ac157474   
   From: mynamespacedbydots@xs4all.nl   
      
   "Roger Jones"  wrote in message   
   news:45d7fd2d-4b67-42d7-b935-d18bfc93929a@n16g2000vbn.googlegroups.com...   
   On Jun 1, 3:57 am, "Phil Allison"  wrote:   
   >>   
   >> Closer inspection showed something odd about the 6X4 wiring, the plates   
   >> both   
   >> went direct to ground and there was a standby switch that linked a 56 ohm   
   >> 5W   
   >> to the cathode. The other end of the 56 ohm went to the negative of a   
   >> bridge   
   >> rectifier made from four 1N4007s.   
   >>   
   >> At idle, the amp drew about 43mA from the 300V HT and the voltage on the   
   >> cathode of the 6X4 was -7 volts.   
   >>   
   >> So the only function of the 6X4 was to create a warm up delay and confuse   
   >> the owner.   
   ?>   
   >> Cute eh ?   
   >>   
   >> ... Phil   
   >   
   >Neat idea for an "iffy" tube... or is that the 6X5? IIRC, one of these   
   >is prone to heater cathode insulation failure, as may be the EZ80 and   
   >EZ81 but you hear less about these two.  Anyway, this scheme puts the   
   >minimal voltage across h-k.   
   >I have to modify a Stewart-Warner table radio containing 6X5 before it   
   >catches fire!   
   >Cheers,   
   >Roger   
      
      
   Although Vk-h is minimal during normal operation, immediatly after power-on   
   the cathode to heater voltage is high and NEGATIVE (heater potential near   
   GND assumed).   
   For 6X4 the paper spec.(RCA) is 450Vk-h max. with cathode either pos. or   
   neg. in respect to heater.   
   Not so for EZ80/EZ81, Vk-h spec.(Philips) is 500V max. but only for cathode   
   positive in respect to heater.   
      
   Latter spec's are for tubes at operating temp., what else, so the question   
   is what negative Vk-h is allowed with a cold cathode or better a cathode   
   that is just starting to be warm?   
      
   Gio.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)