Snubber on power transformer primary

I have built a small amplifier with a pair of ECL82/6BM8 valves in single ended configuration and a EZ81 rectifier valve (with 50uf - 3H - 50uf filter) PSU
I notice occasional pops from the speakers when switching the mains off. I suspect this is coming from back EMF from the power transformer primary, causing short duration spikes on switch-off. My transformer is a Hammond DAX370, with the primary configured for 240VAC
I am considering fitting a 100nF + 100R snubber across the 240V primaries. Has anyone any experience of using a snubber this way?
 
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I'd go with a 400 VAC rated cap for 240 V mains. You might get excursions up to 264 V.

I bet the pop is caused by arcing in the mains switch. You may find that a snubber across the switch works better to prevent the pop than a snubber across the primary. It'll also help prevent wear on the switch points.

Tom
 
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Me, I'd try the snubber-across-load (in this case, across the power transformer primary) first. Only if this arrangement's behavior fails to satisfy you, reluctantly try snubber-across-the-switch as your Plan B, then decide whether it increases your satisfaction. Keep the load-energy-dump as local as possible.
 
I bet the pop is caused by arcing in the mains switch. You may find that a snubber across the switch works better to prevent the pop than a snubber across the primary. It'll also help prevent wear on the switch points.

Tom
Exactly as PR Mallory described the action of the "Quench-Arc"

I have a bag of them from a heavy equipment company in Ohio -- to bad you're in England or I'd send a dozen!
 
I bet the pop is caused by arcing in the mains switch. You may find that a snubber across the switch works better to prevent the pop than a snubber across the primary. It'll also help prevent wear on the switch points.
The source of the transient is usually the inductor, specifically the leakage inductance of the primary, so logically place the snubber across the transformer primary, with short leads/low loop area. However mains voltage is high enough without back-EMF considerations to cause arcing anyhow - some switches are better than others mechanically (break the contact faster) and wear and tear on a switch can affect this.

Placing a snubber across the switch will lead to electricity consumption when the unit is switched off, note: 100nF will leak 7.5mA at 240V, which is 1.8W of apparent power on your bill (about 16kWh per year).

Its also possible to get a shock through a snubber. 7.5mA is definitely enough for that.
 
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I have built a small amplifier with a pair of ECL82/6BM8 valves in single ended configuration and a EZ81 rectifier valve (with 50uf - 3H - 50uf filter) PSU
I notice occasional pops from the speakers when switching the mains off. I suspect this is coming from back EMF from the power transformer primary, causing short duration spikes on switch-off. My transformer is a Hammond DAX370, with the primary configured for 240VAC
I am considering fitting a 100nF + 100R snubber across the 240V primaries. Has anyone any experience of using a snubber this way?
Yes, I do that a lot.

LCR in Wales make/used to make these , in 100nF/100R and 220nF/100R, with Class X2 rated caps for UK mains.
They Work. Try your snubber - and keep the wiring as short as practicable to the Trafo primary connections.

NB - even small caps used in a snubber across a switch, at 240VAC, pass enough current for a sting' ; the limit here would require class Y caps, at 2.2nF; less effective. And still enough to feel ... quite fizzy. But way less effective.
 
The source of the transient is usually the inductor, specifically the leakage inductance of the primary, so logically place the snubber across the transformer primary, with short leads/low loop area. However mains voltage is high enough without back-EMF considerations to cause arcing anyhow - some switches are better than others mechanically (break the contact faster) and wear and tear on a switch can affect this.

Placing a snubber across the switch will lead to electricity consumption when the unit is switched off, note: 100nF will leak 7.5mA at 240V, which is 1.8W of apparent power on your bill (about 16kWh per year).

Its also possible to get a shock through a snubber. 7.5mA is definitely enough for that.
That was my thoughts Mark. I have fitted my R-C snubber across the transformer primary leads and this seems to have stopped the pops on switch-off.
I was not comfortable fitting a snubber across the input and output of the switch (or two, as I have a 2-pole switch for isolating Live and Neutral). Apart from the energy leak you mention, I would probably be tripping the RCD circuit breaker in my consumer unit!
 
Yes, I do that a lot.

LCR in Wales make/used to make these , in 100nF/100R and 220nF/100R, with Class X2 rated caps for UK mains.
They Work. Try your snubber - and keep the wiring as short as practicable to the Trafo primary connections.

NB - even small caps used in a snubber across a switch, at 240VAC, pass enough current for a sting' ; the limit here would require class Y caps, at 2.2nF; less effective. And still enough to feel ... quite fizzy. But way less effective.
Martin, I found a compact R-C snubber from CPC Farnell and have installed it across the transformer primary leads. So far, so good. (Probably manufactured by LRC!).
 
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100nF will leak 7.5mA at 240V, which is 1.8W of apparent power on your bill (about 16kWh per year).
But you don't pay for apparent power, you pay for real power. A snubber's impedance is nearly all capacitive, therefore the leakage current contributes nothing to real power. The units are 1.8VAR, not 1.8W. You pay $0 for VAR, and kWhr is close to 0 per year.