Plate voltage question

[kku]

So according to checkout page on manual the plate voltage(pin3 on 6B4G) should be 308V +/- 5%. But looking at the datasheets of NOS 6B4G, the max rating for class A single tube operation, plate voltage is 250V. Not so good for the lifespans of 6B4G's, is it? Or am I missing something here?

http://www.hebertech.com/datasheet/tung-sol/6b4g.html

[Ty_Bower]

Or am I missing something here?

The datasheet specifies the maximum plate voltage measured with respect to the cathode. In other words, for self-biasing designs you must subtract the cathode voltage to get the correct potential across the tube.

[kku]

I see, I knew I was lacking knowledge.

I'm getting 311V on plates which is normal I know. I'm using another 50 ohm choke in place of R17a/b. Is there any way to lower plate voltage to something around 290-300V other than getting a new lower voltage PT? I know these numbers are OK but I just want to make sure my tubes live a bit longer.

[kku]

I'm thinking of getting couple of Triad C 14X chokes(150 ohm/200mA) to replace R17a/b and the first choke(at J5). I'm guessing doing so will lower the voltage on plates. Any other ideas to lower the plate voltage? Perhaps different rectifier to lower B+? I'm looking for plate voltage of something like 230-240V. Currently it's at 259V.

BTW, This amp is the quietest amp I've ever built. No hum at all; even from few centimeters of my 98 db full-range speakers.

[Shannon Parks]

You could replace the chokes with the higher DCR chokes, but I wouldn't worry about it - you're within +/-5% of 250V. I think the stock GSG is pretty gentle on the old 6B4Gs. Some of the datasheets spec 275V as the max, too. And since we're not running them too hard current-wise, I would leave it. Enjoy!

Shannon