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Old 6th March 2003, 04:25 PM   #1
miguel2 is offline miguel2  Portugal
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Default Hum when volume is turned down

Hi,

I built a preamp based on EF86 tubes. The PSU is equal to the one on this preamp .

At first it had a lot of hum. After careful checking I could not identify any ground loop or something and just cured it with a big electrolytic after the PSU.

But now when I put the volume down, when it is very close to the limit it starts to hum like hell. I am using an alps blue pot. When I put the volume up a bit it no longer hums. Any suggestion how to fix this?

Miguel
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Old 6th March 2003, 05:00 PM   #2
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You may try adding a small fix resistor from the pot to ground say 50 or 100 Ohms (depending on pot impedance) instead of the wire connection to ground.

Other things to look for is to check if the body of the pot is grounded and that it is not forming a ground loop.
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Old 6th March 2003, 05:12 PM   #3
SY is offline SY  United States
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Without question, a ground loop. Using a fixed resistor is just a band-aid; look carefully at your grounding scheme.
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Old 6th March 2003, 05:17 PM   #4
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You may also try a grid stopper to V1 and V3 comming from the pot that I see is not included on the schematic.
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Old 6th March 2003, 05:25 PM   #5
miguel2 is offline miguel2  Portugal
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Ground loop

Maybe it is a ground loop but I can't find it!

For ground I have a very fat piece of wire and all ground connections go to it. Transformer, then first the first regulation cap, then the second, the error amplifier, the pass tube and then the amplifier ground. I believe that now I have the chassis connected after the caps, although I tried other places and with resistors too.

I will try some low value resistors on the volume pot .

Miguel
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Old 6th March 2003, 05:25 PM   #6
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I agree with SY, using a fix resistor is a last resource.
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Old 6th March 2003, 05:31 PM   #7
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Default Hum.

Hi,

Quote:
I will try some low value resistors on the volume pot .
Personally I would not even bother with since it won't solve the problem anyway.

Try to unmount the volpot from the chassis and see if there is still hum.
If it is gone then the shaft of the pot is cause of the loop.

Cheers,
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Old 6th March 2003, 05:45 PM   #8
miguel2 is offline miguel2  Portugal
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Tony,

I only built the PSU from that schematic. The preamp section is this:
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File Type: jpg ef86.jpg (23.5 KB, 410 views)
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Old 6th March 2003, 05:47 PM   #9
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The ground loop may be caused by having the star ground connected directly to chassis (or other parts such as the body of the pot) when C16/R28 are active.

I would try to bypass directly to ground chassis (eliminating C16/R28) and see what happens.
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Old 6th March 2003, 06:05 PM   #10
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You are deeling with a hi impedance circuit so it can get touchy.

I don't have much experiance with pentodes but if is not a ground loop you could also try adding a 100 Ohms resistor from the plate to pins 8 and 1 instead of a direct connection.

Anyway, I feel that other on this thread have more knowledge on this type of circuit so their advise will be more helpful.
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