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Old 25th March 2006, 10:28 PM   #1
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Default heretical unity gain and ccs question

i'm building my own simplified newby version of Sy's heretical unity gain line stage. mine will not have the b+ or heater regulation. i've also dispensed with the relay dethumper and will leave the output coupling cap in. all this is subject to change as hopefully i will learn! this is my first attempt at using cathode ccs and have some questions.

ive built my own single "test" ccs on a fixture per sy's schematic. just ccs and a little PS.in sy's instructions for calibrating it he says to load the ccs with 1k directly from ccs b+ to collector of first transistor.(b+ is taken from highly filtered supply with 24v tranny. b+ adjusted to provide @ 25v for very small current requirment as i'm not running heaters off the string and dont plan to)

i did this and every thing seemed fine- got 10ma in circuit reading from the trim pot to the negative rail after some adjusting. and when pot was measured afterward it was 103ohm. OK so far so good i think. top of red led reads 2v and is lit. i used 1.7 red led so i'm assuming this is good. emitter of first transistor reads about 7v and the second @1v.

per sys instructions he say there should be a 10v drop across afore mentioned 1k loading resistor. i get 13-14v. my meter is accurate. should i be alarmed? i'm attempting to learn more about ccs so please bear with me!

here is sy's thread with schematic(scroll down a little to second schematic.)

A Heretical Unity Gain Line Stage part II

also another concern is after i have this ccs on tubes cathode where should i measure for bias? at cathode as traditionally done in cathode bias since tubes grid is grounded? and is there anyway to predict what the bias will be before i hook this thing to a tube?

your help is appreciated! thanks!
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Old 25th March 2006, 10:37 PM   #2
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The voltage across the 1k test resistor is what you use to set the current. 13V across it means that your CCS is running a little rich. Increase the emitter resistor a skootch and all will be well. If I wasn't clear about it, the test resistor gets disconnected before the CCS is installed into the preamp circuit.

Starting simple is an excellent idea. Everything you removed is gingerbread and frosting; your approach will work fine.
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Old 25th March 2006, 10:55 PM   #3
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thanks much SY. i really liked your idea-no gain-just a volume pot switching and cable driver. all with good isolation and low noise-

i figured i would use it also as a learning tool-i have been reading about followers trying to learn- the way you presented it was really good, i thought! i'm just trying to find out the hows and whys. building(copying from schematics gets kind of old after a while!

those transistors i used are npnbipolars radio shack guys ceo=40v beta 100-300

running rich? to much current through input resistors?

do i adjust emitter by the input divider resistors? the ones connected to the bases of transistors? i thought i adjusted the current by just adjusting the pot? i will admit this is my first project with transistors and after this i will hit the books and try to learn some more about them!

i appreciate your response, thanks
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Old 25th March 2006, 11:01 PM   #4
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ok i think i misunderstood, you mean adjust the trimpot? i tried this and my current measurement off trim pot to rail went way way low, unless it's supposed to be like that? where should i take in in cicuit current measurement?
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Old 25th March 2006, 11:42 PM   #5
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That's what the 1k test resistor is for, measuring the current. Once it's set (10V drop across 1k), you can put the CCS into the circuit and don't have to worry about it any further. If a sketch will help, let me know and I'll try to draw something (won't happen tonight, I'm heading out to demonstrate diy cooking for some visiting dignitaries).

The trimpot IS the emitter resistor, sorry for being unclear.
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Old 26th March 2006, 12:41 AM   #6
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One other thing- either boost the voltage you're using to test the CCS (the preamp power supply will work fine) or temporarily reduce the feed resistor to the LED string to get a few milliamps through the LED. That will ensure that the LED reference voltage is the roughly the same in and out of circuit.
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Old 26th March 2006, 03:18 AM   #7
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i didn't try the boosted voltage, i went back to work on it and missed your second post. here is what happened. my b+ is at 24v under load for the following....

i placed the meter reading ma in circuit in between the 1k and collector. the lowest i can get that current is 12ma-( trimpot is a 1k)- actual voltage reading across 1k resistor is 12v. turning trimpot almost all the way back down i can get 15ma. probably more .i didn't bother going there

i swapped the led and it is at 1.9 volts-

so, i got brave ... so i hooked it up to a tested 6dj8 follower i had ready- i placed the current meter in between cathode and collector, yes i removed the 1k resistor fired it up and it settled in and stayed a very stable 16ma .the plate vol @100v.. bias voltage at 2.3v. i didn't listen to it yet. i'm done for tonite.

i'll try to lower the resistor tomorrow.
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Old 26th March 2006, 01:34 PM   #8
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How are you measuring the current when it's in-circuit (e.g., the 16mA)?
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Old 26th March 2006, 02:38 PM   #9
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i attach + meter lead to cathode of tube, and attach - lead to collector on top transistor in schematic. the meter being the actual connection in circuit-varying the pot did vary the current- so i think it's working ok?

with your calibration setup(the 1k resistor patched in) ,i took the reading from the collector to the 1k. again the meter was the connection. did i do right?

i'm going to mess about with it some more shortly..

thanks for bearing with me- i often stumble before i learn anything!
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Old 26th March 2006, 03:33 PM   #10
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Scratch anything I said about a higher voltage supply. The version you referenced (now that I had a few minutes to look it up) was indeed set for a 24V supply. In the words of Emily Litella, "Never mind!"

If I read you right, you're trying to measure the current directly with a milliameter? That's OK, but the purpose of the 1k test resistor is to allow you to measure the current by putting a voltmeter across the test resistor and setting the trimmer for a 10V drop.
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