JL Sargent Posted April 9, 2012 Share Posted April 9, 2012 Last summer with lots of good help from Craig and others here I rebuilt/upgraded an old Stromberg Carlson tube amp. Here is the thread from that project. http://forums.klipsch.com/forums/t/150359.aspx?PageIndex=1 Really have enjoyed this amp but with only one reservation. The idle noise is high. Or in other words there is audible noise with no music playing. What can I do to reduce this? I've noticed on other amps all kinds of wire twisting and such. Maybe some of that is in order? Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 9, 2012 Share Posted April 9, 2012 can you post a schematic..... Quote Link to comment Share on other sites More sharing options...
mike stehr Posted April 9, 2012 Share Posted April 9, 2012 Is it a hum or a hiss? Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 9, 2012 Author Share Posted April 9, 2012 A nice big schematic is located here: http://forums.klipsch.com/forums/storage/4/1557648/S-C_ASR-120_schem.gif I would describe the noise as a low AC related hum. Also you can see the changes I made to the amp in the thread I referenced in first post. Thanks. Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 10, 2012 Share Posted April 10, 2012 I would add 50uf caps to the 7199 275 volt power supply circuts. after the dropping resister (not on the rectifier side of the dropping resister. You can take a peek at the Dynaco circuts that use 7199's to get a sense of the location and additional resistor required. see c102A and c102b in the attached dynaco schematic. I don't see a hum pot adjustment on your power tube circuts......so you either need to get your power tubes tested for Ip to see if they have any matched pairs, or you might want to add a hum pot. The power supply caps are as big as you can put on a tube rectified amp...but you can add a choke to the power supply. One idea if you still have the original power out put trannies is to use them as chokes....they have the correct power rating and should have enough inductance. use the high voltage primary winding not the secondary speaker connections. Don't over look the obvious like testing your AC line by temporarily using a cheater plug if you have a 3 prong cord. I think the biggest bang for the buck would be to see if you can get a matched tube set in there. Also try the cheater plug on the pre-amp or source. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 10, 2012 Author Share Posted April 10, 2012 Some great suggestions Fritz. The only one I'm not sure about is the cheater plug? Is it simply grounding the amp to house ground? Quote Link to comment Share on other sites More sharing options...
JJkizak Posted April 10, 2012 Share Posted April 10, 2012 Twist together all of the filiment 6.3 volt ac lines and run them at 90 degree angles with other lines. Keep them close to the chassis and short as possible. JJK Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 10, 2012 Share Posted April 10, 2012 cheater plug turns a three prong cord into a two prong cord. make sure the ground terminal is not screwed into the wall outlet. try it on your amp if it has a 3 prong cord, then the pre-amp, then the source of the signal. If the hum is a s a result of ground loops, this will help you find it. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 10, 2012 Author Share Posted April 10, 2012 OK, got ya on the adapter. Mine are all 2 wire from the get go. JJK, I'll certainly look at twisting those wires. That would be easy enough. Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 10, 2012 Share Posted April 10, 2012 is c52 on your schematic new (replaced with a new one?) ? it's being used in an unusual way with the ground reference of heater circuit. If it is bad, it will effect the heater circut's resistive based ground reference and as a result, cause hum. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 10, 2012 Author Share Posted April 10, 2012 Yes that C52 sectoral cap was replaced with these values new: Also next post is of the new caps installed. Looks like that picture was taken before I had quite finished up though. I see some wire ends still open there. Old New 50uf to 100uf 40uf to 100uf 30uf to 50uf 15uf to 50uf Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 10, 2012 Author Share Posted April 10, 2012 Quote Link to comment Share on other sites More sharing options...
thebes Posted April 10, 2012 Share Posted April 10, 2012 1. See if you have a "before" picture and see how the builders ran the wiring and do the same. 2. "reflow" all original solder joints (just briefly heat them up remelt them with your solder gun). 3. Replace all the resistors with 1 watt metalaized, not carbon, resistors. 4. Pick up that little orange worm on the lower left side that got out the case and put him back in Uncle Milton's Orange Drop Farm. But first do all the stuff other told you cause every time I offer a technical opinion on this board, it turns out that I'm usually somewhat (mostly, all the time) wrong. Quote Link to comment Share on other sites More sharing options...
JJkizak Posted April 11, 2012 Share Posted April 11, 2012 Not that it means anything but I just don't like the way that amp is wired. NASA would junk that in a New York Minute. Almost reminds me of the "Fisher" models which did work pretty well but looked like a rats nest. JJK Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 11, 2012 Share Posted April 11, 2012 so what are the prospects of getting the power tubes tested and graded for Ip (match characterization). It may be the case out of 4 tubes, two are a match and the other two are no where near the matched pair. in that senerio, mixing tubes would cause : senerio one 1matched pair (tube A and and two unmatched (tube C and D)=hum in one channel if the matched pair is used in one channel, senerio two tube A and C in one channel and B and D in the other=hum in both channels, senerio three tube A and D in one channel and B and C in the other=hum in both channels, etc. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted April 11, 2012 Share Posted April 11, 2012 It would be helpful to find out where the hum is originating. Pull the 7199s leaving only the output stages active. If you still hear hum, it suggests that the problem is in the power supply. It's very unlikely that the hum is originating in the output stages as the push-pull operation should cancel it, and particularly since the the midpoint of the heater supply has +20 volts DC strapped to it (and be sure that R61 and R62 are measuring very closely in value.) If the hum disappears with this experiment, plug the 7199 of one channel back in and see if the hum returns. If it does, you now have a place to start looking for potential causes. If not, plug in the other 7199 and the hum should then return. Email if you want to discuss the troubleshooting process in more detail. I didn't have time to read everything posted, so apologies if I've restated anything mentioned before. Maynard Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 11, 2012 Author Share Posted April 11, 2012 OK, I just got in and tried there ideas. 1) I swapped one of the ouput tubes from each of the two channels to see if the pairings might make a difference. No change. 2) I pulled the 7199s to see if I still had the hum and it's still there without the 7199s. Power supply? Maybe reroute or shield the AC lines in the chassis? Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 11, 2012 Share Posted April 11, 2012 1) I swapped one of the ouput tubes from each of the two channels to see if the pairings might make a difference. No change. if you swap out unmatched power tubes it won't change.....matched power tubes will cancel hum....swapping around mis matched tubes will not change that they are not matched. So basically, matched tubes cancel hum better.....or..root cause it and clean up the power supply....more caps...chokes...etc. In your last thread during your rebuild...you said the amp sounded good....if now you have hum....the wires did not move around....the tubes however do decay and change over time. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 11, 2012 Author Share Posted April 11, 2012 Right, I had the amp hooked up to some AR4 speakers after the rebuild and it seemed pretty quiet with them. I'm now using the amp with Belles which are, of course, much more revealing. Mind you even with the Belles I only notice the hum when no music is being produced. Maybe I'm being too picky. I can certainly try choking with the old trannies as you suggested but if that did work I don't know how I would wedge those things in the chassis. Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 11, 2012 Share Posted April 11, 2012 Yes, that's the problem, you can only fit so much in a given case in the name of improvements. Quote Link to comment Share on other sites More sharing options...
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