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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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20th May 2020, 6:26 pm | #1 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
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Valve heater wiring - phase?
This may be the wrong section of the forum, but here goes.
I have been considering the way that valve heaters in audio amplifiers are wired. I can understand that identical valve types might benefit from having their heaters wired in phase (ECC8_ types; pairs and quads of similar output valves), but is there any reason for wiring the heaters of non-identical valves in some "phased" system and how might this be done, in practice? For that matter, is there any advantage in wiring the heaters of similar valves identically? Could it be that hum will be kept to a minimum, and if so, is this purely by experimentation, or is there a sound (pun not intended) reason? Given that the heaters of some valves (notably the ECC8_ series) have their heaters constructed with two end connections and one centre-tap, if accurately and identically manufactured, wouldn't these heaters be naturally "out of phase"? I can see that a push-pull pair might have less hum wired "in-phase", but what might happen with a quad of identical output valves were wired with each side pair "out-of-phase"? Wouldn't that further reduce hum? Colin. |
20th May 2020, 6:29 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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Re: valve heater wiring - phase?
Given that in a typical situation you've only got one phase available from the heater-transformer, I don't really understand your question.
[runs away at the prospect of audiophools demanding three-phase supplies for their 'boutique' amplifiers]. |
20th May 2020, 7:33 pm | #3 |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
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Re: valve heater wiring - phase?
I guess with ‘Phases’ you mean different windings of the mains transformer? If this is the case, then, yes, this is to minimise hum. The ‘upper end’ amps did have separated windings for pre amp valves, power valves and rectifier valves.
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20th May 2020, 10:08 pm | #4 |
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Re: Valve heater wiring - phase?
Separate heater windings for varios stages means you can "hum buck" them individually. Perhaps that is what was meant.
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20th May 2020, 10:17 pm | #5 |
Moderator
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Re: Valve heater wiring - phase?
Not sure there's any reason for a difference. Most cathodes are a coated tube, and inside them the wire of the heater is coated in ceramic insulation and zig-zags up and down. So whichever end is which from the heater winding, it's going to be symmetrical. If the heater structure stuffed into the cathode is symmetrical, it cannot matter which end is which.
David
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20th May 2020, 10:47 pm | #6 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
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Re: Valve heater wiring - phase?
What might make a difference is if one side of the heater supply (rather than a centre-tap, or a 'synthetic' centre-tap made using two resistors) is grounded. An ECC83 powered from a 12.6V supply of that kind would have one triode with its heater grounded permanently at one end and with the other side oscillating up and down at 6.3V RMS. The other triode's heater would experience much larger excursions from ground. If the two triodes made up two consecutive amp stages then would we want the first one, handling the smaller signals, to be the one with its heater closer to ground ?
I note Andy's (yestertech) query here https://www.vintage-radio.net/forum/...d.php?t=166821 about the heater supply in his Pye Mozart pre-amp being offset asymmetrically about ground. Cheers, GJ
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20th May 2020, 11:00 pm | #7 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
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Re: Valve heater wiring - phase?
Valve heaters could be 'phased' in the same way that drive units in a loudspeaker are phased.
Take an EF86 for example. A purple wire could be connected to pin 4 on one valve and pin 5 on its neighbour. A yellow wire would then be connected to pin 5 on the first valve and pin 4 on the second valve. Both valves are supplied with AC to the heaters but they are 'technically' out of phase. If the capacitance to the cathode or grid is not identical from each heater 'end', a small voltage imbalance could be created. |
21st May 2020, 1:30 am | #8 |
Heptode
Join Date: May 2005
Location: Adelaide, South Australia
Posts: 541
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Re: Valve heater wiring - phase?
I've seen this 'phasing' comment in diy amp build logs, some say make sure that all pin 4's are on the one wire etc. I have come to the conclusion for me at least it does not make an audible difference to the amount of hum that can be heard, there may be some theory that supports 'phasing' but to my ears and my guitar playing son's ears it made no difference, we tried both phasing and non phasing heater wiring layouts, there was a small amount hum in both instances but could only be heard with the volume set full if you put your ear right up next to the speaker.
What I did find made a difference was to keep the heater wires tightly twisted, run them around the chassis edge in the corner and and on 9BA sockets ground the centre shielding spigot. If you have already built the amp then it probably it not worth changing it, but if you are just starting then then it's no extra work to phase the wiring and you can judge for your self. |
21st May 2020, 1:54 am | #9 |
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Re: Valve heater wiring - phase?
Who says which pin is right for one particular phase when different valve types are involved?
But, as a matter of professionalism, if an amplifier is put into production, it would make sense for all to be wired the same. It is expected to make no difference, it probably makes no difference, but if something unexpected did occur at least there would be less confusion. David
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21st May 2020, 9:05 am | #10 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
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Re: Valve heater wiring - phase?
I am of the opinion that it is external wiring which is the cause of hum pick up ,a twisted pair fitted close to the chassis and routed away from the input stage ,earthing one side may cause problems , Hum bucking pot may help if you have problems ,this re constructed Pye amp gives excellent results , with no hum whatsoever ,I had to use the scope to adjust the pot , Mick.
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21st May 2020, 9:20 am | #11 |
Octode
Join Date: Dec 2017
Location: Morden, Surrey, UK.
Posts: 1,552
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Re: Valve heater wiring - phase?
I used to use solid core black twisted pair to wire heaters when I made valve amps, years ago - no easy way to check phase nor did I expect it to make any difference - in a low noise application use DC heaters.
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21st May 2020, 9:32 am | #12 |
Octode
Join Date: May 2010
Location: Grantham, Lincolnshire, UK.
Posts: 1,172
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Re: Valve heater wiring - phase?
Surely the thermal lag of the heater at 100Hz makes the direction of connection irrelevant? Or, as suggested, use DC.
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21st May 2020, 10:08 am | #13 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
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Re: Valve heater wiring - phase?
OK, when I wrote the first post, I think that I could see some confusion with my use of the term "phase". I'm sorry if that did happen to some. Let me be more specific -
I am rebuilding a Vox AC30 guitar amplifier and I came across two features. The heater wiring is two twisted, solid-core wires. One of these is yellow and the other is blue and there is only one 6.3VAC winding on the mains transformer for this purpose. On the four EL84 output valves, all pins #4 are connected with a blue wire and all pins #5 are connected with a yellow wire. when it comes to the preamp valves (four ECC83 and one ECC82 types), pins #4 and #5 are connected together and to a blue wire, with pin #9 connected to a yellow wire. It makes it look pretty if nothing else. one valve-holder has pins #4 and #5 earthed; I know this isn't best practice for lowest hum, but it is traditional. The second feature is that the twisting of the heater-wires isn't very even and tight, so I am planning on replacing all of the heater wiring with better-twisted wire. My intention is to wire up the heaters exactly the same way that they were originally (except for routing the wires better than they were originally). It looks neat and pretty, so I'll do it that way, but does it offer any practical advantage? Colin. |
21st May 2020, 10:09 am | #14 |
Dekatron
Join Date: Feb 2007
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Re: Valve heater wiring - phase?
There's at least 3 things going on:
Thermal effects - at 100Hz, should be negligible due to thermal inertia (one of the reasons for making cathode tubes with a significant heat capacity). Magnetic effects - due to heater current. Depending how heater is coiled within the cathode tube, an AC magnetic field might exist which could modulate the electron stream. Electrostatic effects - coupling from each heater pin to sensitive electrodes is likely to be asymmetric. So, if one side of heater is grounded, it might be better to ensure that one pin rather than another is grounded (ISTR that Mullard suggested pins 4 & 5, rather than pin 9, were grounded in low-level applications of ECC83 if it was not possible to have a balanced heater supply). And similarly, if using a potentiometer across heater supply with wiper to 0V, rather than a centre-tapped transformer winding, it may just be that minimum hum occurs with the wiper not quite central. As to phasing, then unless the heater supply phase is correlated to the actual signal going through the circuit, no this won't matter. |