• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Faint Hiss/Hum.

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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.
Joe A
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.
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.
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.

Low frequency hums can be deceptive ( particularly if your ears are old and abused like mine). I was swapping around power transformers on an EL84PP amp once when it developed a hum that I just couldn't track down. Eventually I realised the hum wasn't coming from the speakers but was resonating from the transformer itself. The transformer (salvaged from an old console) just didn't have the output current to power it and was making it's complaint known!

The lesson to me was that assumptions are just that - assumptions!

Brgds Bill
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