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Old 4th July 2005, 10:19 PM   #1
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Default Debugging my Aikido

I don't understand why this one channel has so much humm. Before the cap (measuring on the 2mV setting on the scope) I get no humm. After the output cap I get humm 10mV setting? Did not use a flash you there are a few extra sinus waves

Probably simple to some of you ...but I don't get it.

[EDIT] To save you from reading this whole thread...the 2mV reading was bad..for some reason....in the end I had the 100k resistor soldered to pin 6 instead of pin 7

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Old 5th July 2005, 05:16 AM   #2
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Hi Bas. Kind of weird isn't it. I'm definitely not the electronics expert but I did build the Aikido and it worked perfectly first try. NO noise! GREAT sound! I think we can say the 1 meg output to ground resister is unlikely to be generating this noise. Everything to the left of the output cap must be ok. So, what is left? Can it be a bad solder joint on that 1 meg resister rectifying and picking up external noise? OT: Just curious as to why you chose 390 ohms for the 12SN7. At 300 volts B+ you must be running it fairly hot. At 876 ohms it runs 5 ma which I thought sounded about right.
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Old 5th July 2005, 08:52 AM   #3
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Hi Bruce,
Quote:
chose 390 ohms for the 12SN7
I think the example John Broskie had was 430R I took the nearest value I had..

There is a lot less than 300V B+ I think...I just used back to back transformers..nice n cheap..just to see what the aikido could do. I was thinking that because of the inherent noise reduction..the b+ did not have to be perfect...
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Old 5th July 2005, 08:54 AM   #4
EC8010 is offline EC8010  United Kingdom
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If that oscilloscope had been triggered, it would have been possible to see what the frequency of the hum was. And if it really was hum, and not RF...
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Old 5th July 2005, 08:59 AM   #5
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Hi EC,

I don't have any scope experience or knowledge the probe is homemade. But listening tests confirm the humm.. the other channel has some noise on it but it is a lot less audible and has less amplitude. I think it might have been triggered..... but the exposure time of the camera caused that overlapping look on the photo.
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Old 5th July 2005, 09:04 AM   #6
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Hello Bas,

When the scope is triggered, each trace perfectly overlays the previous one, and you could have an exposure time of hours without seeing anything untoward. It's just that that "hum" looks a bit too sinusoidal to actually be hum, and I'm wondering if you've got RF oscillation, which often then causes hum due to the excessive current drawn from the power supply. Have another go with the oscilloscope...
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Old 5th July 2005, 09:09 AM   #7
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Quote:
if you've got RF oscillation, which often then causes hum due to the excessive current drawn from the power supply.
I actually think that the humm is caused by excessive current drawn from the psu ...but somehow because of the little transformers? What I don't understand though is why I can't spot it on the scope.

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Old 5th July 2005, 07:42 PM   #8
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Bas:
I am considering building an Aikido as a headphone amp, so I am not interested in having it hum.

I was able to look at the contruction pictures of your Aikido last night and I could not discern as to where the heater filament wiring ran in relation to signal and power supply in your construction. Is the twisted blue wiring filament? I thought I saw it sandwiched between two resistors at the socket.

When I went to look at it again the link did not work.

Cheers,
dr._sleep
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Old 5th July 2005, 07:48 PM   #9
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Hi Bas. Still off topic (maybe). I think you are looking at the "safer version" example with 6n1p and 5687 tubes. That uses 220V and 430 ohm resisters. I wonder if 300V and 876 ohms (as in the first example he gives) isn't more appropriate for a 12sn7. I'm not familiar with ECC99 operating points. What is your operating current?
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Old 5th July 2005, 08:24 PM   #10
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Dumb question for EC- if this were an oscillation causing the power supply to hummmmmm, then why doesn't it show up on the anode side of the output capacitor?

Bas, how is the 1M resistor returned to ground? Does the ground return pass by anything that could induce hum into it?
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