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Do I have bad pot?

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Posted on October 31, 2001 at 13:14:35
Trippyloaf
Audiophile

Posts: 9
Joined: August 30, 2000
Hi all - I just finished building ther JE Labs 300B Deluxe and it sounds fabulous. Only problem is that at certain spots on my volume control, I get an annoying buzz coming out of the tweeter. These spots occur at all the way off (6 o'clock) to 10 o'clock. 12 o'clock to 3 o'clock, and then right at 4 (the end). I ordered some caps to try snubbers on the filaments as suggested on a post I found. Could it just be my potentiomenter. It is a cheapo Mouser 100K audio taper pot. Thanks!

 

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Re: Do I have bad pot?, posted on October 31, 2001 at 18:12:56
It may be ground soldering problem or ground loop problem. If the noise level would be affected by the pot rotation angle, it might show that buzz would be canceled or not canceled by the dividing ratio of the pot, The pot works as if a hum balancer.

Confirm signal ground wiring between the RCA jack to the pot. It should be connected to the chasis near the RCA jack at a point. (The minus side of RCA jack should not be touched to the chasis directly.)

 

Re: Do I have bad pot?, posted on November 1, 2001 at 02:59:13
JeLL-O


 
This is in addition to Gridleak.

Most people do not realize that you can damage those types of pots with heat during soldering. I used to put those Noble pots all the way off and solder, then find such noise at that position. No amount of regrounding etc would take it out.

Next time you solder to a volume pot, turn it to the MAX position. You almost never use it at that point so even if it gets damaged, u dont hear it. Clip or clamp a heatsink or simply your long nose pliers to the pot terminal while soldering. This will avoid the heat damage.

JeLL-O

 

Re: Do I have bad pot, no I don't think so, posted on November 1, 2001 at 14:35:46
Hi all,
I replaced the pot with a similar one and the exact same thing happens. So i figure it probably isn't the pot. I moved the ground to a spot near the rca inpit jack, that didn't help. Note: Joseph doesn't use a star ground nor bus ground scheme. He uses a local one - power supply, input/driver, and output.
I put in some snubber caps in the filament. see: http://www.audioasylum.com/forums/set/messages/9975.html
and that didn't help.
I don't have my 6.3V CT grounded nor do I have my 5.0V CT grounded neither. Could that be a problem? I also have my 5V heater wire run near the volume pot (about 1/2 inch).
It is definitely a buzzing sound and not a hum.

Worse case scenario, could I measure where the audio pot has the least amount of buzz, take that value and place a grid input resistor of the same value into the circuit and use a preamp?

thanks for any suggestions.

 

got it!, posted on November 1, 2001 at 15:21:45
I found out it was the 6.3V filament CT that I had floating. I looked on JE's site and he had his floating as well. I guess in mine it should be grounded. It looks like he also floats his negative speaker terminal tap...I always thought those should be grounded as well.

 

Re: got it!, posted on November 1, 2001 at 16:36:59
As I don't have the circuit diagram, I suppose that 6.3V-AC supply is used for the driver stage that has indirect heating cathode. In most case hum ballancer or CT is not necessary for the indirect heating tube because heater circuit is isolated.

If there is no CT in your transformer, you can check it by adding hum ballancer in 6.3V. Maybe you won't be able to get the good result. For my experience I got the best (Least hum) point at the both side of hum ballancer. That ment hum ballancer wasn't necessary.

But 6.3V circuit should not be floated, as you wrote. Because floating means being coupled with stray capacitors to the signal line, and it may be affected by noise easily.

I solved similar problem by changing ground wiring and ground point before. You should better try one point grounding carefully.

 

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