PSU caps should connect to the ground bus in the same order as they connect to the supply rail. Get this wrong and the earlier ones can inject hum into the ground connection of the later ones. That is why it is best if this ground is a bus and not a star.
So I reordered the caps on the bar, now I need to move the vol pot ground closer to the input I think. The hum is aaaaalllllmost gone. If I play with the voltage divider resistor values I am sure I can get it gone but I want to keep the values lower.
Thank you all for the help.
J
Thank you all for the help.
J
Attachments
So I shorted the 6sl7 & 2a3's to ground and the hum is still there. I also pulled the ground out of the umbilical cord and notta. Any other ideas?
Hi!
If the hum ist still there when you ground the 2A3 grid, that means the hum is injected in the output stage and you have to check there. This leaves two possibilities: Hum from the filament to hum from the B+ supply. You can verify that by temporarily adding more filter capacitance to the B+ supply. If doubling the cap reduces the hum, you don't have enough B+ smoothing.
Thomas
If the hum ist still there when you ground the 2A3 grid, that means the hum is injected in the output stage and you have to check there. This leaves two possibilities: Hum from the filament to hum from the B+ supply. You can verify that by temporarily adding more filter capacitance to the B+ supply. If doubling the cap reduces the hum, you don't have enough B+ smoothing.
Thomas
No matter what style you do in your grounding if its only a single umbilical wire,it would still be an insufficient grounding because current flowing through chassis will be too small.
one thing you can do is take a bunch of black wires,( about 8 pieces), cut it with the same length,twist it together and put an octal sockets on both ends and assemble this as ambilical cord between chassis and amp. this will serve as your multiple wire grounding as did in computer power supply if you've seen one....
one thing you can do is take a bunch of black wires,( about 8 pieces), cut it with the same length,twist it together and put an octal sockets on both ends and assemble this as ambilical cord between chassis and amp. this will serve as your multiple wire grounding as did in computer power supply if you've seen one....
Two ground wires are needed, not one, not eight. One for chassis/safety ground. One for signal/PSU ground. Multiple grounds may be needed on a high speed ribbon cable (as used in some computers), but that is for quite a different reason which does not apply to audio.
The thing to remember is that best ground practice for audio and best ground practice for RF and high-speed may be opposites of each other!
The thing to remember is that best ground practice for audio and best ground practice for RF and high-speed may be opposites of each other!
So I will check the b+ supply. Maybe hum is being induced running with the filament supplies.
I have 2 grounds 1 safety/chassis & 1 amp/signal. They are tied at one point. Honestly I don't think this amp has a grounding issue.
So how do i know how many or what value caps to add to the b+?
Thank you all for the help.
I have 2 grounds 1 safety/chassis & 1 amp/signal. They are tied at one point. Honestly I don't think this amp has a grounding issue.
So how do i know how many or what value caps to add to the b+?
Thank you all for the help.
I saw that but I am not sure what that means In practice. I don't think it's as simple as throwing another set of caps in it is it? It will change the values of the existing 2/50/20. If it means just adding another set in series or parrellel I'm ok with it but not so sure...
I've gotta 4 section JJ'a laying around....
I've gotta 4 section JJ'a laying around....
Noob question
How much does the value matter? As in 20 or 50 or 100 etc...
I'm also assuming that I should not change the value of the 1st cap
How much does the value matter? As in 20 or 50 or 100 etc...
I'm also assuming that I should not change the value of the 1st cap
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Hi!
Only change the last cap. Changing the first cap would change the voltage.
As mentioned just double the last cap or thereabout. This is just to see if this reduces hum. The value is not critical. If you add too little the effect will not be noticeable. So it least double the cap.
Thomas
Only change the last cap. Changing the first cap would change the voltage.
As mentioned just double the last cap or thereabout. This is just to see if this reduces hum. The value is not critical. If you add too little the effect will not be noticeable. So it least double the cap.
Thomas
Threw a 40uF on the other 20's and didn't seem to make a difference. Really thinking next step is pulling the 330V from the umbilical cord and seeing what happens.
Thank you for taking the time to help it is much appreciated.
Thank you for taking the time to help it is much appreciated.
Hi!
Then it is most likely the AC on the filaments. Is there a hum buck pot across them?
I have AC filament and B+ wiring through the same umbilical all the time, this does not induce hum.
Thomas
Then it is most likely the AC on the filaments. Is there a hum buck pot across them?
I have AC filament and B+ wiring through the same umbilical all the time, this does not induce hum.
Thomas
Hi again,
is the last cap of the PSU located in the PSU or amp chassis? At least the last cap should be in the amp section close to the circuit. I always have the final LC section in the amp section
Best regards
Thomas
is the last cap of the PSU located in the PSU or amp chassis? At least the last cap should be in the amp section close to the circuit. I always have the final LC section in the amp section
Best regards
Thomas
There is a hum pot.
The last cap is in the amp chassis.
Must be the filament. So converting them to dc is an option.
The last cap is in the amp chassis.
Must be the filament. So converting them to dc is an option.
Hi!
How does adjusting the hum pot work with regard to hum?
When using headphones you will most likely not be able to adjust the residual hum from the filaments to zero.
Again it is advisable to do a quick test before converting the amp to DC filaments. Hook up one 2A3's filaments to a lab supply and see if the hum goes away on that channel. If it does, convert the amp to DC filaments
Thomas
How does adjusting the hum pot work with regard to hum?
When using headphones you will most likely not be able to adjust the residual hum from the filaments to zero.
Again it is advisable to do a quick test before converting the amp to DC filaments. Hook up one 2A3's filaments to a lab supply and see if the hum goes away on that channel. If it does, convert the amp to DC filaments
Thomas
Thanks for the help. My lab is a multimeter and not much else.
The hum pots work well and the hum isn't too bad.
I also have a bad output tube so I had to order new ones. 1 channel is barely audible, but no hum lol.
Would using twisted shielded wire reduce the hum?
The hum pots work well and the hum isn't too bad.
I also have a bad output tube so I had to order new ones. 1 channel is barely audible, but no hum lol.
Would using twisted shielded wire reduce the hum?
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