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Old 21st February 2013, 07:19 PM   #41
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It looks like Tinitus' circuit in post 31 will also work if the wiper goes open......

at least to my semi-newbie eyes......
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Old 21st February 2013, 07:34 PM   #42
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I was told to do it that way by a master member
curcuit was drawn by me, but approved as shown
and thats about all I know about it
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Old 22nd February 2013, 02:29 AM   #43
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Better?
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Old 22nd February 2013, 03:26 AM   #44
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Quote:
Originally Posted by mr2racer View Post
Better?
nope. If the wiper lifts, your tubes are "full on". Study the circuit in post #29 or post #31, At least one bias circuit resistor needs to connect to the tube grid without going through the pot wiper. That way, if the pot goes open, the grid is still connected to the -ve bias voltage.

Last edited by boywonder; 22nd February 2013 at 03:32 AM.
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Old 22nd February 2013, 04:04 AM   #45
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And the 37k resistor isn't between the supply and the grid even if the wiper lifts? If the wiper lifts the 37k/22k voltage divider comes into play alone. The output voltage from which is about 27 volts not zero? (Which needs tweaking as it is a bit too high.)

Last edited by mr2racer; 22nd February 2013 at 04:32 AM.
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Old 22nd February 2013, 01:50 PM   #46
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Quote:
Originally Posted by mr2racer View Post
And the 37k resistor isn't between the supply and the grid even if the wiper lifts? If the wiper lifts the 37k/22k voltage divider comes into play alone. The output voltage from which is about 27 volts not zero? (Which needs tweaking as it is a bit too high.)
Sorry, yes. What you have works.....had to turn my head sideways to see it....

I think your R values need tweaking though. I would shoot for an adjustment of about -20V to -35V or so on the grid.

Last edited by boywonder; 22nd February 2013 at 02:06 PM.
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Old 22nd February 2013, 02:59 PM   #47
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Kevin: Here is what I came up with......please check the math. It appears a 10K pot does not give much voltage adjustment unless lots of bias current is flowing to ground. This circuit is using between 2.1-2.8 ma of bias current to ground.
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Last edited by boywonder; 22nd February 2013 at 03:02 PM.
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Old 22nd February 2013, 09:16 PM   #48
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shouldnt there be resistor in series with wiper ?
looks a bit naked
anyway, I thought it would take some of the load ?
I suppose the 'security' resistor in paralel with pot/wiper also does that, as a side effect ?
or are they 'just there' ?
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Old 23rd February 2013, 03:27 PM   #49
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Thanks guys, I'm glad I finally got it right. I already have some nice Bourns 10k PCB mount pots I may return them for 20k if Digikey has them. I'm using an Antek AN0124 transformer here 2 x 24 volts at 210ma. I'm going to build the boards and the rectifier and see what I get for voltage. Then put in the values I need. The bias will drw something like .001125 of current so the transformer output should be near peak.
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Old 25th February 2013, 10:49 PM   #50
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Hello Again All,

Can anyone tell me what wattage the 100ohm pot in the CCS needs to be? And is it just connected on the one end and the wiper? And does all the DC current go to ground through the lower tubes grid?

Kevin
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