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 Post subject: Power supply Cap
PostPosted: 13 Mar 2018, 06:50 
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Joined: 13 Mar 2018, 06:32
Posts: 14
In Bruce Heran's power supply schematic of a push-pull amp it shows a 1uf cap in front of a 100uf cap. Can someone explain what is the 1uf cap's purpose?

Sorry I can't attach an image of the schematic. The cap occurs after rectification using 2 diodes.


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 Post subject: Re: Power supply Cap
PostPosted: 13 Mar 2018, 11:19 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Hi, welcome!

It's to bypass HF noise that might ride on the rail.

Cheers!

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 Post subject: Re: Power supply Cap
PostPosted: 13 Mar 2018, 11:41 
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Joined: 13 Mar 2018, 06:32
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My problem with my amp (new project) is hum!! I have disconnected the 6.3 volts from the heater while amp is running and the hum still shows on the oscilloscope and is heard on the speaker. This was suggested and it worked. The hum stayed so it must be caps or the diode rectifier. Is their a way to check caps w/o pulling them all out one at a time?


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 Post subject: Re: Power supply Cap
PostPosted: 13 Mar 2018, 21:51 
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Joined: 04 Jun 2008, 20:59
Posts: 3791
Location: Arizona, USA
Hi If your scope has AC coupling and will tolerate the B+, you can use AC coupling on the scope input and look at the ripple right as the B+ goes to the output transformer. The ripple will be moderately high as the design utilizes the ability of the output stage to cancel it out. I suspect that your hum it is from some other source. Grounding issues seem most common. Alternatively ground loops that occur when the signal ground is connected to the AC ground (chassis).

Good listening
Bruce

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 Post subject: Re: Power supply Cap
PostPosted: 14 Mar 2018, 11:11 
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Joined: 13 Mar 2018, 06:32
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That was a great idea. My scope can add the 2 signals together and I can see the hum with the input grounded. Perhaps my output tubes are not balanced. The output hum is about .1V. I have 2 Hickok tube testers I inherited from my brother but no manuals. I am waiting on the manuals so I can check the tubes. Thanks for the help.


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 Post subject: Re: Power supply Cap
PostPosted: 14 Mar 2018, 17:17 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Actually if you use the AC coupling feature it will separate the ripple from the DC. It should appear at twice the frequency of your AC mains. Let us know what you find. If the wave form is distorted it could indicate a bad rectifier.

Good listening
Bruce

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 Post subject: Re: Power supply Cap
PostPosted: 15 Mar 2018, 11:25 
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Joined: 13 Mar 2018, 06:32
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I am not sure what you mean by "AC coupling feature". I have attached an image from my scope. It is on the 5mv setting so the signal shows it is 10mv. It is twice the mains freq. I noted this by putting a 120cps signal at the same time and they were both the same freq. I again disconnected the 6.3 volts to the tubes and the hum did not disappear. So am I right that it is not the tubes.

Well it seems I don't know how to put in an image from my iPhone. Sorry, but the hum image on the scope did die out as the voltage from the caps dwindled.


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 Post subject: Re: Power supply Cap
PostPosted: 15 Mar 2018, 12:03 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, The scope has a capacitor in line with the input on AC coupling and it blocks DC. Looking at the trace it would seem that you have some issue with the rectifiers. What I see is only 1/2 wave rectification and not full wave. This will have hum. There ought to be a pulse in between each of the existing ones. So one side of the rectifiers is not working . Check the orientation of the rectifiers to make sure it is correct. If so try to touch up the soldering at the rectifiers. If still no change then a rectifier is likely bad. If you don't have an in circuit tester for rectifiers (few folks do) then you will have to carefully un-solder one end of a rectifier and measure the resistance across it with the probes going in both directions. One way it will be very high or even off scale the other way ought to be fairly low. Do one rectifier at a time. Any that show low both ways or high both ways are bad. If this doesn't find the problem, then I give some additional guidance on what to test.

Good listening
Bruce

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 Post subject: Re: Power supply Cap
PostPosted: 15 Mar 2018, 12:19 
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Joined: 13 Mar 2018, 06:32
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Here is another hum from a different amp (but same design). It is about 1/3 of the 1st one. I have made a sketch of the schematic and shown what the scope traces look like. The problem comes after the B+ for the EL84's. After the next cap there is a bump on the scope that does not go away even after another cap. There is a 36k resistor (not in the sketch) between the 2nd and 3rd cap.

Attachment:
deg1.jpg


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Last edited by gwalters on 15 Mar 2018, 14:40, edited 2 times in total.

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 Post subject: Re: Power supply Cap
PostPosted: 15 Mar 2018, 12:46 
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Joined: 13 Mar 2018, 06:32
Posts: 14
This is the rectifier I am using. It's from mouser. Perhaps its not doing what I thought.
VS-CPU6006L-N3

This pic is how it's used. I don't have a schematic.


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