Overly complicated 1.6A CCS Heater Supply - diyAudio
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Old 31st January 2012, 02:19 AM   #1
dtproff is offline dtproff  United States
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Default Overly complicated 1.6A CCS Heater Supply

I built this unit for myself so I put into it the features that I wanted.
  • Operates from a 6.3A transformer (this complicated things)
  • 1.6A CCS with a 400mA standby
  • The Standby signal is opto-isolated so I can float the whole supply
  • Voltage controlled Soft-start for cold heaters filaments
  • Scalable architecture

So there it is.

Tony
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File Type: pdf CCS Heater Supply Rev B.pdf (27.8 KB, 235 views)
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Old 31st January 2012, 03:56 AM   #2
anatech is offline anatech  Canada
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Hi dtproff,
You have me wondering. Are you running a bunch of heaters in parallel with a constant current source by chance? This raises a number of concerns if that is the case.

I haven't followed the entire schematic (since it's much easier to ask you). I'm going to guess you keep the heaters partially on while in the off mode. Since you do have a soft-start built in, is the partial on state necessary? In these days of high efficiency everything, I'm surprised at the 400 mA delivered in the off state. There isn't much of a plus keeping the heaters warmed up unless you are dealing with mercury vapour rectifiers or thyratrons. Those heaters would be running a little hotter then anyway.

Floating the heaters is a method I use often to reduce heater coupled noise. Are you going negative with that, or positive? Looks impressive, and I am curious now.

-Chris
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Old 31st January 2012, 10:14 AM   #3
DF96 is offline DF96  England
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You do ensure that the HT supply is removed when the heaters are on 'standby', don't you? Otherwise you are setting up the valves for cathode damage.

By the way, what is the point? Most valves work best with a constant voltage supply; that is what they were designed for. However, a set of valves run in parallel from a CCS means that each one individually sees an approximation to a voltage supply so I suppose they will be happy.
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Old 31st January 2012, 02:23 PM   #4
dtproff is offline dtproff  United States
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Chris;

No... this is for a single 1.6A heater coil.

If I were doing a CCS <1A I would use a standard TL431/transistor circuit with a diode/cap/resistor for the soft-start function and use a MOSFET to parallel a resistor on the current sense resistor to get the standby I needed. However because it was 1.6A and a TL431 needs 1.24V the I2R losses would have meant that I was dissipating more power in the enclosure than I wanted.

The soft-start function is for the heaters and not the valve. I have an analog compandor that looks at the audio port (rigged as a dB meter) if I am quiet for too long (I forgot to turn the unit off when I wasn't using it) then it flips me into standby mode. By the same token, when it wakes things up (mutes the input till everything stabilizes out), I delay the HT line until the heaters hit temperature and then turn the AC mains on at the zero crossing. I do the same for disconnecting from the mains in that I wait for the zero crossing to disconnect the mains.


DF96

The HT mains is disabled when it goes to standby.

This is strictly about noise reduction, valve life, and power dissipation in the box.

Not totally sure I agree with you that constant voltage is the bests solution. Think of the filament on a standard bulb. It usually pops because of inrush current. The coil is cold and at a low impedance so if you happen to turn on with a 90 degree phase angle on the AC mains then the inrush current can be quite substantial (remember the voltage regulator is not in regulation yet because the power supply is still starting up). Even ignoring the inrush current at various phase angles the 5ms time it takes for the AC mains to reach it's peak voltage doesn't provide a lot of time for the heater coil to get to the correct temperature and resistance. So for a period of time you put more current through than you would probably want.

Overall, does this make a huge difference? Maybe not. As Chris said just because you can doesn't mean you should. I did this because this is what I wanted. I offer my thought process so you can judge for yourself.

Tony
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Old 31st January 2012, 02:37 PM   #5
DF96 is offline DF96  England
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OK. You could achieve broadly the same result by doubling the voltage and using a fat dropper resistor. Boring, though!
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Old 31st January 2012, 04:29 PM   #6
dtproff is offline dtproff  United States
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Yes... As I said, it is an overly complicated circuit. However, it does accomplish exactly what I wanted using the 6.3Vac power transformer.

I would be interested in hearing how others would handle the same task for the same features using a standard 6.3Vac transformer and little power dissipation in the box. I did a switcher version running as a CCS source. I got it working the way I wanted but ultimately went this way since I didn't want a switching PSU in the box.

Tony
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Old 31st January 2012, 04:31 PM   #7
dtproff is offline dtproff  United States
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Quote:
Originally Posted by DF96 View Post
OK. You could achieve broadly the same result by doubling the voltage and using a fat dropper resistor. Boring, though!
Yes but at 1.6A and a 6V drop would be 9.6W. I would call that pretty exciting actually. 10W power dissipation and another 10W for the heater means 20W total. Yuck!

Tony
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Old 1st February 2012, 12:45 AM   #8
tomchr is offline tomchr  United States
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What filament voltages are you looking to support with this circuit? With the present circuit, you may be able to run a 5 V filament if you have a big a** reservoir cap. 6.3 V would be a stretch.

~Tom
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Old 1st February 2012, 01:39 AM   #9
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A few months ago I tried operating ECC88s with series heaters, dc, higher voltage, better smoothing in the power supply and all that. Did I get a shock! (No, not electrically from the 48V odd). Some lit up like a christmas tree while others were dull. Dashing for the turn-off switch, I went back to data: 300mA acording to some, 365mA (!) according to others. So I measured the heater current for some 8 valves at piously 6.30V on the heater. The currents varied from 290mA to 420mA! - some of the same make. I was forced to shelve my efficient series design and go to a rather hot 6,3V output supply.

I have some 25 x 6L6GCs. I dread doing the same measurements on those - not that I would use series heaters there. But wth is going on here with quality control? Or has it been like that all the time (for me that will be over 50 years)? One never measured heater current; the ubiquitious 6V winding did service and that was that. I am also using a dc heater supply for ECF82 input tubes in a power amplifier, and found the same thing there, though not nearly as severe. Still, again I was forced to stick to a 6,3V (hot) heater supply. (All the above are regulated in a pious attempt to cope with the varying mains supply, load-wise and from town to town, in RSA.)

I would like to know whether others had any experiences in this direction. I cannot imagine this is typical of the heater world; I do not have enough examples of other types to conduct a similar experiment.

Oh, sorry Dtproff - perhaps somewhat of a thread-jack, but I thought since the matter of heater supply is discussed here - and this matter would be important in heater supply design, series operation being more efficient.
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Old 1st February 2012, 02:38 AM   #10
iko is online now iko  Canada
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Your transformer, choke, filter resistor, or all three must have overheated. High resistance power supply is a basic current limiter, no?
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