synthfreek Posted March 28, 2004 Share Posted March 28, 2004 I am getting a static sound out of my left speaker. It comes and goes but lately has been coming more than going. When I tap lightly on the left 7199 it sometimes goes away. It sounds like weak FM reception static. Do I need new 7199's? Should I replace both? It happens even when I switch the 2 out so I think it's more of a connection issue. Should I buy some contact cleaner? Canned air? Should I go SET? Quote Link to comment Share on other sites More sharing options...
NOSValves Posted March 28, 2004 Share Posted March 28, 2004 Usually this would indicate just what you suspect. But also a good cleaning of the tube pins and socket could help. The other option could be a cold solder joint. Craig Quote Link to comment Share on other sites More sharing options...
synthfreek Posted March 28, 2004 Author Share Posted March 28, 2004 So this is what I did. First off I swapped the 7199's again because I got this feeling that I may have mixed them up and installed them right back into the same sockets when I tried troubleshooting the other day. Voila...the static is now in the opposite speaker. I then cleaned the pins on that tube with some light sandpaper(there was a dark colored buildup on the pins) and reinstalled. Dead silent so I decided to do the same to the left tube's pins. This time when I reinstalled I started getting that sound from the left speaker again???!!! I looked at a site that said I should try to adgust the bias slightly because this was sometimes the cause of the prob. I got my meter out and got like a 1.22 from the left and 2.5 or so on the right. Even with the bias knob all the way down on the right the closest i could get to 1.56 is 1.76 on both sides. The noise is now only audible when I turn my preamp past 25% or so. My next step will be to get up under there and resolder any spots that look sketchy. 7199's aren't cheap are they! As a last resort I'll by a set. Any recommends? I want the best. Quote Link to comment Share on other sites More sharing options...
DRBILL Posted March 28, 2004 Share Posted March 28, 2004 I really am uncomfortable with the readings you are getting from the bias set. Is the selenium stack still in use in your unit or has it been replaced with a diode? What about the 50mfd electrolytics? You might want to clean the bias pot, the same way you clean volume or tone controls --spray it with cleaner/lube and work it back and forth several times. I have even had to replace them when the bias would not hold steady. Yes, good 7199's can be pricy. Do you have a friend with a tube tester that can check them for shorts/leaks/gas, etc. I'm a great believer in the old "thump" test. Might as well test the EL-34's at the same time. I have seen some really bad ones lately with the cathode coating falling like dandruff after only 6 months duty. Long distance trouble-shooting is tough! DR BILL Quote Link to comment Share on other sites More sharing options...
NOSValves Posted March 28, 2004 Share Posted March 28, 2004 Yup that amp needs some help for sure ! If you can't get a descent range above and below the designed Bias setteng the something is wrong that will require work beyond simple tube swapping and cleaning. Sounds to me like it needs a overhaul. Craig Quote Link to comment Share on other sites More sharing options...
dlb945 Posted March 29, 2004 Share Posted March 29, 2004 The rebuild of an ST-70 is today, very straightforward. If you can use a soldering gun, the recommeded steps follow with approx prices. 1) Remove all parts from chassis - transformers, sockets, boards etc. 2) Get local auto replate shop to recondition chassis - $130.00 3) Order new driver board kit with parts - $100.00 4) Order new power supply board with parts - $60.00 5) Order new sockets, silver coated wire, pots etc. - $160.00 6) Order/buy new S.S. Pan Head screws/nuts/washers/Lwashers - $40.00 7) Remove end-bells and refinish - $5.00 8) Using ST-70 kit guide and board guides, mount and solder all in place. 9) Purchase and install all NEW tubes - $90.00 to 200.00 Checkout an example at >>> http://home.bellsouth.net/p/PWP-dlblabs Quote Link to comment Share on other sites More sharing options...
DRBILL Posted March 29, 2004 Share Posted March 29, 2004 Well --- It might not be that drastic. Kill the snake closest to you. He is not trying to create a museum piece. DR BILL Quote Link to comment Share on other sites More sharing options...
Thoriated_Tiger Posted March 30, 2004 Share Posted March 30, 2004 That you can't bring the biaset voltage down to 1.5 is very troublesome--I think you have a bias rectifier about to go out on you -- and if it does, it may take the tubes and transformers with it. There are numerous kits out there to freshen and / or upgrade this baby. I chose to use joe curcio's pc3-R board, it's a dupe of the original ckt but made out of modern fiberglass instead of ye olde, spongy phenolic. The circuit remains the same. This board also replaces the selenium bias rectifier with diodes. Main rectification is up to the user, it can do both SS and tube -- I chose to stick to the 5AR4 rectifier. I had noises in mine too -- popping, scrapes, etc. SOme of it was dodgy 7199s, most of it was just old capacitors. The parts dyna used were of rather low quality. That they lived 30, 40 years is impressive. Mine's now rock-solid reliable, the bias changes so slow I now check it once every 3 months. One other change I did to mine was use sds labs' filter cap board. Gave the little beastie a lot more substance. Quote Link to comment Share on other sites More sharing options...
DRBILL Posted March 30, 2004 Share Posted March 30, 2004 There may be a little confusion in Tiger's suggestions as to which board does what. The cap board usually has the bias fix, not the driver board. "If it ain't broke don't fix it." is still reliable advice. I wish I had followed it more often. DR BILL Quote Link to comment Share on other sites More sharing options...
Thoriated_Tiger Posted April 1, 2004 Share Posted April 1, 2004 Quite correct you are. There's one thing, circuit-wise, changed by the Curcio board, iirc (I built it 2 years ago, memory fades ;o) There's a pair of 470k ohm resistors mounted on the chassis of the amp itself -- these are removed, and are now on the board. That's about the only change, and it still conforms to the original ckt. The bias rect is done on the cap board, as the good doctor mentioned. Both boards are of extremely high quality. You can't go wrong with either. Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted April 1, 2004 Share Posted April 1, 2004 I had a pair of dynaco MKIV monoblocks that are basically like an ST70 and I had an intermitent Hum and hiss problem in one that would get better and worse when I would try swapping the 7199 out. I eventually found a Bad ground connection where dynaco grounded a circuit board trace to the chassis. these boards used solder over the traces and over the years the solder deteriorated under the screw holding the board to the chassis. Moving the 7199 just caused the bad connection to clear up temporarily. Like others have said I believe You should check out the Bias Supply Circuit since you can't set a proper bias. Mike Quote Link to comment Share on other sites More sharing options...
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