Joined: May Thu 26, 2011 5:58 am Posts: 786 Location: Sterling- A city in Northern Virginia about 20 miles outside of D.C
I finished up my little three tube amp today and fired it up on the variac. It works well but has a little hum until you get to almost full volume then it goes away. Tube lineup is 35Z5GT, 50L6GT and 12SQ7. Would there be any gains to adding a tone control to the setup? The schematic is based on the amplifier circuit from the venerable AA5. Will post some pics soon. Off now to start the AM transmitter!
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Joined: Jan Thu 01, 1970 1:00 am Posts: 8650 Location: Chesapeake VA
badwaxcaps wrote:
IIt works well but has a little hum until you get to almost full volume then it goes away
I'm guessing you are using a volume pot of around a meg-ohm??? I've found higher resistance volume controls cause that issue, go to a 50-100K ohm, should eliminate the problem... Stems from using a low impedance input source, you can try hanging a 100K(maybe lower) resistor across the pot...
Joined: Jan Thu 01, 1970 1:00 am Posts: 28935 Location: Livermore, CA
Agree with Tom.. Also be sure body of the control is grounded. There are plastic boxes for projects. Using one of these you will need to run a wire from body of the control to circuit ground.
Joined: May Thu 26, 2011 5:58 am Posts: 786 Location: Sterling- A city in Northern Virginia about 20 miles outside of D.C
Tried connecting the volume control body to B-. The hum got a little quieter but still there. I have a 100k pot around here somewhere. I will try swapping the 500K with the 100K and see what happens. Will have to set up a seperate power switch. The 500K pot has one integrated into it.
_________________ "No man goes before his time, unless the boss leaves early"
Joined: Jan Thu 01, 1970 1:00 am Posts: 18099 Location: Detroit, MI USA
Having the power switch on the volume control can also cause a small amount of hum to be picked up. You never see that on high quality amplifier designs.
Also, reversing the AC power plug in the wall outlet can help minimize hum. Sometimes it will be quieter in one position vs. the other.
Joined: May Thu 26, 2011 5:58 am Posts: 786 Location: Sterling- A city in Northern Virginia about 20 miles outside of D.C
Oh I see how it is.. My design is'nt "high quality".. I did kind of figure that the culprit was that combined switch/volume pot. Gonna swap it out and see what happens.
_________________ "No man goes before his time, unless the boss leaves early"
I did kind of figure that the culprit was that combined switch/volume pot. Gonna swap it out and see what happens.
