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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

What is the best way to connect DC filaments?

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Genuine 'very knowledgeable experts' will normally tell you how to do it, and why exactly they disagree with the manufacturer's datasheet. You can then weigh up their argument, and ask others to consider it too.

However, be patient. Someone may pop up here with good advice.

It may help if a mod changes the thread title to 300B DC filaments, as some people may just assume that you are asking about DC heaters for indirectly-heated valves. In the UK we distinguish between filaments and heaters, but elsewhere that distinction is not always maintained.
 
Hi!

The way you did it is the best way how to connect DC filaments if your filamet supply is very clean. That's how I do it as well.

A hum buck potentiometer is only useful with DC filaments if your supply has some ripple left. Then you can tune that partially out with a um buck. However a good, clean DC supply does not need that.

Just be aware that bias voltages are usually given for the mid point of the filament voltage in the datasheet of DHTs, so you need to take this into account for the bias voltage.

Best regards

Thomas
 
Rod Coleman has a lot of posts to dig through! Thanks for the heads up, I will look over them over some coffee. I already built my filament regulators with LM338 voltage source supply (one for each tube), would Rod Coleman's ideas still be relevant? I would prefer not to use new supply when what I already have works fine.

I just want to be able to decouple any noise generated by the filament supply. The 300B is for a preamp and needs to run very quiet.
 
A preamp? Cool. Certainly hum can be kept arbitrarily low with regulation, and excess noise will vary with tube/valve construction. The Rod Coleman circuit is a high impedance current source feed to the filament, with the intent of removing the power supply from the signal path. All low impedance power supplies are inherently in series with signal to ground.

All good fortune,
Chris
 
Thanks for the link Chris.

I've been reading up and thinking a lot. What about:

-floating DC 5V supply from 6.3VAC
-connected to humpot used in AC heaters, with center tap connected to ground

Would this be better for decoupling any noise from the regulated 5V filament supply? It's basically an AC filament heater but with DC and the humpot not really doing anything.
 
I think the goal is to have a dead quiet DC supply and a separate signal path to ground (or wherever). A battery sitting on a sheet of glass, and your humpot, fills the bill.

With a DC supply, the classic old-skool choice instead of a hum pot was two 50 Ohm resistors in series across the filament, signal taken from their junction. The second goal, removing the power supply from the signal path, is the trickier bit.

All good fortune,
Chris
 
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