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Old 10th March 2004, 12:40 AM   #1
giddy is offline giddy  United States
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Default Seeking suggestions on slowing B+ turn on

I am in the process of building a tube preamp with regulated SS filament (based on LM317) and B+ (transistor regulated) supplies. Currently a voltage that reaches 50V shows up at the output of the preamp for about 10 seconds a little while after the turn on/off. I see a couple of choices:
1. Use tubes for B+
2. use a timer and relays to either delay B+ connection or short/disconnect output of the preamp

Does anyone have any better ideas? I am looking for a simpler solution. Adding capacitor to B+ regulation transistor can delay B+ ramp up for another 1-2 seconds, but 10-20 sec ?
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Old 10th March 2004, 05:25 AM   #2
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I haven't tried this myself, but I've seen a couple of designs that insert a negative temperature coef. Thermistor right after the SS rectifiers. Depending on which type is selected, you get a smooth ramp-up over about 30 seconds.

Norman Koren uses one somewhere in his TEANA amplifier design and provides a link to a datasheet for it from that page.

There's a rumour that they are sonically deleterious, so try a before and after listening test (and report back here )

There's always Jukka Tolonen's soft-start circuit from Glass Audio. http://koti.mbnet.fi/siliconf/JukkaT.../amptimer.html

And, as you observed, using a tube rectifier of course too
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Old 10th March 2004, 07:21 AM   #3
SHiFTY is offline SHiFTY  New Zealand
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Umm... Use a capacitor at the output of the preamp? A film cap of a few uF should do it.

Failing that, the 6X4, EZ80 would be pretty ideal for a preamp. Also they have separate cathodes and heaters, so you can connect the heater to your existing heaters.
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Old 10th March 2004, 05:19 PM   #4
giddy is offline giddy  United States
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Default Thermistor

I have a thermistor similiar to what Norman Koren uses in his power supply. I believe that the delay that it adds is not enough. I do not have a scope, so I can't tell you what delay it inserts.

I also do have a capacitor at the output of the preamp. The voltage at the input of the capacitor is changing, therefore it appears at the other end of the capacitor as well.

Any other ideas ?
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Old 10th March 2004, 10:07 PM   #5
Bryan is offline Bryan  United States
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Hi,

Acouple stupid questions, but.....

1. Do you have a bleeder resistor at the output, and how big is it?

2. Also, do your PS filering caps have bleeders?
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Old 11th March 2004, 03:27 AM   #6
SHiFTY is offline SHiFTY  New Zealand
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Quote:
I also do have a capacitor at the output of the preamp. The voltage at the input of the capacitor is changing, therefore it appears at the other end of the capacitor as well.
Ok, are you sure its not an oscillation?

And yeah, a bleeder resistor of a few hundred kOhm across the power supply caps is a good idea. What size are your caps?

Perhaps you could use a relay on the output? Power it off the heater supplies possibly?
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Old 11th March 2004, 10:08 PM   #7
giddy is offline giddy  United States
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I have 1M resitor at the output. My output cap is 1uF

Power supply capacitors are 100uF with 470k bleeder resistors.
I don't think that the bleeder resistors have an affect on the transient voltages. The voltage spike that I am seeing lasts for a number of seconds, and then it goes away, which means it's not an oscillation. For testing I've been connecting my amp after a minute to the tube preamp to avoid burning something in the amp or blowing speakers.
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Old 12th March 2004, 02:56 AM   #8
DougL is offline DougL  United States
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A couple sugestions.

You could use a 6D22S diode after your first power supply cap.
Sort of a "constant voltage" resistor.
It has a 40 sec turn on, will take 6kv across it and will run with either 5 v or 6.3 v. remarkable device.App note for 6D22S

or use a 6as7 as a tube pass regulator. it also has a slow turn on time, good current capability and was designed for this application.

I don'y know if either approch will fit your needs.

Good Luck.

Doug
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Old 12th March 2004, 04:49 PM   #9
giddy is offline giddy  United States
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All good suggestions. Thanks guys. For now I'll go with a timer and relays since I have most of the parts. Later on I might try tube rectifier.
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