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Old 22nd September 2003, 03:42 AM   #1
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Default B+ delay circuit

Hi,
Recently changed my AN DAC 1.1 output tube filament to current regulation. I noticed that the volage supply to the filament ramps up very slowly. About 1 minute before the full 6.3V (6922 tubes) is reached.

Now , I am worried about the effects this would have on the tubes as B+ will be present before the heaters are warmed up.

I want to install a delay circuit to the B+. Something with a timer that switches on a relay to let the B+ through once my heater reaches the required voltage.

Any suggestions ? (Note : I have searched around. Mostly people just add a second switch or talk about heater soft start). In addition, I cannot use a tube rectifier as the chassis is already cramp.

Thanks.

Regards,
Joe Ling
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Old 22nd September 2003, 08:19 AM   #2
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There are a number of options, two of which require extra valves which I think you have stated is non viable for you, but I'll go through them anyway with the other options in my order of preference.

1/ Delay valve on the ground connection of the HT supply

2/ slow warm up valve to complete the ground connection of the HT supply, i.e. HT does not find ground until this valve is warm enough to conduct.

3/ on off switch interrupting HT ground supply

4/ 555 timer chip based delay start for HT as found here
http://www.ultranalog.com/schem/hvdelay.html
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Old 22nd September 2003, 11:50 PM   #3
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Dear Paul,

Thanks for the info. This is exactly what I am looking for. Appreciate.

Another problem cropped up while I was thinking about the HV issue. My current regulated filament cannot get up to the 6.3V requirement. I am quite puzzled. My backup tube did achieve this but my first choice tube (Amperex Bugle boy which I have used for a few months already) can only muster 4.0V. Gues it's back to the drawing board to reinsert the voltage regulation for the filaments.

Regards,
Joe Ling
Malaysia
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Old 22nd September 2003, 11:54 PM   #4
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forgot to mention that I am following the values for the 417 tube in the schematic above. The B+ was not switched on when I did my test. Waited 5 minutes but still cannot get 6.3V. The filament is lifted 40V above HV ground to.

Regards,
Joe Ling
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Old 23rd September 2003, 12:26 AM   #5
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Maybe I'm missing something, but your regulator schematic doesn't look right. The regulator will work to maintain 1.25V between OUT and ADJ. So, to make a current regulator, I think you want the sense resistor to be in series with the circuit you're trying to regulate current for.

For the 417A, it draws 300mA with a 6.3V heater, so your resistor needs to be R = V/I = 1.25 / .3 = 4.16 ohms. Then, this needs to be in series with the heaters, so you need to hook up the heater to the other end of the resistor, not to the OUT pin of the regulator.

At least, that's how I did it.

Take a look at this post which describes this a little better:

http://www.diyaudio.com/forums/showt...871#post226871

Hope that helps.
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Old 23rd September 2003, 04:39 AM   #6
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Dear Saurav,

Sorry for the confusion. What you have described is absolutely correct. The schematic is correct too. It is just that the author of the schematic chose a non traditional way to represent the 3 pins of the voltage regulator. He has reversed the "out" pin with the "adj" pin if you look closely.

I did for my DAC what you did but still the voltage is not right. Sigh.

Regards,
Joe Ling
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Old 23rd September 2003, 06:10 AM   #7
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My bad. Didn't even look at the pin labels

Looking at the LM317T datasheet (http://www.national.com/pf/LM/LM317.html), it looks like it needs a minimum of 3V drop across it in order to function correctly. So maybe that's what your problem is. You're dropping 3V across the regulator, and another 1.25V across the sense resistor (remember that this is in series with the filaments), so that's a total of 4.25V lost right there, with a 9V supply, that leaves you just 4.75V left for the filaments.

Maybe you need more voltage to begin with (which is probably hard to do, might need a new transformer or filter section). Another option is to try the LT1085/LT1086 regulators (that's what I used), those can work with about 1.5V drop across themselves (though I don't know how well they'll work at the lower limit of their dropout voltage) so that might solve your problem.
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Old 23rd September 2003, 08:35 AM   #8
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Dear Saurav,

Thanks for the info. My contention is that my backup tube did achieve 6.33V but my supposely "good" tube can only muster 4V. Kinda weird. However, it was late last night when I did this. Maybe I was not using my common sense & something obvious was not in order but I missed. More checking tonight.

Regards,
Joe Ling
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Old 23rd September 2003, 08:49 AM   #9
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just jumped in here so excuse me for being lazy and not reading all the posts.....but the 317will work fine with a 9v input
It will drop voltage when it goes into a current limit
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Old 23rd September 2003, 03:28 PM   #10
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Quote:
My contention is that my backup tube did achieve 6.33V
Well then, looks like your supply design is fine.
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