km5gn Posted February 21, 2007 Share Posted February 21, 2007 I recently purchased a Scott LK-72-B from a fellow that I think knew what he was doing when he updated it with some new caps. His ebay description of it included the following.. "1 ohm 1/4 watt current sensing output tube resistors installed to makebias and balance extremely simple. I set these at 30-35 ma.per tube " Well, I own a multimeter and know how to use it, but that's about as sophisticated as I am when it comes to electronics, It sounds awesome to me driving my Klipshorns, but I'm clueless as to how to check or set the bias of the tubes and how often this needs to be done. I'm pretty comfortable that it came to me in good shape. Can anyone recommend a source for how I keep it in good tip-top shape? TIA, Steve (Loving the scott and my 'new' 78 K'horns) Quote Link to comment Share on other sites More sharing options...
Speedball Posted February 22, 2007 Share Posted February 22, 2007 I'm surprised no one jumped in to answer this.........sounds like an easy tube question for everyone around here except me, I'm tubeless. Maybe now someone will see your post..........goodluck and enjoy your klipsch and tubes. Quote Link to comment Share on other sites More sharing options...
jcmusic Posted February 22, 2007 Share Posted February 22, 2007 You need to check your bias about once a month, unless there is a problem. Craig at NOS Valves is the guy you want to check with about the proper settings. I have a Scott 299 and set the bias between 250-275 DCV, I prefer to run mine a little on the low side (longer tube life) providing you don't play it at very loud volumes. Jay Quote Link to comment Share on other sites More sharing options...
Guest " " Posted February 22, 2007 Share Posted February 22, 2007 Here is a link that claims a step by step proceedure. http://www.audioasylum.com/forums/vintage/messages/120222.html All the tube equipment I ever owned had meters built in, and very simple to follow proccedures in the owners manuals. As far as the frequency adjustment......definitely are a lot of factors....for example dialy voltage fluctuations could change the measured results....if you have a voltage line regulator....the daily fuctuations won't be an issue.....point being....don't adjust the bias when the daily voltage of your AC is at it's lowest point in the day. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 22, 2007 Share Posted February 22, 2007 I recently purchased a Scott LK-72-B from a fellow that I think knew what he was doing when he updated it with some new caps. His ebay description of it included the following.. "1 ohm 1/4 watt current sensing output tube resistors installed to make bias and balance extremely simple. I set these at 30-35 ma.per tube " Well, I own a multimeter and know how to use it, but that's about as sophisticated as I am when it comes to electronics, It sounds awesome to me driving my Klipshorns, but I'm clueless as to how to check or set the bias of the tubes and how often this needs to be done. I'm pretty comfortable that it came to me in good shape. Can anyone recommend a source for how I keep it in good tip-top shape? TIA, Steve (Loving the scott and my 'new' 78 K'horns) 1 ohm ? That is awful small value resistor which would make the measurements on the cathodes very inaccurate with anything but a high quality prefessional meter. The norm is to install a 10 ohm resistor which would translate into setting each cathode the .30 to .35 volts DC. Does this amp have a test point for each of these cathode resistors above the chassis? Your going to need to confirm the 1 ohm value for me (I think it maybe a typo) and then I can help you bias the amp. Some above and under chassis photo's would help especially if you don't have 4 test points above the chassis. Craig Quote Link to comment Share on other sites More sharing options...
km5gn Posted February 22, 2007 Author Share Posted February 22, 2007 I went back and checked, and it ends up that what he said about the resistor and the settings were for another amp, not this one. It does have test points on the top of the chassis, I don't remember how many or how they were labeled. I'll check when I get home and maybe post a picture. I'll also take a look at the link listed above. Thanks for the help! Steve Quote Link to comment Share on other sites More sharing options...
km5gn Posted February 22, 2007 Author Share Posted February 22, 2007 Hopefully I've done this correctly - my first time to try an upload... I've attached a picture of the bias adjustment section of the top of the amp. Sorry about the flash reflection, but it's pretty dark in my listening room, and this is the best I could get without removing it from the system. Much of the lettering on the case top is pretty faint. There are two phono plugs marked Bias Test Points, one marked R, a Bias Adj. Switch, and at the bottom, 4 pots. marked P5 - P8 . Those at the top are labeled Balance, can't read the label at the bottom, and the ones on the left of this shot are marked R. Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted February 22, 2007 Moderators Share Posted February 22, 2007 Hopefully I've done this correctly - my first time to try an upload... I've attached a picture of the bias adjustment section of the top of the amp. Sorry about the flash reflection, but it's pretty dark in my listening room, and this is the best I could get without removing it from the system. Much of the lettering on the case top is pretty faint. There are two phono plugs marked Bias Test Points, one marked R, a Bias Adj. Switch, and at the bottom, 4 pots. marked P5 - P8 . Those at the top are labeled Balance, can't read the label at the bottom, and the ones on the left of this shot are marked R. The photo came through fine and as soon as Craig from NosValves sees it you are going to be in great shape. Travis Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 23, 2007 Share Posted February 23, 2007 I need another picture of the area where the 4 7591 output tubes are located. Hopefully you will find two small pieces of wire sticking up and insulated from the chassis. These are cathode test points for each channel. If they are not present then you will have to take the amp out of your system and remove the bottom cover to properly bias it. Craig Quote Link to comment Share on other sites More sharing options...
km5gn Posted February 23, 2007 Author Share Posted February 23, 2007 OK Craig, I've attached two more pictures, but I don't see anything like you're describing. Odd, since it seems obvious that the intent is that the measurements could be made from the top. There ar some little holes below the pots marked p5 - P8, with a wire showing... Thanks for your help, Steve Quote Link to comment Share on other sites More sharing options...
km5gn Posted February 23, 2007 Author Share Posted February 23, 2007 Oops, Gues I can only do one at a time. Here's the other Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 24, 2007 Share Posted February 24, 2007 Sorry to say your amp has to be biased from under the chassis. Scott gave some funky way of doing it with those RCA Jacks and switch but its highly inaccurate and confusing to perform. Not sure if you would be comfortable messing with it under the chassis with 450 Volts DC present? I'm curious if anything has been done to the amp under the chassis. Craig Quote Link to comment Share on other sites More sharing options...
km5gn Posted February 24, 2007 Author Share Posted February 24, 2007 The fellow I bought it from said " The bias supply rectifier has been replaced with modern equivalent as well as supply cap for safety". Quote Link to comment Share on other sites More sharing options...
33klfan Posted February 25, 2007 Share Posted February 25, 2007 My 233 is .7V DC or 700mv. Quote Link to comment Share on other sites More sharing options...
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