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Weird voltage w/ LM1084 regulators / #27 filaments

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So I'm working with some #27 triodes, and I'm dealing with some heater hum, which I gather (from some research) isn't too uncommon even though the #27 triode is indirectly heated. Apparently due to poor heater / cathode insulation.

I figured I'd put together some simple voltage regulators using the LM1084 regulator. I put together a bridge rectified DC supply which feeds a LM1084 configured in the way they show in the datasheet.

I'm feeding 2 x #27 triodes in parallel. So the voltage needs to be 2.5V, and the the draw on the supply is 3.5A (each #27 is 1.75A).

My raw DC supply is sitting at 4.5V and that gives me enough headroom for the LM1084 to output 2.5V.

I've attached a schematic of how it's all wired up. Now I'm experience some very odd voltage inconsistencies that I can't figure out.

With the #27 filaments connected to the output of the regulator, I attach one multimeter directly to the filament pins of one of the #27 tubes and with the output of the regulator maxed, I only measure as high as 1.9V. The really weird part is that while I'm measuring this, I also have another multimeter connected directly to the output of the regulator, right at the output pin of the LM1084, and it measures 2.84V..

How can this be? I've been racking my head, and confirming how everything is hooked up. Measuring directly on the filament pins of the tubes I get 1.9V, measuring directly on the output of the regulator I measure 2.8V.. The tube filaments are connected to the same location that I've got the multimeter connected which is measuring directly at the regulator.

What the heck is going on here??


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First, are you sure that the dc supply is 4.5v fully loaded?

I don't understand the bit where the transformer is marked 9v in your schematic. 4.5v at your first filter suggests a transformer AC RMS voltage of closer to 3.2v.

I don't know the source impedance of your transformer, but if i assume that it's source impedance is like half an ohm with 3.2vac output, that filter network thrown into psud2 with a 3.5A constant current load pulls the DC down to 2.1v.

If this is the same transformer as you were using for AC heat, well, there's your problem, as they say.

Also i think that choke might do better work on the other side of the regulator? *shrug*
The voltage is low because it's basically a choke input raw DC supply.. It keeps the current demands down on the power transformer and rectifier.

The 4.5V output of the raw dc supply is fully loaded, I'm dropping a significant amount of voltage with that 1 ohm resistor in the supply.. I have the 9V power transformer already, so I figured I'd just use it.

Anyways, turns out it's these stupid little alligator clips I was using.. I swapped out for some heavier wire, and I'm measuring exactly the same at the tube pins now, that I measure at the regulator output.
I mean it makes sense that if the voltage was lower at the tube pins than at the output of the regulator, it had to be voltage loss through that wire.. I don't know why I didn't try that before posting.

Thanks for the suggestion.

I set my true RMS Fluke meter to measure AC on the mV setting on the output of the regulator and I measure 0.000mV of AC.. So I'd say the DC output is pretty clean with this supply :)
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