youngjj Posted June 17, 2012 Share Posted June 17, 2012 My LK-72 recently got a new output tube, new 7591s and four tested Fisher Telefunkens. It was sounding great. I turned it on tonight and with the volume at 0, as soon as it warmed up, it sounded like something from the Exorcist. It was a fairly loud electronic growl. The only thing I took time to do was try switching the input selector, but that did nothing. Craig rebuilt the amp a couple of years ago. I know audio in general pretty well, but have very limited technical electronics knowledge. This development is out of the blue. Any ideas? Quote Link to comment Share on other sites More sharing options...
Guest " " Posted June 17, 2012 Share Posted June 17, 2012 was the bias checked when the tubes replaced? Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted June 18, 2012 Share Posted June 18, 2012 One or both channels? Any change in the loudness or tone of the "growl" when you changed the input selector? A number of possibilities come to mind; however, you mentioned having limited technical knowledge so going through a "differential diagnosis" may be of dubious value. But, if you want to check a few things, send me an email and I can take you through a number of tests which will help to isolate the section of the amp which is causing the problem. Quote Link to comment Share on other sites More sharing options...
youngjj Posted June 19, 2012 Author Share Posted June 19, 2012 There is no change by moving the input selector. Only the right channel is affected. The continuous growl has stopped, except for the moment when I turn off the amp. I tried resetting the bias and that right channel is very erratic. There is no way to set the bias for that channel. The left is fine. One of the new 7591 tubes is pulsating, so I tried replacing it with one of the old ones, but nothing changed. The sound out of the right channel is very muffled, like the bias was off, but as I said, it cannot be adjusted. Quote Link to comment Share on other sites More sharing options...
mark1101 Posted June 19, 2012 Share Posted June 19, 2012 I have an LK-72. It sounds like you have an output tube issue. Try swapping output tube pairs. Swap the pair on the left with the pair on the right and see if the problem moves. I bet you failed a 7591. Also, have you tried replacing the rectifier tube? Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted June 19, 2012 Share Posted June 19, 2012 There is no change by moving the input selector. Only the right channel is affected. The continuous growl has stopped, except for the moment when I turn off the amp. I tried resetting the bias and that right channel is very erratic. There is no way to set the bias for that channel. The left is fine. One of the new 7591 tubes is pulsating, so I tried replacing it with one of the old ones, but nothing changed. The sound out of the right channel is very muffled, like the bias was off, but as I said, it cannot be adjusted. Hi JJ- From your description, it's not the power supply as that is common between the 2 channels. Since the volume control is after the front end (12AX7), the problem is between that and the output transformer. Before anything else, swap the phase inverter tube between the 2 channels (either a 6U8 or 6GH8 depending on the version that you have) and see if that cures the problem. As far as the erratic bias pot, did Craig install it or was it factory installed? Did Craig make any modifications to the amp? Are you technically inclined enough to be able to work under the chassis in a safe manner with the amp powered? The voltages under there are quite lethal and I hesitate to give you any instructions about measuring there unless you know your way around. Lastly, what kind of test equipment do you have? In any event, feel free to post again if you wish and I'll try to assist you as time permits. Btw, why haven't you just called Craig about the problem since only he knows what was done to it? All the best---- Maynard Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 21, 2012 Share Posted June 21, 2012 You have full lifetime email and phone support warranty if I rebuilt the amp even if you purchased it second hand. Email me or give me a call I'm sure we can track down the problem in short order. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted June 21, 2012 Share Posted June 21, 2012 Send me the tubes...I'll test 'em for ya...[] Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 24, 2012 Share Posted June 24, 2012 [] funny..... in this case that might do the trick I think he installed some bad tubes but I bet the bad tubes may have damaged something. I wonder where he purchased them. Jeff, I called you and left and message. Your email keeps kicking back. Quote Link to comment Share on other sites More sharing options...
youngjj Posted July 8, 2012 Author Share Posted July 8, 2012 Guys, thanks for the suggestions. I talked with Craig, did what he said and determined two new output tubes had failed. I put two of the old ones in and everything is great now. I had ordered the new output tubes when a no-name one failed. The ebay seller I bought the tubes from is replacing them, but I plan to keep what I have in place for now. The two olds ones back in are HH Scott original tubes. Quote Link to comment Share on other sites More sharing options...
Audio Flynn Posted July 13, 2012 Share Posted July 13, 2012 Guys, thanks for the suggestions. I talked with Craig, did what he said and determined two new output tubes had failed. I put two of the old ones in and everything is great now. I had ordered the new output tubes when a no-name one failed. The ebay seller I bought the tubes from is replacing them, but I plan to keep what I have in place for now. The two olds ones back in are HH Scott original tubes. That has saved me audio pain and suffering for over 10 years on more than a few occassions! Quote Link to comment Share on other sites More sharing options...
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