From: not.here@invalid.invalid   
      
   On Wed, 13 Apr 2011 16:23:18 -0500, flipper wrote:   
      
   > On Wed, 13 Apr 2011 13:42:13 -0700, Big Bad Bob   
   >  wrote:   
   >   
   >>On 04/13/11 13:32, flipper so wittily quipped:   
   >>> On Wed, 13 Apr 2011 13:05:54 -0700, Big Bad Bob   
   >>>   wrote:   
   >>>   
   >>>> On 04/12/11 10:37, Nordic Breeds WA4VZQ so wittily quipped:   
   >>>>> "Big Bad Bob"   wrote in   
   >>>>> message news:SOadnWoLsZTQ4DnQnZ2dnUVZ_qqdnZ2d@earthlink.com...   
   >>>>>>   
   >>>>>> perhaps a switching power supply for tubes _could_ have a 'warmup   
   >>>>>> cycle' on the filament supply, ramping it up from 2VDC to 6.3VDC   
   >>>>>> over a period of 10 seconds.  It's a 'feature' worth considering.   
   >>>>>> A similar feature might turn the bias supply on FIRST, then ramp up   
   >>>>>> the HV side.   
   >>>>>>   
   >>>>>> Anyway, this kind of thing gives me a reason to consider that sort   
   >>>>>> of stuff in any future designs.   
   >>>>>   
   >>>>> I have a signal generator with a rather exotic (read as nearly   
   >>>>> unobtanium) oscillator tube.  To protect this tube, I built a   
   >>>>> regulated DC supply to power it using the ancient Fairchild uA723   
   >>>>> regulator with a pass transistor.  I set the current limit to 50%   
   >>>>> more than the normal filament draw.  When first turned on, the   
   >>>>> supply regulates the current for about 10 seconds with the voltage   
   >>>>> rising slowly.  Once the current starts dropping, the supply   
   >>>>> switches to regulate the output voltage. This supply not only   
   >>>>> protects the tube during startup but it also increased the stability   
   >>>>> of the oscillator about ten fold.   
   >>>>   
   >>>> every time someone confirms the idea of proper voltage regulation in   
   >>>> tube amps I raise the project's priority level.  If I put such a   
   >>>> beast into hardware I suppose I oughta 'open source' the board layout   
   >>>> at least (after getting ideas from this newsgroup).  Start filaments   
   >>>> before B+, ramp up filaments, bias on first, and sbort-circuit   
   >>>> protection like a computer P.S..  Yeah.   
   >>>>   
   >>>> /me pondering how to do all this with a single toroid.   
   >>>   
   >>> The time I went 'whole hog' with DC heaters I put a CCS under each   
   >>> tube so there isn't any 'surge' at all.   
   >>   
   >>I'd guess about an extra $1 per tube?   
   >   
   > All the parts, including heatsinks, were junk bin and, so, 'free'.   
   >   
   > CCS was a pass transistor and current sense resistor under each heater   
   > with a 324 opamp closing the loop. The one 324 holds 4 for 4 tubes.   
   >   
   >>  how was your warmup time?   
   >   
   > That was a long time ago and I don't recall. But I don't remember it   
   > being 'much' longer.   
      
      
   I suppose you could just use a LM317 & 1 resistor for each heater now.   
   Those things are stupidly cheap and no op-amp supply needed. Allow 1.25v   
   across the resistor and another 2-3v across the 317 to let it regulate   
   properly. Although where you get a 4.16666666666666667 ohm resistor for a   
   300mA heater from I've no idea!   
      
   --   
   Mick                      (Working in a M$-free zone!)   
   Web: http://www.nascom.info   
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