It's a bad idea because you want to have low output impedance connecting to high input impedance.
Put a 50Kohm or 100Kohm pot (log or audiotaper) in front of your 6J7 tube and use shielded cables. Connect the input ground to the pot (left finger -not the wiper), and also connect the shield of the cable running to the grid here. Take a seperate run of wire from this point to your starground. Don't connect the other end of the shield (near the grid) to anything.
Put a 50Kohm or 100Kohm pot (log or audiotaper) in front of your 6J7 tube and use shielded cables. Connect the input ground to the pot (left finger -not the wiper), and also connect the shield of the cable running to the grid here. Take a seperate run of wire from this point to your starground. Don't connect the other end of the shield (near the grid) to anything.
Thanks Disco and Joe Roberts about pot advices, I'll do that.
Plate is fused in case something goes wrong with a tube, I thought it was a good idea.. but I did not find a good value (fast 10-15mA ?).
About chokes you're right, probably not a very good choice but it seems to do the job, I don't hear any 100 Hz noise.
First I tried with IGPW chokes (4H 100mA) but the sound was unpleasant, flat.. so returned to the Hammond 156R and sound opened. I wonder why..
Plate is fused in case something goes wrong with a tube, I thought it was a good idea.. but I did not find a good value (fast 10-15mA ?).
About chokes you're right, probably not a very good choice but it seems to do the job, I don't hear any 100 Hz noise.
First I tried with IGPW chokes (4H 100mA) but the sound was unpleasant, flat.. so returned to the Hammond 156R and sound opened. I wonder why..
Following on from your initial success with the earthy bar adjustment - can I suggest you try this easily reversed experiment.
-Snip the wire between your small signal ground and your power supply ground.
-Attach a small gauge wire to the mid point of your small signal ground.
-Attach the other end to the PSU ground bar, then try it attached to the safety earth - see if either configuration gives a reduction in the hum.
If not rejoin the bus bar at the snip point.
Shoog
-Snip the wire between your small signal ground and your power supply ground.
-Attach a small gauge wire to the mid point of your small signal ground.
-Attach the other end to the PSU ground bar, then try it attached to the safety earth - see if either configuration gives a reduction in the hum.
If not rejoin the bus bar at the snip point.
Shoog
I've never used a plate fuse before and to be honest here I wonder if it is of any value. I have seen it in output tubes but then again I think it would be better to use a cathode resistor of a small wattage that would simply go open. I mentioned the choke size because I still do not know how to properly size a choke for an application. I have built some line stages that had specified a choke of 30H @ 40mA so I have a few of them on hand.
As my english is not good, I am not sure to understand 100% of your suggestions.
So I drew a ground schematic, could you please tell me if it's ok ?
A few more questions
- do I have to connect shield between pot ground and bus PS ground wire ?
- same question for output ground wire..
- is it better to connect the grid leak resistor (300K) directly between the wiper and pot ground ?
About plate fuse, why not remove it, in this case a 300R 1/4W should be enough (2V on cathode, 6-7mA).
Thanks
So I drew a ground schematic, could you please tell me if it's ok ?
A few more questions
- do I have to connect shield between pot ground and bus PS ground wire ?
- same question for output ground wire..
- is it better to connect the grid leak resistor (300K) directly between the wiper and pot ground ?
About plate fuse, why not remove it, in this case a 300R 1/4W should be enough (2V on cathode, 6-7mA).
Thanks
Attachments
Here you go: a starground.
It's in its most extensive version. Often the input side collapses to one feed (to starground) from the orange dot. Also, normally the tube has one feed to starground. If the output is located at >15cm distance, run an extra feed to starground.
The last capacitor of the PSU connects to starground, as does your case (make 1 connection only). The heaters connect to the starground, either through two resistors, a humpot or through a capacitor. The mains safety ground goes to your case and connects to everything one can touch on the outside.
So, there's one central hub we call ground, which defines the only 0,000 volt reference in your amplifier. All voltages are either above or below this potential.
Your groundbar may also be used but it takes practise to implement as disrupting large currents can interfere with small audio signals and multiple ground points may introduce alternative paths to ground.
An externally hosted image should be here but it was not working when we last tested it.
It's in its most extensive version. Often the input side collapses to one feed (to starground) from the orange dot. Also, normally the tube has one feed to starground. If the output is located at >15cm distance, run an extra feed to starground.
The last capacitor of the PSU connects to starground, as does your case (make 1 connection only). The heaters connect to the starground, either through two resistors, a humpot or through a capacitor. The mains safety ground goes to your case and connects to everything one can touch on the outside.
So, there's one central hub we call ground, which defines the only 0,000 volt reference in your amplifier. All voltages are either above or below this potential.
Your groundbar may also be used but it takes practise to implement as disrupting large currents can interfere with small audio signals and multiple ground points may introduce alternative paths to ground.
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Hi,
unfortunately, no improvement, hum still there..
https://dl.dropbox.com/u/425350/6J7%20Preamp/IMG_1633.JPG
I'll try to shield the auricap red wire and use it as signal ground & connect it to the star ground.
Normally is a 6J7 AC heated quiet ?
Volume pot is now a 50K instead of 10K, I wonder if it can be related or not but the sound is more "flat" and "uninteresting" than before (wider, deeper, "fatter").
unfortunately, no improvement, hum still there..
https://dl.dropbox.com/u/425350/6J7%20Preamp/IMG_1633.JPG
I'll try to shield the auricap red wire and use it as signal ground & connect it to the star ground.
Normally is a 6J7 AC heated quiet ?
Volume pot is now a 50K instead of 10K, I wonder if it can be related or not but the sound is more "flat" and "uninteresting" than before (wider, deeper, "fatter").
The 6J7 should be quiet so it looks like it is a problem of implementation.
Remove the connection from the power supply ground buses to the star ground, and introduce a short wire from the common point on the extreme right to the safety earth. Even if this doesn't work out it cannot hurt your implementation.
Even a tiny imbalance in resistance across your power supply earth "ring" could be enough to create significant earth currents.
Shoog
Remove the connection from the power supply ground buses to the star ground, and introduce a short wire from the common point on the extreme right to the safety earth. Even if this doesn't work out it cannot hurt your implementation.
Even a tiny imbalance in resistance across your power supply earth "ring" could be enough to create significant earth currents.
Shoog
At this point I have to ask: are you certain that the input jack is insulated from the chassis? Are you using "shoulder" washers on the input jack? I know most of the jacks come with a washer with a tiny shoulder and I find I have to install the jack in a "sloppy" hole and even wrap a thin strip of electrician's tape around the jack where it passes through the panel. I really like the Amphenol jacks that are well insulated from the chassis.
Amphenol RCA jack
Amphenol RCA jack
Hi,
Thanks for your suggestions.
https://dl.dropbox.com/u/425350/6J7%20Preamp/IMG_1634.JPG
- bus bar is now "short" and PS ground connected to earth socket
- pot metal case to starground
- output signal wire with shield used as ground
- plate fuse removed
I checked the cinch connectors, they are insulated from chassis, but I added electrician tape just in case.
No improvement, hum still there
Before connecting the preamp to my system, I tried again the following:
- input shorted, output to line-in laptop (on battery), outpout to headphones with "playthrough" option and very high sensitivity.
Observations:
- input shorted or not, no difference
- no hum when only heaters on (that was not the case before)
- B+ on, hum immediately appears
- B+ off, hum immediately disappears
Isn't it strange ? PS capacitors takes about a minute to go down to 0V
Thanks
Thanks for your suggestions.
https://dl.dropbox.com/u/425350/6J7%20Preamp/IMG_1634.JPG
- bus bar is now "short" and PS ground connected to earth socket
- pot metal case to starground
- output signal wire with shield used as ground
- plate fuse removed
I checked the cinch connectors, they are insulated from chassis, but I added electrician tape just in case.
No improvement, hum still there
Before connecting the preamp to my system, I tried again the following:
- input shorted, output to line-in laptop (on battery), outpout to headphones with "playthrough" option and very high sensitivity.
Observations:
- input shorted or not, no difference
- no hum when only heaters on (that was not the case before)
- B+ on, hum immediately appears
- B+ off, hum immediately disappears
Isn't it strange ? PS capacitors takes about a minute to go down to 0V
Thanks
- between 2 et 10 mV, sometimes 20 mV, sometimes 50 mV, it continuously change, I don't know if it's normal or not ?
- yes
- low microphonic, I only hear it when testing with laptop + headphones + very high sensitivity.
even if it's not yet fixed, you help(ed) me
Guys, don't you think it's strange when I switch off the B+ the hum immediately disappears ? capacitors are not discharged in one second..
Magnetic or electric field issue
OK, ripple is almost flat so that can be ruled out.
No microphonic tubes, so we can also rule out picking up of transformer resonance.
- Where is your transformer located? Is it radiating into a sensitive part of the amp?
Last option before I throw in the towel is to check for HF resonance. Did you put a resistor in series with your audio signal, close to the grid?
No microphonic tubes, so we can also rule out picking up of transformer resonance.
- Where is your transformer located? Is it radiating into a sensitive part of the amp?
Last option before I throw in the towel is to check for HF resonance. Did you put a resistor in series with your audio signal, close to the grid?
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