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Old 10th July 2007, 04:36 AM   #1
eeyore is offline eeyore  Australia
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Default 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!
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Old 10th July 2007, 04:53 AM   #2
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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.
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Old 10th July 2007, 05:10 AM   #3
eeyore is offline eeyore  Australia
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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?
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Old 10th July 2007, 05:15 AM   #4
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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!
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Old 10th July 2007, 01:00 PM   #5
jeapel is offline jeapel  Canada
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the sweet spot for long life is 6.1v from my research
in internet
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Old 10th July 2007, 01:05 PM   #6
jeapel is offline jeapel  Canada
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but below 6.0v it s bad for the cathode
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Old 10th July 2007, 01:10 PM   #7
EC8010 is offline EC8010  United Kingdom
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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.
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Old 10th July 2007, 01:55 PM   #8
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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.
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Old 10th July 2007, 02:24 PM   #9
Yvesm is offline Yvesm  France
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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.
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Old 10th July 2007, 02:25 PM   #10
EC8010 is offline EC8010  United Kingdom
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Default 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?
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