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Old 5th March 2013, 07:34 PM   #21
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Try this. Change the AC filament voltage going to the 12AX7 to filtered DC. Use like 2200 uF or more for the new design.

The 6L6 filament voltage potential should be elevated above the voltage of the cathode for minimum hum. 30V or more.

You may need to change that to DC as well. I know it's a real pain but sometimes hum can be a bitch!!! Especially with SE designs.
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Last edited by TubeHead Johnny; 5th March 2013 at 07:57 PM.
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Old 5th March 2013, 07:52 PM   #22
jrenkin is offline jrenkin  United States
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Quote:
Originally Posted by vcelkamaja View Post
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.
ADD the filter segment. CLCRC.
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Old 5th March 2013, 08:25 PM   #23
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Does the hum change when chassis is propped open, like your pictures? If so, it's from magnetic coupling transformer-to-transformer. This is consistent with the observed warmup - when the valves aren't (yet) conducting current can't flow in the OPT primary.

All good fortune,
Chris
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Old 6th March 2013, 10:55 AM   #24
N4BBQ is offline N4BBQ  United States
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If you can fit it in there, I'd add another R & C in the line and increase C as I go.

Bridge->C(100u)-Choke-C(390u)-R(30ohms to 100ohms)-C(390u)-C(0.01u)
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Old 6th March 2013, 11:09 AM   #25
DF96 is offline DF96  England
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If it is 100Hz then it could be poor ground design on the PCB. If it is 50Hz then it could be poor heater supply routing on the PCB. The pictures on the website show the latter, but the PSU grounding is not visible so difficult to comment.
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Old 6th March 2013, 12:28 PM   #26
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Actually if I open the chassis as shown on the photos the hum is still the same - so I guess it is no coupling between output and power transformers.

I have checked the circuit ones again and it actually has CRC (CLC) filtering for output tube but CRCRC (CLCRC) for input tube. Do you think it is still not sufficient that it can produce audible 100 Hz hum?
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Old 6th March 2013, 12:36 PM   #27
chrish is offline chrish  Australia
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I would bet the problem is the first thing mentioned by the first reply - ground loop. Did you short out the input to ground like the first poster suggested and see if the hum was still there?
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Old 6th March 2013, 12:43 PM   #28
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Yes, I have tried this. I have connected input to GND (inside the input CINCH connector and there was no difference.

One more thing I did - I disconneced AC heating voltage from one channel while having the AC heating of the second channel connected. The hum was only in the channel with AC heating connected. Of course the sound was not there in the channel without heating. Does this help in identifying the source of hum?
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Old 6th March 2013, 12:56 PM   #29
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I believe the GND is ok - please see attached picture. There is one single star GND point at power supply capacitors. Input GND gos here as well and input jacks are isolated from chassis. Earth (chassis) is near the power transformer and single wire from start GND goes here + power transformer shield + PE mains goes here as well.

I have tried different connection / disconnection of Earth GND, output transformer GND wires and input GN point but basically there was no improvement. Sometimes it was a bit worse but never beter. Therefore I thought it it not the ground problem. But I'm not sure. Maybe I'm doing something wrong.
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Old 6th March 2013, 01:01 PM   #30
N4BBQ is offline N4BBQ  United States
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The fact that as soon as you remove AC power, the hum goes away tells me that either that power supply has a leaky cap, needs a bit more capacitance, or you are picking up noise from "AC ground" if it is somehow tied/shorted to the B(-) line.
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