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Old 23rd November 2009, 09:37 AM   #191
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Quote:
Would this be needed as well as the IXYS cascode? If so any designs you have in mind?
Well not needed. But try it.
CCS -> glow tubes = good shunt reg psu.

The CCS before the glow tubes should be IXYS cascode as well. But should be set for the circuit current draw plus 10-20mA extra for the glow tubes to shunt.

Last edited by Bas Horneman; 23rd November 2009 at 09:47 AM.
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Old 23rd November 2009, 02:05 PM   #192
Sheldon is offline Sheldon  United States
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Originally Posted by Bas Horneman View Post

The CCS before the glow tubes should be IXYS cascode as well.
Now that's overkill.

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Old 23rd November 2009, 08:55 PM   #193
mach1 is offline mach1  Australia
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An IXYS cascode feeding the glow tubes is nice, however the max current one can dial in is less than when using single device.

I was forced to use use a single device in my 12B4 linestage (attached). From memory I think around 45-50 mA is tops for a cascode.
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Old 23rd November 2009, 09:26 PM   #194
mach1 is offline mach1  Australia
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The big advantage over using a resistor is the increased PSR and the consequent ability to use only a relatively small amount of L & C in the PSU. I like using small oil caps in this application.
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Old 23rd November 2009, 09:28 PM   #195
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True, the IXYS part in cascode has a limited current range. You can get a little more with the DN2540. I use them at 65 mA with no problems, and have gone as high as 75.

I really like the CCS-gas tube method. Three out my four tube stages use it with excellent results. Reliable firing, ultra low ripple let through, and quite compact mounting. 4W dissipation is no problem at all with the right heatsink. And that beautiful purple, pink, or orange glow....
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Old 24th November 2009, 06:45 AM   #196
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In the 12B4 schematic you are posting above, it seems like you are using one single pair of OC3 for both channels. I posted a question regarding a setup like this in one of the 12B4 threads some time back, and was told that I would probably need a pair for each channel. I never understood the logic behind this since the current is not passing through the VR tubes. Please, share your thoughts regarding this.
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Old 24th November 2009, 07:25 AM   #197
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point is that current must pass through VR tubes ; or you can leave them in drawer

just imagine them as super duper big zenners , maybe you'll dig their role that way
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Old 24th November 2009, 07:58 AM   #198
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Originally Posted by Zen Mod View Post
point is that current must pass through VR tubes ; or you can leave them in drawer

just imagine them as super duper big zenners , maybe you'll dig their role that way

Yes, but only approx 22mA (5ma+40ma/2), and NOT the current that passes through the 12B4 tubes, right? If so, the VR tubes would be out of limit at 62mA (22mA + 20mA for each of the 12B4 tubes).
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Old 24th November 2009, 08:11 AM   #199
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OK - so I missundertood your post

so - just to "complete" what you are saying .....

in any case - best praxis is when shunt chain ( be it gass tubes , or sand ) is drawing at least same current as load ( in this case 12B4) .

ages ago I "developed" ( same as reinventing the wheel) VR stabilization in 12B4 thread , with plain resistor as series element in reg ; boyz just refused anything more complicated

VR tubes were biased in mid of their operatuing current range , and that's just enough for one channel of 12B4 .

I saw several times shunt regs with shunt element underbiased for their role - not underbiased looking at their own operating range , but comparing to load current . that's exactly the case in schematic posted above .
beat me if I know what's logic behind that ...... even added shunt resistor in parallel to load/VR combo will strighten things , considering that regulation tasks for constant current load aren't that much critical
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Old 24th November 2009, 09:13 PM   #200
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bequerel,

In the 12B4 schematic, 18 - 20mA is shunted down the glow tubes. This safely allows for one 12B4 to be pulled or fail without glow tube failure. If two tubes are pulled or fail simultaneously, then the glow tubes will be operating outside their max range of operation and will probably fail. If this scenario bothers you, then use a separate shunt for each channel.

A shunt reg only needs to be able to vary its current conductance over the range which the load fluctuates: its effectiveness depends on its ability to respond to change in source or the load.

If the shunt current is large with respect to the load current, then any change in the load current requires a proportionally smaller change in the shunt current, thus increasing its effectiveness (given equal shunt transconductance). Taking this situation to extremes, shunting 1000 times the load current through an entirely passive device (a resistor) can achieve an efficiency equivalent to a shunt based on an active device. However, one must remember that the effective transconductance of the glow tubes changes over their range of operation (from memory it peaks at around 14mA for a 0C3), and is reduced at higher currents. In this instance, the loss of transconductance resulting from biasing the glow tubes higher effectively swamps the proportionality gains referred to above, and also renders the glow tubes liable to failure should one linestage channel fail.

The answer: If you really need better shunt operation, use a separate set of glow tubes for each channel, and run around 14 - 20mA through each string. I didnít have the space to do this, and seriously doubt the cost/complexity effectiveness, given that the glow tubes are being fed by and feeding highly stable constant current sources. If the current load was one which varied significantly over its operational range Ė eg class AB push pull or even regulating screen voltages, then extra shunt complexity would be warranted.
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