My take on a heater supply...

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Paul Barker
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#16 Re: My take on a heater supply...

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cressy wrote: Wed Apr 12, 2017 9:36 pm That reminds me ed, ive still got pauls pair of sv811-10s
I drew up a cct using a cathode follower to drive them then stalled with the heater supply

I was looking at shoving an smps straight up them bit then got sidetracked by the cold cathode resistance being a potential problem with the smps struggling to start the heaters. Did some reading, but i couldnt find any concensus as to wether it was an issue at all.

These type valves are a pig to heat, a board like nicks is ideal
We used to start them through a resistor and switch it out once heater warm. Didnt take long, 30 seconds or so. I say we, because I did it but after phone calls with Nick if I am remembering correctly. fallable mind I have though, that is true. So much time has elapsed I'd need to relearn all these coaping stratergies. But I was in the realms of 833a smps heated. How dod Phil get on? Think his smps didnt require any assistance. I learned a lot by catastrophy before I arrived at reliability. Until I realised it was all vanity.
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#17 Re: My take on a heater supply...

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Paul Barker wrote: Fri Apr 14, 2017 7:47 am
cressy wrote: Wed Apr 12, 2017 9:36 pm I was looking at shoving an smps straight up them ...
We used to start them through a resistor and switch it out once heater warm.
Oo-er example of convergent evolution - didn't know anything about that yet I was thinking of doing this too. Got a couple of notebook power suppy packs to play with, but not done anything yet.

How dod Phil get on? Think his smps didnt require any assistance.
As far as I know it doesn't, can't remember why but probably cos of internal overcurrent protection. But I'm guessing.

.
 
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#18 Re: My take on a heater supply...

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My SMPS was 12v @ 30A, and I used two 100W resistors and caps to lower the voltage to 10v.

So, I suppose the two resistors are a load.
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#19 Re: My take on a heater supply...

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pre65 wrote: Fri Apr 14, 2017 10:21 am So, I suppose the two resistors are a load.
In series with the heaters, yes.
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#20 Re: My take on a heater supply...

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AH! Ta Phil that makes sense.
 
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#21 Re: My take on a heater supply...

Post by jack »

cressy wrote: Wed Apr 12, 2017 9:36 pmI was looking at shoving an smps straight up them bit then got sidetracked by the cold cathode resistance being a potential problem with the smps struggling to start the heaters.
Trying to directly start a heater from cold with an SMPS will be a problem - most DECENT SMPSs have short circuit protection that drop the output to pretty much zero for so long as the "short" persists...

In this scenario, the SMPS will shut off immediately with a cold, high current, heater and because the heater will then never warm up, the "short" stays so the SMPS does not restart.

This is one of the reasons why an SMPS pre-regulator followed by a slow-start LDO works fine - the initial current is limited by the slow-start h/w, so the SMPS doesn't drop out.

There is a lot of subtlety to this and I'm skipping over a lot, but that's the essence of it.
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#22 Re: My take on a heater supply...

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If you're going to make it current-sourced to the filament, ie. high Zout , then I think the key thing we ( Andrew, Nick and I ) learned from the DHT fil supply project is that the Zout has to be high( ish ) but more importantly STABLE across 10 octaves.
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#23 Re: My take on a heater supply...

Post by jack »

IslandPink wrote: Sat Apr 15, 2017 11:45 am If you're going to make it current-sourced to the filament, ie. high Zout , then I think the key thing we ( Andrew, Nick and I ) learned from the DHT fil supply project is that the Zout has to be high( ish ) but more importantly STABLE across 10 octaves.
High-Zout is a prime requirement - it's a mandatory requirement - Rod and everyone else agrees that you must have a high dynamic output impedance.
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#24 Re: My take on a heater supply...

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Yes but unfortunately I don't think Rod's manage this. I found the treble a bit shut-in on his, and with a lot of faff and experiments we found a way of getting them better so the top-end tone and sparkle came through.
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#25 Re: My take on a heater supply...

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I thought the "secret" of Andrews units (like wot I use on GK-71) was that they were current and voltage regulated ?
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#26 Re: My take on a heater supply...

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pre65 wrote: Sat Apr 15, 2017 1:52 pm I thought the "secret" of Andrews units (like wot I use on GK-71) was that they were current and voltage regulated ?
No, the "secret" is Marks beta testing.
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#27 Re: My take on a heater supply...

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Nick wrote: Sat Apr 15, 2017 2:25 pm
pre65 wrote: Sat Apr 15, 2017 1:52 pm I thought the "secret" of Andrews units (like wot I use on GK-71) was that they were current and voltage regulated ?
No, the "secret" is Marks beta testing.
In that case, well done Mark. :D
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#28 Re: My take on a heater supply...

Post by Andrew »

jack wrote: Wed Apr 12, 2017 3:21 pm Seems that I can only add three attachments to a post, so here is an LTspiceXVII simulation run of the ramp gen - the output current of the board follows this precisely.
Hi Nick,

Couldn't you use a buck-boost switcher that has a ramp generated slow start on it?

cheers,

Andrew
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#29 Re: My take on a heater supply...

Post by jack »

Andrew wrote: Sat Apr 15, 2017 6:44 pm
jack wrote: Wed Apr 12, 2017 3:21 pm Seems that I can only add three attachments to a post, so here is an LTspiceXVII simulation run of the ramp gen - the output current of the board follows this precisely.
Couldn't you use a buck-boost switcher that has a ramp generated slow start on it?
I had thought of this and did some tests last year - I've designed a few high efficiency (in the 90%+ bracket) SMPSs in my time (there's one on my website), and it's tricky to meet all the design parameters without compromise. Also, whilst you could use the ramp generator to drive the FB input of the SMPS, that only gives you voltage, not current control. Integrated SMPS devices tend to implement OC conditions by stopping the regulator and restarting when the condition clears - that just does't work in our scenario.

In an audio application, even at 500KHz or 1MHz switching rates, residual noise and ripple at high current is difficult to manage, especially when you look at the bandwidth and power distribution of the resulting EMI; everything is critical, including the PCB design.

A postfix linear regulator (or buck-boost pre-regulation) neatly addresses most of these compromises, but it's very hard to get the balance right... one of the challenges being that none of the ultra-low noise linear regulators can deliver more than about an amp, so you have to get that up to 8A or more whilst still keeping the low uVolt noise levels. There are standard ways of addressing the low current issue, but they tend to magnify the noise and ripple proportionally as well...

...and as Mark says, it's not about the technology (though I do like a challenge), it's actually all about the sound. In order to achieve high output currents, high efficiency, total and absolute stability and deterministic acoustic performance over 5 or 6 octaves as well as sounding good... it's hard.

BTW., one of the biggest challenges is with loop stability - there's a fair chunk of control theory at play here - the maths and modelling goes only so far - there's a lot of testing & measuring, i.e. a reasonable amount of AOT.

...hmmmm (or not, as the case may be :) )
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#30 Re: My take on a heater supply...

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IslandPink wrote: Sat Apr 15, 2017 1:45 pm Yes but unfortunately I don't think Rod's manage this. I found the treble a bit shut-in on his, and with a lot of faff and experiments we found a way of getting them better so the top-end tone and sparkle came through.
Mmmm interesting….exactly what I thought, on a 300b amp I built. They were a bit 'dull'….

My ha'penny's worth, there….
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