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Old 24th November 2010, 07:11 PM   #1
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Default Faint Hiss/Hum.

I've built Eli Duttman's El Cheapo PP power amp, with 6BW6 outputs, most components coming from Jim McShane. The amp sounds nice except for a faint noise (between hiss and a hum). It's not that "efffff-ing" valve sound either and it doesn't interfere during play but can be slightly annoying without a signal coming through. It's not the preamp, because it's still there without it.

I've just made a "major surgery" to isolate each section's grounding (including power supply grounds) and brought them to a star-earth connection. The faint noise remained. BTW the heaters are nowhere connected to earth.

Is this normal for a valve amp? Is there anything to do to eliminate such a noise?

Thanks for any comments/suggestions.
Regards,
Joe A
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Old 24th November 2010, 07:29 PM   #2
DF96 is offline DF96  England
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What are the heaters connected to? They need a reference voltage from somewhere. Heater supplies should not be left floating.
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Old 24th November 2010, 07:33 PM   #3
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Thanks DF96, I'm not sure I understand your question properly, but the heaters are daisy-chain connected in parallel to a 6.3V secondary winding.
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Old 24th November 2010, 07:53 PM   #4
SY is offline SY  United States
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Is that 6.3V winding referenced to ground? If you leave it floating, annoying sounds can happen.
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Old 24th November 2010, 08:10 PM   #5
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This is a good read


The Valve Wizard
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Old 24th November 2010, 08:15 PM   #6
DF96 is offline DF96  England
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If the 6.3V secondary has a centre tap, ground that to somewhere near the HT supply ground. If not, try grounding one side or the other side. One may give less noise so experiment. An ungrounded heater supply can pick up noise via capacitive coupling in the mains transformer.
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Old 24th November 2010, 08:32 PM   #7
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Quote:
Originally Posted by SY View Post
Is that 6.3V winding referenced to ground? If you leave it floating, annoying sounds can happen.
Not necessarily. I left the heater winding floating for one project that uses a power xfmr with multiple secondaries (5.0V / 3.0A for 5U4GB; 6.3V / 6A; 650Vct / 150mA). The positive rail is supplied with a 5U4GB, and has a solid state negative rail. The negative rail hits 450Vdc within a couple of seconds, and would really bust the spec for Vhk for the 6FQ7s it feeds.

I tried it with floating heaters before resorting to Zener protect diodes, and all I get is a 6.0mVp-p, 60Hz, residual at the output. You can barely hear it, and if not for the glow, you'd never know it was on.
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Old 24th November 2010, 08:52 PM   #8
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Either ground one side of the heater or connect it to a positive voltage of up to +80volts. If output tubes are cathode biased, simply connect the heater to the cathode.
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Old 25th November 2010, 08:02 AM   #9
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Thanks a lot for all that useful information. In my case the filament winding (one of multiple secondaries) is not centre-tapped. It is connected as Miles Prower describes (before the Zener protection) and although I don't have the necessary test equipment to measure it, the noise I get is something similar.

I also read the Valve Wizard post (thnx Soonerorlater!) and I will experiment with the various methods described by contributors.
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Old 25th November 2010, 02:36 PM   #10
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I connected a potential divider network (220R each leg) across the heater winding and connected the junction point to ground as proposed in the Valve Wizard link. While the noise has imperceptibly been reduced there is still some residual noise. Nothing serious but it's still there. To my ears it sounds like it is transformer noise, which is a toroidal affair carrying all secondary voltages.

OTOH, other members may have a different view.
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