DHT Heater using VCCS - PCB or Kit (Possible Group buy)

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RhythMick
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#916 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Ta muchly
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#917 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

Most of what the sites says won't apply to a VCCS, since it will deliver exactly the amount of current required to match the reference voltage, regardless of the cathode tolerances and age. I do agree with the point about switching on the HT slowly, there are plenty of ways to achieve this.

Equally, and I think Nick already said this, I don't see any reasonable suggestion on what you should use instead, perhaps I am missing something?

AC will inter-modulate and any voltage source will see the music signal as a varying voltage and attempt to regulate your precious music signal.

I have run the "bad idea" quite happily with these boards without any noise. I won't argue you can't do better, just saying its the lesser of evils, or in engineering terms, a better set of compromises than either AC or a voltage source.

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RhythMick
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#918 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Nick wrote: Sat Mar 24, 2018 4:34 pm
a couple of 10R or 22R would be my choice in most cases.
Sorry I'm having a bit of a mental block thinking about the virtual cathode thing. I might have untangled it in the process of typing the question and editing it as I've worked through it - grateful if someone could check I haven't gone off the rails. It's been known !

Say we have a single DHT, let's say it's a 45, with a DC board across the cathode pins. The board is set to deliver the required 2.5v @ 1.5A across the filament. There's only 1 path for that current to take, which is up through the pins and across the filament. All is good.

If we now connect a pair of resistors (say 10R) across the pins to form a virtual cathode, we've provided a parallel path for the current delivered by the DC board? So heater current is going to flow through those resistors?

The resistance of the valve filament when hot is around 1.7R, much lower when cold. As the filament heats up isn't current going to increasingly flow through the resistor pair? How does the DC board react when that parallel path is put in place? Based on Andrew's comment above "it will deliver exactly the amount of current required to match the reference voltage" I think the board will deliver that additional current? 2.5v across 20R = 125mA through the virtual cathode resistors?

The board doesn't know what's connected across it's output pins, it sees a resistance which is the filament resistance in parallel with the 20R. So the 1.7R has dropped to 1.54R and the board will increase the current to achieve the 2.5R.

Same applies whether DC board or AC transformer, and whether its a pair of resistors or a Humbucker. I've been using a 10R Humbucker (with 22R wings) on my SE amp without issue. 2.5v across a 54R resistance should draw 46mA through that path? Is that right?

I think I have this right now partly because I've typed out the question and thought it through better, I guess I just never considered heater current going through those resistors (or humbucker) before, which obviously affects the dissipation calculation. Instead of 10R * 15mA^2 (I have 30mA going through the 45) giving .00225W, we have 10R * (.125 + .015)^2 = 0.2W. I use a safety factor of 4, so I need >= 1W at that point. It also slightly increases the power dissipated on the DC board itself, from 6.8W for the 45 to 7.4W. Not an issue I think, but I just hadn't considered it.

Or I'm missing something, not for the first time? Do I need to stand in the corner with the pointy hat again sir?
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#919 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

I think you have it right. I could kick in with why I think a centre tap is worth doing and in fact why a hum pot works at all (not something I have ever seen written down), but it will be a distraction.
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#920 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Nick wrote: Wed Mar 28, 2018 8:48 am I think you have it right. I could kick in with why I think a centre tap is worth doing and in fact why a hum pot works at all (not something I have ever seen written down), but it will be a distraction.
Thanks - 2 years ago I was a total duffer as regards elastic-trickery. Could NOT get my head around the difference between volts and amps! It's intensely rewarding for me when I can tease my way through something like that, especially when someone says I have it right.

PLEASE kick in with VC/Humbucker explanation, I'd be very interested. Perhaps a PM or separate thread.
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#921 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

RhythMick wrote: Wed Mar 28, 2018 7:48 am Instead of 10R * 15mA^2 (I have 30mA going through the 45) giving .00225W, we have 10R * (.125 + .015)^2 = 0.2W. I use a safety factor of 4, so I need >= 1W at that point.
Actually there will be 3 sources of current going throug those 10R virtual cathode resistors...

Heater current as above : 125mA DC
Operating point : 30mA /2 = 15mA DC

Signal current : At peak volume the 45 could swing from 55mA to 15mA, so 40mA peak to peak which would be 28mA RMS. Let's call it 30mA / 2 = 15mA.

So for dissipation purposes I should plan on 155mA through each resistor. 10R * 0.155^2 = 0.24W. 1W would still be ok, but it's higher than I'd thought it would be.
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#922 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Andrew wrote: Fri Mar 23, 2018 7:07 pm A 5v tx, if you can get one, will generate less heat.
It's PCB mount, but the Hammond 162H10 at the link below has 115/230v primary plus 2 secondaries, each 5v @ 2.0A. They are quite small (approx 60mm x 50mm x 50mm) but I've ordered some and will give them a go when I get the boards.

http://www.hammondmfg.com/162.htm
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#923 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

Yep, I would agree, you have a parallel path. The VCCS won't care, but it will cost you more current and more heat.

Despite agreeing with Nick on the virtual cathode argument and the potential benefits that he is alluding too, I remain unconvinced on why this set of compromises, presented in so-called the "bad idea" isn't actually the least worse engineering solution for this particular problem.

Especially, if you have a) a decent VCCS and b) one which is pretty darn quiet in the first place.

Happy to be proved otherwise, of course as that would imply room for improvement, which is a good thing.

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#924 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Andrew wrote: Thu Mar 29, 2018 12:46 pm Despite agreeing with Nick on the virtual cathode argument and the potential benefits that he is alluding too, I remain unconvinced on why this set of compromises, presented in so-called the "bad idea" isn't actually the least worse engineering solution for this particular problem.

Especially, if you have a) a decent VCCS and b) one which is pretty darn quiet in the first place.
I plan to start simple, by taking the cathode tail from joining either the + or - cathode pins. Once I've got a decent sound to compare to I'll try the virtual cathode approach. For example this is the 26 pair.
Capture.JPG
Last edited by RhythMick on Thu Mar 29, 2018 1:53 pm, edited 2 times in total.
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Nick
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#925 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

Especially, if you have a) a decent VCCS and b) one which is pretty darn quiet in the first place.
Agreed, but the quiet VCCS wont protect you from external common mode noise sources, thats what I was trying to reduce.
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#926 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

RhythMick wrote: Thu Mar 29, 2018 8:32 am
Andrew wrote: Fri Mar 23, 2018 7:07 pm A 5v tx, if you can get one, will generate less heat.
It's PCB mount, but the Hammond 162H10 at the link below has 115/230v primary plus 2 secondaries, each 5v @ 2.0A. They are quite small (approx 60mm x 50mm x 50mm) but I've ordered some and will give them a go when I get the boards.

http://www.hammondmfg.com/162.htm
Just as a check I mailed Hammond UK to check that the secondaries can be used independently, with this reply.

"Yes the secondary’s can be used separately but the insulation breakdown is very weak.

We would say about 100 - 150 volts. It’s definitely high enough to allow the rated voltage to be isolated but may not be useful for anything further.

Please also note there is really poor isolation in doing so."

Given that these will be at the same voltage and with subsequent smoothing and regulation I see no issue.
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#927 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Mike H »

Dare I suggest you don't really need two resistors to make a centre tap at all, because it's not an AC heater supply but DC, no hum nulling is required, so just choose one side of the DC heater (aka one end of the cathode filament) to be the cathode connection for anode and signal currents.

Or should I get my coat?

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#928 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

That's where I started Mike, but Jac (Lundahl reseller) has my respect and for the most part I listen to his advice, even if I don't always take it. Also Nick seems to believe there is merit for common mode noise cancellation. I'm willing to try it when I have a comparable system.
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#929 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by pre65 »

On my 811a amp rebuild I will be using a Tentlab module (845 type) and I will be using grid bias.

Can I still use the two resistors centre tap and a 1R sensing resistor to ground ?
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#930 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

pre65 wrote: Thu Mar 29, 2018 8:20 pm On my 811a amp rebuild I will be using a Tentlab module (845 type) and I will be using grid bias.

Can I still use the two resistors centre tap and a 1R sensing resistor to ground ?
Yes.
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