| eeyore |
| 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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| Conrad Hoffman |
| 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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| eeyore |
Thanks Conrad, so let me do some quick maths... :whazzat:
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. ::xeye: 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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| Conrad Hoffman |
| 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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| jeapel |
the sweet spot for long life is 6.1v from my research
in internet |
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| jeapel |
| but below 6.0v it s bad for the cathode |
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| EC8010 |
| 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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| sayang001 |
| 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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| Yvesm |
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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| EC8010 |
| 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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| Yvesm |
| quote: | Originally posted by EC8010
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? |
Who know ?
Car amplifiers ;) |
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| richwalters |
| quote: | Originally posted by eeyore
The stated heater current draw for each KT88 is 1.6A. |
Yvesm is right..
Some years ago I got some white based Tesla KT88 replica GL and these take 1.8A. One has to allow increased currrent spread esp at high heater volts, bear in mind heater volts spec is from +/- 0.6V from 6.3 V nominal.
The Svet 6550 winged C heater is 1.65A.
richj |
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| eeyore |
| Thanks for the replies. So in conclusion, it is safe. |
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| zacster |
Not really, you are stressing your transformer. At best it'll run hot, at worst it'll melt.
Your voltage and resistance are fixed. The transformer provides 6.3 volts due to the winding, your tubes' heaters have a fixed resistance. At 6.3v at whatever your resistance you will have fixed amperage. If your transformer can't provide that you'll get heat.
But, given all this, you are probably OK only because there is a little headroom and you are on the margin. I know I stress my Dynaco ST-70 transformer's heater windings and its been running 2 years now that way, and it is always hot. |
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| Johan Potgieter |
To sound like a salesman (oops, .....person )
There is no sweet spot or ideal value. 6.3V is simply the heater value for which all the specs would hold, including tube life. A slightly lower voltage will yield a somewhat lower max. anode current, but longer life, and vice versa.
The Philips Special Tube Manual specifies that a heater voltage outside 6,0V - 6,6V will lower tube life. This (after careful calculation) signifies 10%. Also it must be kept in mind that a transformer also has regulation. Ideally 6,3V will only be present if the winding is loaded to its full current capacity. This is also influenced by other windings yielding exactly their currents.
The bottom line: There is unfortunately often enough variation around to bring one close to these limits. But for now, as Yvesm said, the "overload" is negligible really. (The variation in load with mains variation, on the transformer that is likely to be used for this project will likely be some 10 times 0,65W!)
EC8010
:bigeyes: :bigeyes: :bigeyes: Really now....
but it will of course depend on just how powerful an amplifier you want to install in your 1950 Bentley..... |
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| EC8010 |
| quote: | Originally posted by Johan Potgieter
The Philips Special Tube Manual specifies that a heater voltage outside 6,0V - 6,6V will lower tube life. This (after careful calculation) signifies 10% |
10% spread, or +/-5%.
PP KT66 driving LS3/5a in a blown Bentley? Now that would be an interesting twist on the typical rubbish car with add-on big exhaust and stereo more powerful than the engine... |
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| Johan Potgieter |
Ah yes! I always had the suspicion that you were a bright gentleman EC8010.
Just imagine the possibilities with p.p. KT88s driving an on-Bentley full frequency system. You could play back your synthesised exhaust sound tweeked to your liking, baffle the next driver ("Yeo man, where did you get them mufflers?") Then blind tests could be done on whether the sound has really changed, or perhaps has 0,173% of 11th harmonic distortion, a thread will be opened here .....mmmm...zzzzz. |
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