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Old 21st April 2005, 01:03 AM   #11
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Quote:
Originally posted by Yvesm
and since the current source is tied between the grid and the cathode, it adjusts the bias automatically.

Quote:
Originally posted by DonJuan
if you look at el84 curves, with the current plate voltage and 40ma, you hv the grid-cathode voltage. ta da!
OK...

The current parts i get...

How the voltage gets to the appropriate value still seems magic to me.... guess i'll just have to build it and experience it

dave
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Old 21st April 2005, 03:16 AM   #12
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Ah, that is set by the tube. Run a horizontal line at Iccs and vertical line at B+. Where they cross (minus grid bias, which being derived from B+, subtracts from it) is the operating point. The nearest grid voltage curve is your bias.

Tim
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Old 21st April 2005, 05:58 AM   #13
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I really appreciate all this great info. I definitely will want to try the CCS version. But first I need to debug my prototype. It plays, not as loud as my SE EL84/6EU7. It also seems to have a couple bugs to work out. I put a switch in series with the 100k treble pot. I didn't expect to find the volume drop to 0 as the R dropped past 1/2 way(treble pot). Even worse, there was initially an occasional pop at full volume. I also noticed with the tone pot dialed low, the B+ voltage went sky high! (500V ) I also noticed only 7 volts on the 120 ohm cathode resistor I used. Maybe my supply lacks steam? 250V present less the bias voltage. Will debug later,....again thanks
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Old 21st April 2005, 09:02 AM   #14
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Default Magic resistor

Hi Dave,

Quote:
How the voltage gets to the appropriate value still seems magic to me.... guess i'll just have to build it and experience it
It is magic !!

You may look at the CCS as a "magic resistor" wich adjust itself to maintain a constant current THROUGHT it whatever could be the voltage ACCROSS it.

In this context (CCS in the cathode) the voltage accross the "magic resistor" will be exactly the bias needed by the tube for it passes the current you wish.

The CCS does not know how much voltage it must set, but the tube KNOWS !

Does this help ?

Sparky,
Quote:
It also seems to have a couple bugs to work out. I put a switch in series with the 100k treble pot. I didn't expect to find the volume drop to 0 as the R dropped past 1/2 way(treble pot). Even worse, there was initially an occasional pop at full volume. I also noticed with the tone pot dialed low, the B+ voltage went sky high!
your PB looks like parasitic oscillations.
Have a clean short wiring and try to use grid stoppers.

Yves.
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Old 21st April 2005, 09:26 AM   #15
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Default Re: Magic resistor

Quote:
Originally posted by Yvesm
You may look at the CCS as a "magic resistor" wich adjust itself to maintain a constant current THROUGHT it whatever could be the voltage ACCROSS it.

In this context (CCS in the cathode) the voltage accross the "magic resistor" will be exactly the bias needed by the tube for it passes the current you wish.

The CCS does not know how much voltage it must set, but the tube KNOWS !

Does this help ?
Yes thanks... i was getting the idea from the 2 earlier posts, but yours is the icing...

dave
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Old 21st April 2005, 09:59 AM   #16
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Default Exemples

Hi Dave,

Have a look at
http://perso.wanadoo.fr/yves.monmagnon/SSCCS.GIF
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Old 21st April 2005, 05:30 PM   #17
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Thanx, i'll enlarge that, print it, and take to the "library" for further study.

dave
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Old 22nd April 2005, 03:07 AM   #18
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I think you are getting it, but if the horse isn't totally dead I can whip a little more:

> How the voltage gets to the appropriate value

Forget the current source.

Build a cathode follower with resistor load. Say +300V on the plate, grid resistor to 0V, and a 50K resistor to a -1,000V supply.

As a rough guess, tube grid-cathode voltage is "small" compared to 1,000V. So the resistor sucks 1000V/50K = 20mA of current from the tube.

Now we look on the tube curves. At ~300V, we might find that it takes -20V grid-cathode to pass ~20mA. That is our next approximation.

We are close to 280V and 1020/50K = 20.4mA. Squinting at the tube curves, our new guess is -19.5V grid-cathode. We could now try 280.5V and 20.39mA, but clearly the tube WILL find an operating point "about" 20V grid-cathode.

And if we change the 50K to 100K, "about" 10mA, the tube will find a new operating point "about" -25V grid-cathode.

Or consider the good old Cascode, beloved of TV tuner designs. The lower tube is biased to 10mA-1mA (variable to adjust RF gain). The upper tube is (sometimes) fixed-biased with +100V on its grid. Its cathode will find an operating point: maybe -1V (+101V) at 10mA, -10V (+110V) at 1mA.

The sand-state current-limiter will do the same thing, but without the extra 1,000V or 100V voltage drop. For (almost) any current we set the CSS at, the tube will find its grid-cathode voltage that will (at that plate/screen voltage) pass the current being sucked through it.

Of course the only reasons to do the "Self Inverting" trick are to use a cheap push-pull transformer and power filter instead of beefy SE iron and filter; and to increase the tube-count to look good on the name-plate. You could get 6 Watts easy with one 6L6, or by beating the heck out of one 6BQ5. But the push-pull savings might pay for the extra tube, and "8 tubes!" looks better than "7 tubes!"
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Old 23rd April 2005, 12:42 AM   #19
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Default just to be sure I get it right...

using the LM317 CCS, which leg is ground? Vin or Vout...a 50 50 shot, and I'm sure it's obvious to most. Yes the attraction is using inexpensive iron, few parts, to see how it compares with my PP and SE amps (6BQ5).....thanks!
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Old 23rd April 2005, 01:06 AM   #20
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Default Re: just to be sure I get it right...

Quote:
Originally posted by Sparky OR
using the LM317 CCS, which leg is ground? Vin or Vout...a 50 50 shot, and I'm sure it's obvious to most.
For a CC sink, as in this thread, "In" would go to the sense resistors and "Out" to ground (or V-).

For a CC source, "In" to V+, and "Out" to the circuit.
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