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That is kind of mysteries to me and very none expected. From the people who use DHT I heard many stories that 2.5V filaments driven by AC are less noisy then 5V or 7.5V. That is kind of self-explanatory as with higher AC voltage the more noise. Still what I am witnessing is that that my 4V tubes have less or identical noise with my 2.5V tubes.

I have as now 6 types of 4V tubes:  RE604, KL71403, AC044, YO186, LK4110 , RE614 all of them single plate tubes and I see absolutely no noise increased when I run them despite I it almost twice higher voltage. What I am talking about “noise” I mean the absolute numbers of AC noise measurement not the auditable noise. I was wondering why it is so.

From my Russian sources I learned about the history of YO-186 tube and it was said that the tube it was made for instrumental use to measure low-levers inductance and the tube was designed especially to be driven by AC on it's filaments.  Later on the tube was used during 30s in cinema amplifiers and with AC on it's hears only. Interesting...

In the Yamamoto article:

http://www.jacmusic.com/Yamamoto/html/Amps/A06/description-ready-product.html

…they say: “The 4Volt technique gives best ratio between diameter and length of the filaments”. It does not sound very credible to me, in fact is sound more like marketing BS.

Anyhow, does anyone have any explanations why my 4V tubes have no more AC noise then 2.5V tubes?

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