I want to power up the heaters of a pair of 27 tubes off a 4A 6.3V CT winding. I'd like to keep this as simple as possible using AC heaters, and am leaning towards series connecting them, with a dropping resistor between them and grounding the CT. Ballpark value on the dropping resistor would be nice to verify what I am thinking I will need.
Other ideas are welcome, especially anyone with experience working with 27 tubes. I'm planning to use them as driver tubes for a SE amp using 47 output tubes. I also have some 56 tubes if 27's don't stand a chance of being able to drive a 47. TIA
Other ideas are welcome, especially anyone with experience working with 27 tubes. I'm planning to use them as driver tubes for a SE amp using 47 output tubes. I also have some 56 tubes if 27's don't stand a chance of being able to drive a 47. TIA
Heater is listed as 2.5V @ 1.75A, and two in series would be 5.0V @ 1.75A.
From a 6.3V winding, the series resistor would be (6.3 - 5.0) / 1.75 = 0.743 ohms @ >2.3W.
A couple of 1.5 ohm 5W resistors in parallel should work ok.
You could use an adjustable 1 ohm resistor to account for AC line and transformer variations.
Some power resistors are made with a movable wiper lug/clamp for adjustment.
If you have a good junk box, there might be one in there.
From a 6.3V winding, the series resistor would be (6.3 - 5.0) / 1.75 = 0.743 ohms @ >2.3W.
A couple of 1.5 ohm 5W resistors in parallel should work ok.
You could use an adjustable 1 ohm resistor to account for AC line and transformer variations.
Some power resistors are made with a movable wiper lug/clamp for adjustment.
If you have a good junk box, there might be one in there.
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Strange my data sheet gives the voltage for a type 27 @ 2.5V at 1.75A that would make the dropping resistor 0.743 ohms.
Cool, I came up with the same value 🙂 Thanks! And unfortunately my junk box isn't that good.
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Oh yeah, they're a very old type of part. Sort of a linear rheostat, but easier to manufacture.
They just start with a standard resistor body, mask off a rectangular area when enamel coating them
to expose the winding, and then you add the sliding clamp lug. Here's an old ad.
They just start with a standard resistor body, mask off a rectangular area when enamel coating them
to expose the winding, and then you add the sliding clamp lug. Here's an old ad.
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