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Old 8th April 2012, 05:46 AM   #1
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Default Heater supply

I have 2 questions, they might seem very rudimentary but I can not find any direct answer to them.

1. The heater of the EZ81 is rated at 6.3V (6-6.6V spec). I'd like to use a toroidal transformer to power it, however I can only find one rated at 6V. For most tubes it'll most likely be fine, but I read that it is important to to underheat rectifier tubes because often they run close to their current output limit. My question is will this be a problem?

2. I am thinking of using a C-RC filtered unregulated DC to supply the heater for 6H30. From PSUD2 simulation the ripple is at 0.3V. My question is, will this be ok?

Thank you.

Last edited by Navyblue; 8th April 2012 at 06:35 AM.
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Old 8th April 2012, 08:02 AM   #2
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1. This is quite easy to fix, in practice. Just choose a toroid with a (say) 5A secondary. Trafos are usually rated to give the rated voltage at full load - so at one fifth load or less, you should get plenty of extra voltage. If you get too much, add some series resistance.

If you connect the heater to the dc output though, please remember that the secondary winding of most toroids is the OUTER winding, and that you should take care that the insulation is not compromised. Especially: mount the trafo on some insulating material between the chassis and the trafo.

2. 0.3V ripple for phono stage may be too much, but for other applications, it's OK.
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Old 8th April 2012, 11:28 AM   #3
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Thanks Rod.

1. My initial plan was to use a 15V toroid to power them in series (12.6V), and use a high powered 2.4 ohms resistor to reduce the voltage (I am not even sure if I can assume the heater as a fixed resistive load). If it is as you said I guess I would just use a larger toroid and risk it. The rectifier tube is is feeding a 20mA supply, which I guess is on the low side.

2. It is for a "spud" headphone amplifier which tend to be more sensitive than say a pre amp.
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Old 8th April 2012, 11:40 AM   #4
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The heater load will be different COLD than HOT but once HOT it is pretty constant.

Why not use an LM317 and regulate the heater supply. You could even use it in CCS configuration which will give the valves a degree of safety against heater element failure.

You only need a Bridge Rectifier, a relatively small cap 2200uF or so, the LM317 and a few resistors.
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Old 8th April 2012, 02:43 PM   #5
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I am open to all suggestions actually, given how little I know. Initially my plan was to use commercially available board kits, like this:

H-PS-1 low-voltage regulator

But it is out of stock now. I tried to look for alternatives, or schematic that is not too complicated, but have not found any. So I took the easy way out.

Basically I need 850mA at 6.3V per tube. A brief search finds me this:

LM317 High Current Voltage Regulator - Electric Circuit

Please point me to a schematic if you have any.
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Old 8th April 2012, 03:13 PM   #6
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Hi!

6V will be fine for the rectifier, as suggested, if you pick a toroid with a higher current rating than required, the output will be a bit more and closer to 6.3V

I would suggest to try AC on the 6H30 first. If laid out neatly, it should not hum. You could still convert to DC if necessary

Best regards

Thomas
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Old 8th April 2012, 03:52 PM   #7
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I am lazy and I kinda like that idea, it's like cutting the gordian knot.

I am looking at this 6V 8A toroid (12V 4A CT), it might just work to supply all 4 tubes (6.3V 3.7A). I never seem to observe my mains go below 230V, which I suppose I should be able to hit 6V which is the lower bracket.

If it hums, I guess I could use the 12V CT and make me a 6.3V DC?
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Old 8th April 2012, 04:36 PM   #8
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Hi!

I'd keep the heaters of the rectifier and signal tubes separate! Connect the rectifier heater to the rectifier cathode at one point.

Often it is beneficial to bias the heater of signal tubes slightly positive say 20-25V. This is done by connecting one end of the heater to a voltage divider across B+. Connect a cap in parallel to the resistor to ground. This biases the parasitic diode between heater and cathode positive and minimizes leakage

Best regards

Thomas
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Old 9th April 2012, 02:59 AM   #9
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Hi Thomas,

Please pardon me as I know quite little of these stuffs.

For the rectifier tube, may I know what is the purpose of biasing the heater?

As for the signal tube, can you please correct me if I am wrong?

So I wire both ends of the heater to both end of the heater transformer as usual. And only on one end of the heater I have an extra wire that branches out, one end to the B+ supply (with a resistor in series) and another to the ground (with a resistor in series and a capacitor in parallel to the resistor)?

Thanks.
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Old 9th April 2012, 06:52 AM   #10
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Hi!

Quote:
Originally Posted by Navyblue View Post
For the rectifier tube, may I know what is the purpose of biasing the heater?
It avoids a large voltage difference bewteen heater and cathode. Although the tube is rated for this, it eases stress on the tube



Quote:
Originally Posted by Navyblue View Post
So I wire both ends of the heater to both end of the heater transformer as usual. And only on one end of the heater I have an extra wire that branches out
I think you got it ok, just to be sure:

Say you have a 250V B+. Connect two resistors in series from B+ to ground, a 100k and a 10k. The 10k on the ground side. Add a 10uF/40V or higher parallel to the 10k. Connect one end of the heater to the connection point of the two resistors.

Best regards

Thomas
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