tube fanatic Posted April 11, 2012 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? In your picture of the underside it looks like you installed a toggle switch at the lower left corner to serve as the on/off switch. Leaving the switch wired in, take it out from its location and fold it back so that the wiring is nowhere near the output tube sockets (fold it over the side of the chassis) and see if the hum persists (with the 7199s removed also.) Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 11, 2012 Author Share Posted April 11, 2012 OK, with the 7199s out and the toggle switch pulled out and away the amp is 90% better. So I put the 7199s back in and tried it with the toggle switch still out and away and most of the noise is back? Note: My "on" indicator lamp is 120v too. Maybe I should go with a DC lamp and move the switch to the back reducing the AC wires all together? Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 11, 2012 Share Posted April 11, 2012 OK, with the 7199s out and the toggle switch pulled out and away the amp is 90% better. So I put the 7199s back in and tried it with the toggle switch still out and away and most of the noise is back? Note: My "on" indicator lamp is 120v too. Maybe I should go with a DC lamp and move the switch to the back reducing the AC wires all together? don't forget about adding caps to the 7199 275V circuts on the load side of the resistors (not on the rectifier side) Quote Link to comment Share on other sites More sharing options...
Greg Oshiro Posted April 11, 2012 Share Posted April 11, 2012 I'm with tube fanatic and JJKIZAK on this one. I think the physical location of the wiring ("lead dress") is at the root of the problem. Does the hum occur if nothing is connected to the amp inputs? How about if RCA shorting plugs are connected to the inputs? Once it is certain that the amp (not the interconnection of the amp with other equipment) is humming, try this: Connect amp to speakers. Connect shorting plugs to amp inputs. With amp upside down, move wires around with a non-conductive tool (plastic ballpoint pen barrel?) and see if you can change the loudness of the hum. The lead dress as it stands now does not appear to be optimized for minimum hum, so it could be coming from any number of sources and pickups. The power switch wiring and the filament wiring, as mentioned by others are likely culprits. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted April 11, 2012 Share Posted April 11, 2012 OK, with the 7199s out and the toggle switch pulled out and away the amp is 90% better. So I put the 7199s back in and tried it with the toggle switch still out and away and most of the noise is back? Note: My "on" indicator lamp is 120v too. Maybe I should go with a DC lamp and move the switch to the back reducing the AC wires all together? Try just a plain old switch with no indicator light and see what happens. Keep the switch and AC lines as far away as possible from any signal parts of the circuit. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 11, 2012 Author Share Posted April 11, 2012 Some great suggestions guys! I will be able to get after it this weekend. I'll go by the electronic supply and pick up those caps and I'll reduce my AC wiring to the bare minimum and see what we have then. Thanks a lot! Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 11, 2012 Share Posted April 11, 2012 another observation....you have AC heaters....you can only take that so far.....DC heaters are much quieter......but alot of folks think, myself included...that you get better dynamic's with AC heaters. You can temporarily by pass the AC heater supply and use a 6V lantern battery instead. If that meets your noise goal, then you have to convert to DC heaters. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted April 12, 2012 Share Posted April 12, 2012 Some great suggestions guys! I will be able to get after it this weekend. I'll go by the electronic supply and pick up those caps and I'll reduce my AC wiring to the bare minimum and see what we have then. Thanks a lot! The coaxial leads from the input jacks to the 7199 (g1) grids could be configured so they don't snake around the JJ capacitor and run in parallel with a output transformer primary lead. The line signal input wires need to be as far away from AC and HV DC as much as possible. It may or may not help with noise. The best approach with line signal leads is to mount the input jacks as close to the tube's input (g1) grid as possible. However, it's not the most esoteric and practrical procedure over having the input jacks at the back of the amplifier. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 13, 2012 Share Posted April 13, 2012 JL, Give me a call the amount of information I would have to read to get up too date on this would take me hours... 810-287-6135 Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 21, 2012 Share Posted April 21, 2012 so how did this story end? Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 21, 2012 Author Share Posted April 21, 2012 After trying several of the suggestions and with some troubleshooting help from Craig (phone call and several emails) the hum was not in the wiring dress or circuit per sey. The problem ended up being that I hadn't put a bottom on the amp. I was listening to the amp with bottom open and stacked on my tube pre. So the pre amp was picking up AC noise from amp. I had some thin stainless sheet laying around, so I fashioned a bottom for the amp, screwed it to the chassis and presto. It's so much quieter now. I can only hear it idle with my ear against the squawker at this point. I needed a "ground shield" between the pre and the amp. Last summer when Craig helped me rebuild this amp I was using it with a SS NAD preamp. In this setup the tube amp's excessive gain seemed ok. But with the tube pre amp I had no volume control. About 1/4 turn would be very loud. Craig suggested we add more feedback to the circuit by reducing the feedback resistance. I tried several different resistors until I had a good volume control and I couldn't be happier. We have been listening to the system every day since. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 22, 2012 Share Posted April 22, 2012 Jeff, Don't forget to contact me in about 2 weeks and I bet we can bring the sound up another notch... fine tuning is a good thing and fun to boot when the results bring a smile to your ears. Quote Link to comment Share on other sites More sharing options...
Guest " " Posted April 22, 2012 Share Posted April 22, 2012 thats a happy ending.... Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted April 22, 2012 Author Share Posted April 22, 2012 Will do Craig, got those caps ordered. Thanks to everybody. Lot's of generous help around here. [Y] Quote Link to comment Share on other sites More sharing options...
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