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Old 25th April 2008, 07:41 PM   #1
wicked1 is offline wicked1  United States
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Default lm317 CCS question

hi,
Im trying to use an lm317 for a CCS for a LTP. Right now I'm just experimenting, and starting w/ the basics, so I'm trying to do the most simple connection w/ the 317 and one resistor to set the current.

So, despite this being a simple 2 component circuit, I can't get it to work!
Pin 1 - adj - goes to resistor which connects to pin2 - out to set the current
pin 2 - out - is connected to ground (as well as the resistor from the adj pin)
pin 3 - in - goes to the LTP

Power it on, and nothing. There isn't much that could go wrong w/ this circuit! Can anyone think of anything I might have missed? Is there a way to test this lm317 w/ a multimeter?

thanks!
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Old 25th April 2008, 07:54 PM   #2
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Check your pinouts ... sometimes the pin numbering on the data sheets isn't clear ... check a few ! (from experience)
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Old 25th April 2008, 07:56 PM   #3
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Default Re: lm317 CCS question

Quote:
Originally posted by wicked1
Is there a way to test this lm317 w/ a multimeter?
Take it out of the bigger circuit and hook it up with a 9V battery and a current meter. My guess would be that you have the wrong size resistor which is resulting in very very low current.
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Old 25th April 2008, 08:34 PM   #4
wicked1 is offline wicked1  United States
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Ok,
I finally found a 9 volt battery, and 200ohm resistor is definitely giving me 7ma.

What else could be wrong? I guess I'll briefly explain the rest of the circuit.

There's a triode input stage w/ LED bias. That goes to this 12at7 LTP, which goes to triode wired 6v6 outputs.

W/ the CCS in place, the LED does not light on the input triode.
I put the resistor back in the tail, and the amp works fine again, so I'm sure I didn't mess something else up.
The plate resistors are of equal value. (and that even seems to work w/ the resistor instead of the current source)

Any ideas?
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Old 25th April 2008, 08:34 PM   #5
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what tube is in question , what is voltage difference between cathode and output of CCS ?

you need at least 5-6 V difference from in to out of CCS
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Old 25th April 2008, 08:38 PM   #6
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Hi Wicked

Which valves are you using for the LTP? Are the grids at ground level? Are you using a negative voltage?

If the grids are at ground level and you have no negative voltage (which I imagine is your situation) you can only work with low mu valves (6SN7, etc), as they cab have a high enough Vgk for the LM317 to work properly (the LM317 needs at least ~3,75VDC across it to work properly) Also the LM317 needs to conduct at least 5mA!

But, well, just change one thing in your CCS: connect the adjust to ground, and disconnect the output. Yes, the output should not be connected to anything with exception of the current setting resistor.

Chech figure 26 in this datasheet

Good luck, Erik
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Old 25th April 2008, 08:48 PM   #7
Salas is online now Salas  Greece
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If it was a Schmidt like in the top schematic, you need to do it as in the bottom schematic so to work with a CCS. Better use a -V instead of ground at the CCS tail to work properly, with headroom.
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Old 25th April 2008, 08:51 PM   #8
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Here's the schematic. It's a modified ampex 612. Some of you probably answered my question a few weeks ago about what I should do to modify it. This is what I've done so far.

I didn't completely finish modifying hte schematic. The plate resistors on the LTP are both 110k ohm, there's an LED in place of the cathode resistor on the input tube, and the LTP tube is a 12aT7, not aU7.

thanks again
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Old 25th April 2008, 08:57 PM   #9
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So you want to substitute R2 (82k) for the LM317 CCS? The LM317 can only withstand 37V across it, and in this application you have surely way more than that (the grids are at 80V, so the cathodes are even somewhat higher.

You still can use the LM317 however, for example by connecting a 60V zener (look at dissipation: P = V*I) in series with it, or otherwise a resistor (R = 60V/current CCS).

Good luck, Erik
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Old 25th April 2008, 09:03 PM   #10
wicked1 is offline wicked1  United States
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Thank you so much.. I was just realizing this myself, but wasn't sure.
I was about to pull out the datasheet.

Hurm.. Maybe I'll remove the first tube, and try going from my audio source directly into the LTP, as someone mentioned in my previous thread.
That was the first thing I tried, but it didn't have enough gain. I've since made several other changes, so it might be worth trying again.
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