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Help with hum and rustling sound on tubes4hifi ST-120

Greetings,

This is my first post, so apologies if I missed something or am not supposed to ask for help here..

I built a tubes4hifi ST-120 (tubes4hifi amplifier KITs page) 9 years ago. It's got a faint 60hz (or 120hz?) hum on both channels, makes a rustling noise, and sometimes I hear a windy sound. It's been doing this for at least 8 of the years i've owned it.

If I pull out the three driver tubes in the middle and turn it on, the above noises still occur. So that should rule out the driver tubes, right?

I have cleaned the power tubes contacts with deoxit and tightened them up.

I have swapped the locations of the four power tubes and that doesn't seem to change anything.

In the instructions for building this kit there is a list of about 20 values that I can check with a multimeter. All of those values are in the correct ranges.

Is this just a case of faulty power tubes? Or is there something else going on? Could it be more than one thing?

Let me know if there is any other information I should be providing.

Thank you very much,

Jesse
 

baudouin0

Member
2019-03-22 5:41 pm
UK
Both channels at the same time?


It might be worth re-checking/pulling/re-soldering the HT connections particularly around the valve sockets. Sometimes you can get a dry joint or one where the wire is just poking through the hole. With HT it can arc a little.
 
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baudouin0

Member
2019-03-22 5:41 pm
UK
The hum is on both channels.. The rustling sound seems to be on one channel..

OK the hum is a separate issue. Unplug the input valve and then driver valves in turn. See if it disappears. If you have a scope see if its 50Hz, 100Hz or rectifier current.

The rustling sound could be a number of things, dry joint, bias pot, valve - even output transformer. Unplug pre-amp valves to remove NF and if you have a scope see if you can see where it appears (try grids of output valves first).
 
Thanks for the help.. While I've got you, is this scope good enough for audio purposes? I'd rather not spend 400 for one of the bigger ones.. https://smile.amazon.com/SainSmart-Portable-Handheld-Digital-Oscilloscope/dp/B074QBQNB7/

I just ordered a new set of KT88 tubes, so that may provide some more troubleshooting info.

What is NF (I tried google!)? Is the input valve the middle one on the driver board? And then the driver valves are the ones on either side of the input valve on the driver board?

I have tried running it with only the KT88 power tubes installed and it still made the hum and the rustling.

I'm beginning to think I should just order a new driver board since those are only ~100.

Thank you for your help!
 

baudouin0

Member
2019-03-22 5:41 pm
UK
Don't know much about scopes maybe others can offer advise. Sorry I meant NFB (negative feedback). If you have remove the driver valves then there's not much left on the driver board to go wrong. Trouble with the rustling is it could be just anything from valves to resistors to dry joints. Have you tried tapping a few things with a pen. You say you swapped the output valves over and the rustling stayed on the same channel. Keep the new KT88s until you have found the issue just in case.

Ok if the hum is present when the driver board is missing that odd in a push-pull amp as the same currents go down both sides of the transformer. Maybe there's a mismatch in current (check voltages across cathode resistors). The HT electrolytics could be drying up.
 
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I have thoroughly tapped everywhere possible with a chopstick. I have poked all the wires and nothing seems to change anything... When I tap on the driver tubes I can hear it in the speakers, but I think that's to be expected. Here's hoping the power tubes (output tubes?) will shed some light.. If not, then it's some random bad solder joint...

I can send it in to the tubes4hifi technician for $60 an hour plus shipping.. but I'm determined to figure this thing out.. It's a pretty simple design!

Thanks again...
 

baudouin0

Member
2019-03-22 5:41 pm
UK
If the crackling does not move channel when you swap the output tubes over they are probably not the issue. It would not take that long to just freshen up the joints with a bit of new solder in all your wiring harnesses. Sorry cannot be more help - remote debugging is not the best. Anyway tubes shed more heat than light.:D
 
New power tubes (KT88) made the rustling go away, but the buzz remains. Initial startup with the new tubes did make some static. Is that normal on new tubes? Buzz is louder on one channel than the other, but definitely audible on both.

I re-soldered some iffy looking joints on the driver board.. I'm getting new driver tubes tomorrow. I suspect that will not fix anything.

At this point, rather than sending it off to Bob's technician for $60 an hour and postage each way, I am going to order a new driver board kit ($90) and take the entire thing apart and completely redo it. This was my first real kit and in retrospect it was probably a little above my skill at the time. At least now I know how to make a proper solder. Hakko on order as well.

If someone thinks I could somehow troubleshoot this with an oscilloscope, let me know.. Otherwise any advice on ferreting out hum/ground loop is appreciated.

Thanks for the help.

PS- I did de-oxit and tighten all the power tube sockets.
 
New power tubes (KT88) made the rustling go away, but the buzz remains. Initial startup with the new tubes did make some static. Is that normal on new tubes? Buzz is louder on one channel than the other, but definitely audible on both.

I re-soldered some iffy looking joints on the driver board.. I'm getting new driver tubes tomorrow. I suspect that will not fix anything.

At this point, rather than sending it off to Bob's technician for $60 an hour and postage each way, I am going to order a new driver board kit ($90) and take the entire thing apart and completely redo it. This was my first real kit and in retrospect it was probably a little above my skill at the time. At least now I know how to make a proper solder. Hakko on order as well.

If someone thinks I could somehow troubleshoot this with an oscilloscope, let me know.. Otherwise any advice on ferreting out hum/ground loop is appreciated.

Thanks for the help.

PS- I did de-oxit and tighten all the power tube sockets.
I have experienced hum ( or buzz) from driver tubes in a vta70. Replacing with new ecc82 ( 12au7) solved this. Try this before changing the board, you might save a large expence and a risky operation.
 

jfetter

Member
2012-06-25 12:57 pm
na
From the inital description sounds like its in oscillation.

Hum aside, put original tubes back in and short the inputs.
Make a simple rf detector using a diode,cap and wire loop then connect to a dc millivolt meter. ( cheap AM battery powered analog radio works good too)
Make sure no vf lamps or switching power eqpt nearby.

Simple RF Detectors


I had a led bench lamp case all sorts of grief.
 
I will say that having replaced all the tubes has solved any rustling sounds.. and after replacing the driver tubes it's quieter than it has been.. but there is still a buzz on one channel, and a much quieter buzz on the other channel. It's definitely a 60hz buzz. On the one channel I can hear it out of the tweeter and the woofer, on the other I can really only hear it out of the tweeter. Does that mean it's a 60hz hum along with another frequency hum also?

The power cord only has two poles on it (both of which go into the power transformer..) I wonder what would happen if I tried to ground the grounding lug to my house ground.

Thanks as always..
 

baudouin0

Member
2019-03-22 5:41 pm
UK
So you have a buzz rather than a hum. It could equally be 120Hz as its harmonic without a scope you won't be able to tell. A buzz implies capacitive rather than hum loop. Could be rectifier getting in input maybe by heater on first valve. Could equally be poor grounding between chassis and circuit ground. I tend to connect the two with 10R+100nf in parallel but there are a few different ways. Could also be position of input cables.

Looking at the negative feedback it may be better to take the ground returns from the speaker connectors straight back to the driver board.
Hard to know.
 
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What is the grid leak resister? I did some searching and it sounds like the resister that is connected to pin 5 of the power tubes.. On my amp there is a separate wire coming from the driver board to each of the four power tube's pin 6. Pin 6 is connected to pin 5 with a resister. Pin 6 doesn't actually connect to the power tube, as pin 6 is absent on my power tubes (and presumably all KT88 tubes.) I'm assuming they just wanted to have the pin 6 connector connected to something for some reason. Or perhaps they were just using the blank pin 6 connector as a convenient place to mount the resister.

When you say AC heater noise probably from bad wiring, which bad wiring do you mean?

Thank you very much for any help. I will post photos soon.
 
I don't have the 120 schematic but on the 70 schematic it appears that there are no screen grid resistors. This may cause the screen grid to exceed max dissipation shortening the tube life and possibly causing the noises. Usually 220R to 1K depending on supply volts. Also I don't see any path for the heaters to ground so they are floating.