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Old 22nd December 2012, 07:52 PM   #11
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Thanks for all your comments.

I think it is 100 Hz hum (didn't measure it, just compared by ear with tone generator) and it didn't appear immediately after power one but a few seconds later (warming up?). But it disappears immediately after powef off (no fading out)


I believe I have all grounds connected right (no grond loops).
The resistor in power supply filter is 100R, not 10R as I mentioned before.
So do you think replacing this resistor with choke will help?

Thanks
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Old 22nd December 2012, 08:21 PM   #12
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I had a similar problem a few years back involving a SE guitar amp. The OPT was picking up hum electromagnetically through the Power Transformer. I had to move the OPT to the opposite side of the chassis to fix it. SE designs are much more susceptible to this than PP. An easy way to find out is to just disconnect the P/S secondary and run the transformer w/o a load. If there is electromagnetic coupling you will still hear hum through the speakers. I think in your case the best thing to do would to install the amp and P/S on two different chassis.
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Old 5th March 2013, 05:57 PM   #13
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Default Still haven't found a solution...

Well, after a couple of months I'm trying agan to sovle this issues with hum. A few observations:

- the hum is 100 Hz
- it doesn't appear immediately after power one but rather a few seocnds lates - maybe when heating of tubes gets to the right temperature?
- it disappears immediately after power off
- I have tried choke instead of power resistos but there is no difference
- I have tried hum pot parallel to CT resistors (connected to GND) but it doesn't help (it influences 50 Hz hum but not 100 Hz that I'm getting)
- I have twisted all pairs of wires tightly
- all grounds are star grounded - no loops
- I have tried shielding of power transformer but again no difference

Any other suggestions?

Last edited by vcelkamaja; 5th March 2013 at 05:59 PM. Reason: typos
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Old 5th March 2013, 06:16 PM   #14
jrenkin is offline jrenkin  United States
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I had this on an amp that was not supposed to have this, but did due to my choice of rectifier tube and the required low value 1st capacitor. I solved it by adding an additional filter stage. Worked great. Now CLCRC. with a low value R, you lose very little B+.
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Old 5th March 2013, 06:46 PM   #15
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Well if there's a delay, it's not electromagnetic coupling. You can rule that out. Do you have a schematic for the P/S? That would help us a great deal!
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Last edited by TubeHead Johnny; 5th March 2013 at 06:54 PM.
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Old 5th March 2013, 06:53 PM   #16
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So you think that CRC filtering - 100u-100R-200u is not enough?

I have tried CLC filtering as well - 100u-10H-200u but there was no difference.
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Old 5th March 2013, 06:57 PM   #17
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How bout a photo?
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Old 5th March 2013, 07:05 PM   #18
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I'm trying to attach some pictures. The circuit is realized on PCB - see the first page with link to supplier's web page.
All pictures show top plate of amp with output transformers open but there is not difference in 100 Hz hum if the top plate is open or closed.
Attached Images
File Type: jpg amp1.jpg (141.6 KB, 322 views)
File Type: jpg amp2.jpg (147.6 KB, 310 views)
File Type: jpg amp3.jpg (124.8 KB, 306 views)
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Old 5th March 2013, 07:06 PM   #19
N4BBQ is offline N4BBQ  United States
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I do not like that power supply design. It reeks of 100Hz/120Hz ripple. Add the choke, re-measure the ripple, then go from there.
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Old 5th March 2013, 07:08 PM   #20
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Well, I tried to move from CRC to CLC by replacing 100R resistor with choke (I think 10 H / 250 mA) but there was no audible improvement.
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