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Old 18th February 2012, 05:06 PM   #1
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Default Rectifier injecting noise into heater supply

Hi folks.

I'm currently building a mid-high gain guitar amp, and I'm struggling with power supply noise. The amp has 4 6V6's and 5 12AX7's, current draw is somewhere around 120mA. I'm using a Hammond 270HX which has a 550Vac centre-tapped HT secondary. I'm using a series rectifier with two 1N4007's on each phase, 47uF as my main reservoir smoothing cap. My HT sits at about 380 on stand-by and has maybe 0.5% ripple. As soon as I switch on I get about 11V p-p of ripple, which is about 3%, and I can live with that, but here's the issue. With series diodes it's a good practice to put capacitors across each to ensure even voltage but when I add those caps, I get audible switching noise from those caps (as in right at the caps!), and also a lot of noise out of the speakers. When I watch my heater voltage (running 6.3Vac heaters with 70Vdc offset) on the scope and switch on the amp I see the sine wave get a nasty sharp edge near the peaks and valleys. Without those capacitors the amp is pretty quiet. I first tried 22nF caps and theorised that they were large enough that they were pumping current back into the secondary and that noise was coupling onto the heater windings, so I tried 10nF and they were also noisy, but somewhat less.

What am I missing here? Is 10nF still too large? I'll go throw some 330p's on there right now and see... Any help would be appreciated!

Thanks

-Dave
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Old 18th February 2012, 05:26 PM   #2
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With 330pF caps the noise is significantly reduced, although I can still see the distortion of the sine wave on the heaters. As another test I removed the caps and looked at the heater supply and the distortion is there, so it must be coming from the diodes switching.

Any thoughts?

-Dave
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Old 18th February 2012, 05:37 PM   #3
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are you sure you are using the 6.3 V winding for your heater supply (6 amps available) and not the 5 V winding (3 amps available). The total heater current of all your tubes is 3.2 amps. Which is more than what the 5V winding can handle.
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Old 18th February 2012, 06:04 PM   #4
DF96 is offline DF96  England
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You could try snubbers instead of capacitors. Make sure the capacitors are linear (so no harmonic generation) and not piezoelectric (no sound). Also, ensure they are rated to withstand constant AC voltage stress.
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Old 18th February 2012, 06:14 PM   #5
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Quote:
Originally Posted by speakerfritz View Post
are you sure you are using the 6.3 V winding for your heater supply (6 amps available) and not the 5 V winding (3 amps available). The total heater current of all your tubes is 3.2 amps. Which is more than what the 5V winding can handle.
Yes, I'm using the green winding (with Green/Yellow CT to my 70V divider, which is smooth as glass). Yellow (5V) will be used later for relay control.

QUOTE=DF96;2912320]You could try snubbers instead of capacitors. Make sure the capacitors are linear (so no harmonic generation) and not piezoelectric (no sound). Also, ensure they are rated to withstand constant AC voltage stress.[/QUOTE]

You mean RC parallel snubbers? I'm using standard 500Vdc ceramic caps. http://www.vishay.com/docs/28513/dseries.pdf

-Dave
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Old 18th February 2012, 06:52 PM   #6
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I just rerouted my main reservoir and rectifier to further reduce my loop area and this seems to have made a marked improvement. The distorted heater supply is still present, but overall amp noise is definitely reduced.

I'm also struggling with control pots picking up noise in my second stage. I'm using a very old chassis (from a Traynor Mark 3 guitar amp, mid-1970's) for my build so the control holes are not large enough for insulators, so I'll be enlarging them all I suppose. I've already enlarged the input and speaker out jacks.

I still suspect there's something else going on causing so much buzz. This old amp (and my Fender Super Reverb) were never terribly buzzy, and I'd like to think that my circuit layout is better than theirs as I've got all my decoupling caps local to each stage right on the point-to-point board rather than in a box on the bottom or way down in one area. I'm going to go through it again looking for ground loops, but I think I've done a fairly good job so far...

Any suggestions are welcome!

Thanks

-Dave
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Old 18th February 2012, 07:05 PM   #7
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If the amp runs fine without snubbers then I'd just leave them off. 500v sounds too low for snubber caps, you need like 1-2kv caps.
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Old 18th February 2012, 07:20 PM   #8
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I thought they seemed low, but I wasn't sure. With about 750V peak across both in series, 375V across each one.
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Old 18th February 2012, 07:27 PM   #9
DF96 is offline DF96  England
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Standard ceramic caps at nF values are likely to be piezoelectric, as you have found. They are also unlikely to be rated for continuous AC stress so may fail. Use X or Y rated caps, film type rather than ceramic. Try adding a lowish value resistor in series with each cap, to make a snubber.
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Old 18th February 2012, 09:54 PM   #10
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try ac referencing the heaters instead:

remove the circuit that is referencing the heater winding to a b+
the heater circuit should be now just the winding and the heaters.

now install a .1 uf @250V or higher voltage to one side of the heater to ground.

remove caps across B+ diodes.

power on and enjoy.
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