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Old 14th September 2014, 02:28 PM   #1
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Default 6C33C and Computer PSU

For an OTL Power Amplifier with 4 6C33C, i'm using an ATX Computer PSU for the heaters. I'm running each 6C33C at 12V@3.3A which means for four units, they'll consume approx 160W, a number i believe my PSU is capable of supplying.

One problem: cold starts. With 4 6C33C turned on at the same time, the ATX PSU just refuses to start. I believe it's because the heaters are cold and have very low resistance, causing surge to the PSU and then the PSU reads them as short circuits. I can plug just one 6C33C and the PSU will turn on and supply 12V normally.. but once i plug another one, same problem occurs.

I'm thinking of connecting the heaters in series with some resistors for initial startup. Then maybe after 1-2 minutes, i will bypass the resistor for full heater power. Does this sound feasible? What should the resistance value for each 6C33C? Perhaps 100R/5W --> meaning at inital starts, at worse there is only 12V/100R = 120mA flowing for each tube? I'm still not sure if the PSU will continue to work if i then bypass the resistors one at a time.
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Old 14th September 2014, 03:04 PM   #2
Yvesm is offline Yvesm  France
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Are you sure your PSU is able to deliver 13 Amps on its 12V output alone ?

Anyway . . .
Transformateurs - Transformateurs toriques - VELLEMAN 16012 51012 160VA TRANSFORMATEUR TORIQUE 2X12V/6,66A

Yves.
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Old 14th September 2014, 03:26 PM   #3
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If your PSU is capable to give that current try 15R/10W in series with each tube
You need more current to heat the heater and a small voltage drop on res
After 20-30sec bypass
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Old 14th September 2014, 03:26 PM   #4
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It would be far easier to use a bargained 24V 7A transformer with only a slight change to your filaments wiring. A parallel/series combination will do the trick, without a huge wires section. I've been using these satisfying combination for years.
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Old 14th September 2014, 03:33 PM   #5
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Quote:
Originally Posted by Blaireau View Post
It would be far easier to use a bargained 24V 7A transformer with only a slight change to your filaments wiring. A parallel/series combination will do the trick, without a huge wires section. I've been using these satisfying combination for years.
I use the same way, 2*24V/4A splitted
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Old 14th September 2014, 05:40 PM   #6
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My concern is space, there is no more space to put a transformer. I'll have to find a way to make this work. Besides, you can't beat the price. Cost me less than USD12 for the PSU. I'm sure it could work.. I just haven't found the way yet.

It seems my problem is not the overcurrent protection (as my PSU turns out has no such feature). It's actually the undervoltage protection. With 2 tubes or more, the voltage would drop and trigger the protection. My PSU uses EST7502 PWM chip and i have tried disabling the over/undervoltage protection and i got 24v at the 12v output (i think it's now an open-loop PSU as in there's no regulation done by the PWM IC). Tried connecting 4 tubes and it dropped to 18V. It's now a question of how to get 12V without tripping the undervoltage protection. Any ideas? I will try posting in other forums, especially SMPS forums.
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Old 14th September 2014, 05:55 PM   #7
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Could try an inrush current limiter, something like a CL-101, it has 0.5 ohms cold and 0.029 at 13.2 amps. Put it in series with all the rest of the filaments and the power supply will see 0.5 ohms plus the filaments. After it's warmed up the CL-101 will still be dropping 0.3 volts, unless you bypass it with a relay.

Last edited by DeathRex; 14th September 2014 at 05:58 PM.
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Old 14th September 2014, 05:56 PM   #8
Koifarm is offline Koifarm  Netherlands
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I use also smps for my GU81m heater supply. Just ad NTC resistor with the right value in series with the heater supply.
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Old 14th September 2014, 06:05 PM   #9
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Toroid transformers are not more cumbersome than your SMPS.
My 300 VA is 110 mm diameter by 65 mm height...
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Old 14th September 2014, 06:11 PM   #10
Koifarm is offline Koifarm  Netherlands
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SMPS is cheap and works great for heater supply. Just try it. I had with my 12v 13A heater(GU81m) only 25mV rimple at 50khz. Not measureble on the audio output.
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