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DHT4ME wrote:

This is true. It comes down to thermal cycling. I love the #26 tube but it is real hard to get quiet.  It has a 1.5v 1a filiament. The heavy draw causes thermal cycling which is very hard to null out. There is also more ratiated garbage from the current. A tube with 2.5V is far easier to get noise down. Go the opposite direction and it starts to get bad again. The #10 is a Thoriated Tungsten filament 7.5v and it is a bitch. It has a thin wire filament. The #50 has a 7.5v filament but is oxide coated and much thicker. The #50 is less hum prone than the thin wired #10.

Ok, it is might be true but it would presume that all tubes have the same length and type of filament. In reality it is not accurate as there is 5-6 different type of filaments not to mention that there are a few materials the filaments were made from. So, I think the “4Volt technique gives best ratio between diameter and length of the filaments” but it is NOT the only criterion that determines the AC noise. At least it is how I think.

DHT4ME wrote:

I have never bought any euro DHT's but have heard nothing but high praise for them. I one day would like to try the RS241. It is supposedly stunning.

I am very glad that I put the idea of 4V into the Milq channel that was my parley into the world of European DHT tubes. My initial sentiment was to put the YO186 into the game but then all those Telefunken, Klangfilm (which is BS name), Siemens, Masda, Ferranti, Marconi, Triotron, Osram , Valvo, Philips, Tungsram and others begin to pop up.  I always was a fun of German tubes of Telefunken/Siemens and I figured out that it might be a good chance to dive into it.  In the tube of the power level that was interning Germans had RN604 and AD1. The AD1 has own socket but RN604 has the same socket with YO186. So it was no brainer what to do next.  BTW, the RS241 is replaceable with RN604…

BTW, the 10Y/VT25 tube that you mention was the only tube that I would like to try (I got a stunningly good party of them – probably the best tubes that I ever bought) and this is why I initially wanted to keep 6.3V for my MF channel. (It is 15A transformer and it might be driving 10Y with 7V). Then I discovered that 10Y would need very different operation point to do it’s best. So, I kept my 10Y objectives out for now. Still, the preliminary results I am getting from the 2.5V and particularly 4V tubes, the tubes that I already have, suggests me that I will be a very content pussy with this amp.

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