voltage. I bought a pack of 6 and 4 measured (as) exactly (as possible) 10, 2 measured 10.1, so it seems they are extremely close. (Of course I used the 4 that measured 10.0.) Mark, you have detailed how an analog ammeter cannot have zero internal resistance and it makes sense, but what about digital meters? I realize that even they have an internal circuit, but does it come closer to zero? If my actual real world results are true, it must not come close enough Ryan, I haven't had the chance to listen yet as I finished the mod late last night and am at work now, so I won't get a chance...but I don't believe the sound will change much unless I've been way off on my previous settings. I can say that the hum is still present in one channel, though it is slight. I did replace the whole network of the balancing resistors when I did my resistor changes a few months ago except for the pots. But you mentioned I can remove the balance pots? Why do that??? It seems the purpose of the mod (in addition to convenience) is to better see how each tube is biased individually, but if off, what good would it do if there is no balance pot...would that not be counter productive? As to tolerance of parts, when I hear that my tube pairs need to be biased to 44mA, it sounds accurate to me. Further that by saying they can be biased to 46mA to stay in class A a bit more, it sounds even more accurate. I'm sure you are correct when you say they can be off a bit and when we hear these figures we need to take that into account. I went through a lot of work to replace a bunch of resistors (mostly plate resistors and such) with newer more accurate ones and everyone said it's probably not worth the trouble. Having done this, I can't say I noticed an improvement in the sound. The voltages have gotten closer to being balanced from channel to channel, yet I still have my hum, so I must look for another possible cause. But this is why this is a hobby...what else would I have done with my time?? It is great that you guys with the knowledge take time to post these interesting explanations for us (like me) who know just enough about electronics to be dangerous. Thanks guys! Quote Link to comment Share on other sites More sharing options...
ben. Posted June 27, 2003 Share Posted June 27, 2003 If your amp is still humming, maybe it just doesn't know the words. (RIMSHOT.....collective groan) Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 28, 2003 Author Share Posted June 28, 2003 Mark, if it were a heater problem, it would be in both channels. All filter caps are new and it appears to be coming from the pre-amp portion. I say this because I bought a set of 12AX7s and when I plugged them in and turned her on, the hum was gone! Unfortunately, so was the signal in the same channel. One of the tubes was DOA and when plugged into the noisy channel, the noise was gone. Once switched, the noise was back. Methinks I need to look at the solder connections, but I'm open to suggestions... Quote Link to comment Share on other sites More sharing options...
lynnm Posted June 28, 2003 Share Posted June 28, 2003 In addition to your own thoughts about possible poor solder joint(s) and the issues Mark has raised check the routing of any signal lines in the amp as the humming channel may be inductively picking up hum from a transformer,inductor or the AC line itself. When comparing the two halves of the circuit ensure that any wires in the humming side are routed the same way as the wires in the non humming side. Have you tried swapping the 12AX7's from channel to channel to see if the hum follows one or more tubes ? Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 28, 2003 Share Posted June 28, 2003 Lynnm, In addition to your own thoughts about possible poor solder joint(s) and the issues Mark has raised check the routing of any signal lines in the amp as the humming channel may be inductively picking up hum from a transformer,inductor or the AC line itself. I have been advising Andy this same thing for what seems like a year ! This amp has a phase reversal switch, speakers On/Off switch, Power center channel and Headphone jack which I figured the hum was being induced by somewhere. If I remember right Andy pulled the phase inverter tube and the noise was still present which usually means its in the output section which means the 7189 tubes , transformers and all those switches and bells a whistles. Now it disapears with a Dead 12AX7 seems strange ! Craig Quote Link to comment Share on other sites More sharing options...
Deang Posted June 28, 2003 Share Posted June 28, 2003 My brain is "flibbering". Mark, surely you made up that word!! Quote Link to comment Share on other sites More sharing options...
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