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Thread: Finding Heater Winding Current

  1. #1
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    Finding Heater Winding Current

    Beginner question I should probably know the answer to: I have a few PTs of unknown heater winding specs. Is it okay to wire up ONLY the heaters of several tubes (power and any planned preamp) for the sake of seeing how much current the PT can handle? As in, no plate or cathode connections. It would only be to load the winding to see if the voltage drops to unsafe levels.

    I'd like to use some power tubes (6BK5, 6W6) that are different from what came in the amps, maybe in different quantities. These are all old PTs from organs, and are pretty hefty for the sizes of amps they originally powered. I know about the "when the voltage falls below 6.3V, it's too much" rule.

    Thanks,

    Justin
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  2. #2
    Supporting Member mozz's Avatar
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    Chances are with older transformers, they are going to be over 6.3vac anyway due to higher input voltage. When i strip something for parts, i try to write down how many tubes (current)it was supplying originally. That being said, maybe measure the wire gauge or measure the trans temp with one of those cheap infrared thermometers. Some of those 1.2 amp tubes do look interesting to work with.
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  3. #3
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    Thanks for the heads-up on the over-voltage... I guess I came along a little late, always been 120 to me. So maybe if it drops below 6.3V then I call it overloaded? Some I have a manufacturer tube layout on the chassis, some I don't... I didn't pay for anything, but I don't want to blow anything on purpose! They were all already stripped, for the most part. I guess I'll give it a go & see what happens? Or wait til later to press my luck!

    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  4. #4
    Supporting Member eschertron's Avatar
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    The filament windings on tubes are just fragile, expensive, wirewound resistors. All the tube knows - assuming it could know anything - is that it's powered on in stand by mode.

    You may want to see what the unloaded voltage is, and compare to wall voltage. Then calculate what the unloaded voltage would be with line at 110vac or 115vac. As you load the winding down and take actual voltage reads, use your 'de-rating' ratio (110vac/today's line voltage) to estimate 'would have been 60 years ago' filament voltage. When those numbers reach 6.3-ish, whatever current you're drawing may have been the design maximum. That's how I might do it.

    Come to think of it, I recently acquired a PT that's an organ pull. 2x 6L6 and several dozen 12A_7 tubes from what I've learned. I should take a dose of my own medicine and see what I come up with!
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  5. #5
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    Assuming a 10% drop in voltage under full load you can calculate the current this way;

    Accurately measure the DC resistance of the secondary (R). Then power up the transformer and measure the unloaded heater voltage (V). If the voltage drops by 10% then the current is V*0.9/R.

    Doesn't take into account any other factors, but its a starting point.
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  6. #6
    Supporting Member Chuck H's Avatar
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    What they said^^^^^^^^^^^^^^

    And, WRT organ amps, many only used the filament winding on the power amp PT for the power tubes or the PI and power tubes and had a separate filament transformer for the preamp tubes. Because, as noted, some of those old organs had A LOT of preamp tubes. Dozens. With this in mind it's possible that the filament winding on the power amp power transformer could be of modest spec. More likely it will be a typical spec of a couple to a few amps, but something to be aware of.

    I'd probably do what you propose and run a burn test checking for the calculated voltage.
    Justin Thomas likes this.
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