Rectifier injecting noise into heater supply - Page 2

trobbins · 12:49 AM

To alleviate the HT supply rectifier noise issue then quite rightly you need to look at 'loop area', and also in that context I suggest bypassing each half HT secondary winding with a small 10nF cap as close as possible to the Tx - this reduces the loop area of the transient current to the winding itself, rather than taking a longer path through the associated diode and supply filter caps. The 10nf cap needs to 630VAC type rating. If possible, twist the 3 secondary HT wires as much as possible from the transformer out to the diodes and 0V (and the bypass 10nF caps if using them).

The elevated DC bias for the heaters could introduce noise from the B+ supply if the bias is not well ac bypassed, or star grounding introduces noise between the bias supply 0V and the input stage 0V reference.

You can dampen the diode noise level by inserting some series resistance with the diodes. Imho I'd remove the capacitor bypass (or RC) across each diode, and focus on the other measures first.

tauro0221 · 19th February 2012, 01:39 AM

Hi,
I think your over loader the 1N4007's diodes. The max. current for the 1N4007 is 1 amp. If you wire all the filaments in parallel your load is about 3 amps. I will use the 1N5408 instead the 1N4007 that is 1000 volts 3 amps. That's maybe what it is causing the noise that your experiencing.

DavesNotHere · 19th February 2012, 01:49 AM

Quote:

Originally Posted by tauro0221

Hi,
I think your over loader the 1N4007's diodes. The max. current for the 1N4007 is 1 amp. If you wire all the filaments in parallel your load is about 3 amps. I will use the 1N5408 instead the 1N4007 that is 1000 volts 3 amps. That's maybe what it is causing the noise that your experiencing.

yea I forgot to mention diodes. the best overall is to have low impedence. 1n4007 are for solid state circuits. I use 6A10 for my ss rectification. also I see 4 6v6 that is somewhat of a current demand. you need 220uf of capacitance to have a low enough output impedence of the power supply (you'll need 100uf just alone in a pair of 6v6's, not including the drive circuit demands).

trobbins · 19th February 2012, 04:37 AM

The OP is running normal AC heaters - his issue relates to the diode rectifiers for the B+ supply.

jane · 19th February 2012, 07:48 AM

Quote:

Originally Posted by doozerdave

Hi folks.
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.

Why not use resistors (200-500k) to ensure even voltage over the diodes?

Jan E.

DF96 · 19th February 2012, 12:44 PM

Yes, resistors will do the job just as well.

doozerdave · 19th February 2012, 02:06 PM

Thanks for all the input. A few points:

I need to keep the 70Vdc offset (which is confirmed to be super clean) to protect the cathode followers.

My calculations for 4 6V6's in push-pull-parallel, plus the triodes shows that 47uF is plenty to get me lower than 10% ripple, and I'm currently only seeing 3% ripple. I don't want to go too high with the main reservoir and risk overloading the xfmr.

I'm currently in the middle of improving the layout of stages 1&2. Once I'm finished that, if the problem persists I'll try some of your suggestions.

Thanks everybody!

-Dave

doozerdave · 25th February 2012, 02:09 AM

Okay, so frustration is reaching its peak here... I've tried everything that has been suggested and still the noise is there. Right now I've got UF4007 diodes in there (which significantly reduced switching transients) with 1nF poly caps in parallel with each of them. I also increased the main reservoir cap to 70uF total, and this made a small difference to the ripple (obviously) but had no effect on the noise.

What's happening is the HT sine wave, just before it goes into the diodes is getting distorted. I think this is because each 1/2 cycle, 1/2 of the xfmr is unloaded, and the collapsing field is being nasty. I suspect this noise is coupling into the heater supply, and the noise on the heaters is coupling into the grids.

Here are some scope captures.

HT top:

HT bottom:

Heater supply:

Noise spikes (120Hz, 20mV/div) at stage 3:

Heaters when on standby:

Any ideas on this? FYI, both phases on the HT show the same noise pattern.

Tomorrow I'll try wiring up a second xfmr to drive the heaters separately to prove the noise coupling into the grids theory, or at least to prove that the noisy heaters are the noise source.

Thanks for any help. I'm at a loss here... considering punching a new hole in the chassis and wiring in a 5U4 but that seems like admitting defeat. Lots of amps use CT xfmrs with series SS rectifiers without trouble...

doozerdave · 25th February 2012, 02:24 AM

One more thing, I'm pretty sure I'm not saturating the HT winding. It's rated for 230mA and I've measured 160mA on the 6V6's and there's about 20mA total for the pre-amp.

trobbins · 25th February 2012, 04:52 AM

The HT diode switching transient has been shown to easily couple in to the heater winding and get amplified by the input stage - so yes that is highly likely to be the mechanism for noise injection.

I suggest you should remove the cap bypasses on the UF4007s, they just aren't needed, and may be exacerbating the problem. Adding a bypass cap across the HT winding itself, and/or adding some series resistance to the HT winding are likely to be your best path - and perhaps adding a bypass cap across the heater winding as close as possible to the transformer. The aim is to minimise dI/dt at diode switching spike, and constrain the energy in the leakage inductances to just the transformer windings.

19th February 2012, 12:46 AM	#11
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	To alleviate the HT supply rectifier noise issue then quite rightly you need to look at 'loop area', and also in that context I suggest bypassing each half HT secondary winding with a small 10nF cap as close as possible to the Tx - this reduces the loop area of the transient current to the winding itself, rather than taking a longer path through the associated diode and supply filter caps. The 10nf cap needs to 630VAC type rating. If possible, twist the 3 secondary HT wires as much as possible from the transformer out to the diodes and 0V (and the bypass 10nF caps if using them). The elevated DC bias for the heaters could introduce noise from the B+ supply if the bias is not well ac bypassed, or star grounding introduces noise between the bias supply 0V and the input stage 0V reference. You can dampen the diode noise level by inserting some series resistance with the diodes. Imho I'd remove the capacitor bypass (or RC) across each diode, and focus on the other measures first. Last edited by trobbins; 19th February 2012 at 12:49 AM.

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19th February 2012, 01:39 AM	#12
tauro0221 diyAudio Member Join Date: Nov 2008 Location: Victoria,TX	Hi, I think your over loader the 1N4007's diodes. The max. current for the 1N4007 is 1 amp. If you wire all the filaments in parallel your load is about 3 amps. I will use the 1N5408 instead the 1N4007 that is 1000 volts 3 amps. That's maybe what it is causing the noise that your experiencing.

19th February 2012, 04:37 AM	#14
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	The OP is running normal AC heaters - his issue relates to the diode rectifiers for the B+ supply.

19th February 2012, 12:44 PM	#16
DF96 diyAudio Member Join Date: May 2007	Yes, resistors will do the job just as well.

19th February 2012, 02:06 PM	#17
doozerdave diyAudio Member Join Date: Feb 2012 Location: Kitchener, Ontario	Thanks for all the input. A few points: I need to keep the 70Vdc offset (which is confirmed to be super clean) to protect the cathode followers. My calculations for 4 6V6's in push-pull-parallel, plus the triodes shows that 47uF is plenty to get me lower than 10% ripple, and I'm currently only seeing 3% ripple. I don't want to go too high with the main reservoir and risk overloading the xfmr. I'm currently in the middle of improving the layout of stages 1&2. Once I'm finished that, if the problem persists I'll try some of your suggestions. Thanks everybody! -Dave

25th February 2012, 02:09 AM	#18
doozerdave diyAudio Member Join Date: Feb 2012 Location: Kitchener, Ontario	Okay, so frustration is reaching its peak here... I've tried everything that has been suggested and still the noise is there. Right now I've got UF4007 diodes in there (which significantly reduced switching transients) with 1nF poly caps in parallel with each of them. I also increased the main reservoir cap to 70uF total, and this made a small difference to the ripple (obviously) but had no effect on the noise. What's happening is the HT sine wave, just before it goes into the diodes is getting distorted. I think this is because each 1/2 cycle, 1/2 of the xfmr is unloaded, and the collapsing field is being nasty. I suspect this noise is coupling into the heater supply, and the noise on the heaters is coupling into the grids. Here are some scope captures. HT top: HT bottom: Heater supply: Noise spikes (120Hz, 20mV/div) at stage 3: Heaters when on standby: Any ideas on this? FYI, both phases on the HT show the same noise pattern. Tomorrow I'll try wiring up a second xfmr to drive the heaters separately to prove the noise coupling into the grids theory, or at least to prove that the noisy heaters are the noise source. Thanks for any help. I'm at a loss here... considering punching a new hole in the chassis and wiring in a 5U4 but that seems like admitting defeat. Lots of amps use CT xfmrs with series SS rectifiers without trouble...

25th February 2012, 02:24 AM	#19
doozerdave diyAudio Member Join Date: Feb 2012 Location: Kitchener, Ontario	One more thing, I'm pretty sure I'm not saturating the HT winding. It's rated for 230mA and I've measured 160mA on the 6V6's and there's about 20mA total for the pre-amp.

25th February 2012, 04:52 AM	#20
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	The HT diode switching transient has been shown to easily couple in to the heater winding and get amplified by the input stage - so yes that is highly likely to be the mechanism for noise injection. I suggest you should remove the cap bypasses on the UF4007s, they just aren't needed, and may be exacerbating the problem. Adding a bypass cap across the HT winding itself, and/or adding some series resistance to the HT winding are likely to be your best path - and perhaps adding a bypass cap across the heater winding as close as possible to the transformer. The aim is to minimise dI/dt at diode switching spike, and constrain the energy in the leakage inductances to just the transformer windings.