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RE: an interesting journey

I can't find anywhere what you (vinnie2) wrote about the 5V and 10V filaments, but the following factors come into play when "taming" direct heated filaments:
1) voltage
2) current
3) tube gain.

Thus, a 2A3 at 2.5V (square: 6.75) is quite benign and easy to deal with. Besides, it has rather low gain.
A 300B at 5V (square: 25) is more difficult to deal with, but still relatively easy. The gain is also a little higher (if I'm not mistaking) than for the 2A3 but similar enough.
The 6B4G is basically a 2A3 on 6.3V filaments (square 39.69) thus it's more difficult to handle than the 2A3... but the gain is the same, i.e. low.
Now a 2E22 has 6.3V filaments too, but it draws more current (1.5A) and has much higher gain if used as pentode: as a result, a lot more hum than the 6B4G with the same filament voltage requirement.
The 813 has 10V filaments (square 100) and besides drawing 5A it has quite high gain! This literally means that it is very difficult to handle the hum on these.

I have tried operating the 813 on standard mains 10V AC, and the hum was very much audible in the whole room :( so this is definitely out of the question.

BTW the GM70 has 20V on the filaments (square 400 !) but it is not that much more difficult to handle than the 813 because the gain is much lower, being a triode.

You should not have issues with "edginess" but than again this could depend on the amp...
******

http://rh-amps.blogspot.com/

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