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Old 9th February 2006, 08:02 PM   #1
Cassiel is offline Cassiel  Libya
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Default 6sn7 Ccs

Hi,

I'm trying to build a SE 6as7 but i'm having some problems. Main reason is the driver tube- a 6sn7. OK the output stage is solved, i am running the 6as7 with 250 volts at the plate(maximum allowed according to data sheet) and low current and i am getting a lot of power (good) but i need a good sounding driver stage...
I tried direct coupling , SRPP and 12at7 to 6sn7....i didn't like any of them

As i said before the driver is a 6sn7...why? Because i like them and got them cheap. The 6sn7 needs lots of volts hence my 330V B+. Yes i know, too high for the 6as7 but i also got them cheap.


Then i found this schematic...
http://www.jogis-roehrenbude.de/Lese...7/Endstufe.gif

I've never tried a CSS but it seems to me a great idea for this amp. I am not paralleling the 6as7's (don't need many watts) but i want to copy the driver stage. I have bought all the parts for the ccs but now i am in doubt:

- does it have to be same B+ or it could be 20 volts more or less.

-the LED and the diode in the schema...can i use any diode, any LED?

Any other advice about ccs's welcomed... i know very little.
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Old 9th February 2006, 08:09 PM   #2
SY is offline SY  United States
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The LEDs can be anything, but you have to know their forward voltage to properly calculate the emitter resistor. There are whole masses of posts here about just that subject- a few minutes searching ought to turn up all the formulas and schematics you could ever want.
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Old 9th February 2006, 08:46 PM   #3
Cassiel is offline Cassiel  Libya
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Quote:
a few minutes searching ought to turn up all the formulas and schematics you could ever want.
Umm i really dont want any but ooook i'll do some homework tonight.

Progress so far. Wine box.
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File Type: jpg 6as7 1.jpg (96.5 KB, 846 views)
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Old 10th February 2006, 01:46 PM   #4
Cassiel is offline Cassiel  Libya
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It doesn't work. 0 volts at the 6sn7 cathode. No music, nothing,nada.
CCS= 310 VOLTS IN, 307 VOLTS OUT.


LED's light up. What could be wrong? If i had miswired a transistor would i get 307 volts at the plate?

Any ideas for troubleshooting the circuit?
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Old 10th February 2006, 01:53 PM   #5
SY is offline SY  United States
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Clearly, there's no current being drawn. Disconnect the CS from the tube and verify its operation- from the CS to ground, connect a 20K resistor. Is there 200V across it? If so, the CS is working.

Most likely, you've got a wiring error.
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Old 10th February 2006, 02:09 PM   #6
Cassiel is offline Cassiel  Libya
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Thanks SY, i get 232 volts with a 22k resistor to ground.

Is it working?
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Old 10th February 2006, 02:13 PM   #7
Cassiel is offline Cassiel  Libya
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It works ....10v at the 6sn7 cathode.



Thanks again SY.
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Old 10th February 2006, 03:07 PM   #8
kevinkr is offline kevinkr  United States
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What operating point have you chosen for the 6SN7?
Cathode resistor value?

Kevin
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Old 10th February 2006, 03:10 PM   #9
SY is offline SY  United States
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His schematic is linked in the first post. 10mA, 1k cathode resistor.

I wouldn't go that route, but it's not my design.
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Old 10th February 2006, 03:35 PM   #10
kevinkr is offline kevinkr  United States
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Hi Sy,
What do you think of the operating point chosen? I suspect that there is very little voltage headroom to spare here..

Imho this seems like a good place to try transformer coupling if he was so inclined...
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