KT88 Heater Current
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 10th July 2007, 03:36 AM #1 eeyore   diyAudio Member   Join Date: Aug 2005 KT88 Heater Current Trying to finalise the power transformer in the Uniamp circuit, and it has a pair of KT88 in P-P. The stated heater current draw for each KT88 is 1.6A. Will there be a big issue if the transformer can only supply 3.0A overall, so each KT88 will only have 1.5A of current @ 6.3V? Thanks! __________________ DF http://diy-audio-blog.blogspot.com
 10th July 2007, 03:53 AM #2 Conrad Hoffman   diyAudio Member   Join Date: Mar 2007 Location: Canandaigua, NY USA If the transformer can't supply enough current to produce 6.3 volts across the load (the heaters), you'll get less voltage. You can't reduce the current and expect to maintain same voltage- blame George Simon Ohm and his darn law. You'll also likely end up with an overheated transformer. It's desirable to operate the heater at its rated voltage, but others here are for more knowledgeable about what happens if you don't. There's certainly some tolerance, as line voltage goes up and down. I'd run a test to see if the transformer has enough margin in its design to do the job anyway.
 10th July 2007, 04:10 AM #3 eeyore   diyAudio Member   Join Date: Aug 2005 Thanks Conrad, so let me do some quick maths... V = IR 6.3 = 1.6 x R => R = 3.9375 If I = 1.5, R remains constant, => V = 3.9375 * 1.5 => V = 5.90625 V If there is 5% in line voltage variation, meaning that 6.3V is somewhere between 5.985V to 6.615V. : If there is a 10% line voltage variation, then the potential voltage is 5.67V to 6.93V. Am I doing something wrong, or on the right track? Further, I have read on this and other places, that running the heaters at a slightly lower voltage is not a detriment? I think that some even suggest that it sounds better and that it could prolong the life of the tube? __________________ DF http://diy-audio-blog.blogspot.com
 10th July 2007, 04:15 AM #4 Conrad Hoffman   diyAudio Member   Join Date: Mar 2007 Location: Canandaigua, NY USA You are on the right track. There may be some variation in R with current, but I don't know how much. I know high heater voltage is bad for tube life, but far less about where the "sweet spot" might be!
 10th July 2007, 12:00 PM #5 jeapel   diyAudio Member   Join Date: Sep 2003 Location: quebec the sweet spot for long life is 6.1v from my research in internet
 10th July 2007, 12:05 PM #6 jeapel   diyAudio Member   Join Date: Sep 2003 Location: quebec but below 6.0v it s bad for the cathode
 10th July 2007, 12:10 PM #7 EC8010   diyAudio Moderator Emeritus     Join Date: Jan 2003 Location: Near London. UK Hasn't is occurred to anybody that the valve manufacturers probably knew what they were doing? When they said 6.3V, that's what they meant. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
 10th July 2007, 12:55 PM #8 sayang001   diyAudio Member   Join Date: Mar 2003 Location: Amsterdam 6.3V comes from the days that you were powering the heaters from 6V batteries, manufacturers choose 6.3V to compensate for fresh batteries. I have read this in a dutch book from philips about their MiniWatt tubes.
 10th July 2007, 01:24 PM #9 Yvesm   diyAudio Member     Join Date: Jul 2004 Location: Ardeche Yes, yes and yes to all ! But, who really measured how much current the tubes draw in true word ? Allow some room for manufacuring tolerance . . . as usual. Here: http://frank.pocnet.net/sheets/084/k/KT88.pdf they explicitly write Ih (approx) ....1.6 A Conservative engeenering practice should be to consider that ALL the tubes you will use will keep 10% more than average, so design for 1,8A xeye: Optimists or bean counters will design for 1.44 ! But consider that you transformer will be overloaded by only 0.65 watts. Plug and go ! Yves.
 10th July 2007, 01:25 PM #10 EC8010   diyAudio Moderator Emeritus     Join Date: Jan 2003 Location: Near London. UK Evolution As you say, 6.3V originally came from batteries, but you're not proposing that GEC intended that their KT88 should have its heater powered from a battery? __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

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