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Old 24th Nov 2019, 11:55 pm   #1
Join Date: May 2005
Location: Adelaide, South Australia
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Default Filament wiring 'Phasing'

In the lead up to my recent Fender amp build I checked out a lot of other scratch amp builds just to see how others had done it and more than once other constructors mentioned correct phasing of the heater/filament wiring i.e. bridged pins 4 and 5 of the pre-amp valves are all on the same daisy chained twisted filament wire similarly pin 9 of the pre-amp valves are all on the other daisy chained twisted filament wire - I hope my explanation is clear, the idea being that this aids in reducing hum. I can't see that it would make any difference as the filament voltage is AC and indeed I did not bother to do this and both of my recent amp builds were very acceptably quiet - standing around 1m away from the amp with volume on 10 I could hear a faint hiss put my ear next to the speaker grill I could hear a faint hum.

Is this 'correct phasing of the filament wiring' just another keyboard expert idea or is there something to it ?
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Old 25th Nov 2019, 12:46 am   #2
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Default Re: Filament wiring 'Phasing'

There is a lot to "Filament" polarity, in vintage radios, as the filament is the cathode emitting surface and this affects the average voltage with respect to the control grid.

You are talking about "Heater" phasing, all of the valves your are using are indirectly heated. In this instance the phase of the applied AC is usually academic. Generally in many amplifiers, hum minimization is done by not connecting either side of the filament supply directly to ground (common), with a wire wound potentiometer placed across the heater supply, with its slider earthed, and a position will be found with minimum overall hum.

Most of the coupling happens in the early high gain pre-amp stages, especially the input valve. One way it gets in is varying amounts of heater-cathode leakage/coupling in the input valve, which varies with different specimens of the same valve. It is very important that the input stage and perhaps the second stage, that it has a cathode that is properly bypassed at low frequencies with a suitable electrolytic capacitor, usually 25 to 47uF, this eliminates the varying amount of coupling at that point.
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Old 25th Nov 2019, 8:58 am   #3
David G4EBT
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Default Re: Filament wiring 'Phasing'

Not to muddy the waters, the topic of AC v DC heater supplies, especially for pre-amps, often crops up on internet forums such as 'DIY Audio'. It's not something I have a view on and I'm not into amplifier building or restoration, but there seems to be pros & cons to each approach.


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Old 25th Nov 2019, 11:12 am   #4
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Default Re: Filament wiring 'Phasing'

As Argus25 says, the 'traditional' way to do this is with a 'hum-dinger' potentiometer (centre-tap to earth) wired across the heater winding to provide an adjustable 'virtual centre-tap' and let the hum be balanced-out.

In some cases the centre-tap will not be earthed but instead connected to a positive voltage (something around 50V, often derived from a potential-divider across the HT supply) - this is to stop the exposed-parts-of-the-heater-that-can-see-the-anodes from also acting as a supernumary cathode and providing a hum-modulated supply of spurious electrons.
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Old 26th Nov 2019, 12:02 am   #5
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Default Re: Filament wiring 'Phasing'

Another very neat and highly effective method is to use the cathode voltage of the output pair. This only works with Cathode biassing of course.

Instead of a "hum-dinger" I also wire my 12##7's from 12 volts going to 4 and 5, I then earth pin 9 of the phase splitter as my "earthed" heaters.

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Old 26th Nov 2019, 1:31 pm   #6
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Default Re: Filament wiring 'Phasing'

I did not really give it much thought but as Argus pointed out the preamp valves are indirectly heated so it shouldn't make any difference which way around the filament wires are connected and yes H/K leakage does cause hum - I have a few shoe boxes of ECC83 preamp valves and simply grabbed a hand full and plugged them in without testing any - on powering up the amp for a sound trial I noticed a slight hum that got louder with each minute - replacing the first preamp valve cured the hum - on the valve tester the suspect valve came up with around 1/2 meg H/K resistance slowly decreasing as the valve warmed up.

If I wind a mains transformer for an amp build I always (if possible) use a 3.4V bifilar wind for the filament supply - that way I'm sure of an exact centre tap.
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Old 26th Nov 2019, 2:42 pm   #7
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Default Re: Filament wiring 'Phasing'

An exact centre tap is handy if that's what you need. But I've come across designs where the 50Hz hum from the heaters (adjustable via the humdinger) is used to null out 50Hz leaking in from other sources e.g. magnetic flux from the mains transformer, or even ground loops. For that you need to be able to offset the heater contribution away from the centre position, where all it would really be doing would be minimising itself.

The problem then arises with higher harmonics - commonly, for symmetry reasons, odd ones. Now we can set the humdinger to null the 50Hz component, but there may well be 150Hz and 250Hz ones too. The heater waveform itself may (roughly) replicate the mains waveform, which these days can be very far from a pure 50Hz sinusoid. Flux leakage from a transformer can have its waveform strongly modified by the iron's nonlinear B-H curve. Contact resistances which convert ground loop currents into noise voltages can be nonlinear too. So we end up with a number of hum sources each of which has a different waveform and harmonic spectrum. The question becomes whether to null the overall 50Hz component, which might have the largest amplitude, or, say, the 150Hz one, to which the ear might be 16-17dB more sensitive This is when the alternative approach of eliminating the hum at its sources by using DC heaters, well-behaved and positioned mains transformers, ground loop current reduction etc really pays dividends.


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