300B PP bias and expected life time

[jogas]

I am almost ready to start building a 300B PP from a Svettana tech bullettin #56.
I just have a question referred to the 300B bias at 400V and 80mA per tube.
This is 32W against the max 40W plate dissipation.
What is the expected tube life under these (extreme?) biasing conditions?

[audiousername]

It seems that the (old) Svetlana site has gone down - so I'm not exactly sure what circuit you're referring to, but operating a 40W valve at 80% max dissipation isn't particularly extreme. Operating at 140% dissipation is extreme!

[Brett]

Originally posted by jogas
This is 32W against the max 40W plate dissipation.
What is the expected tube life under these (extreme?) biasing conditions?

Lifetime seems to depend on a lot more than just the %Pd, even though ~ 80% is where I bias for domestic amps, and about 70% for MI amps for a not too bad combination of lifetime and sonics. Other aspects of the design, such as how quickly/sharply the B+ and/or heater voltages are applied, the actual heater voltages, temperature of the tubes and how hard they're actually used also factor in. The manufacturer does too.

In all my amps the heaters come up slowly in a controlled manner, are regulated to the correct voltage and about 30 secs later the B+ comes up in the same controlled way.

[ThorstenL]

Konnichiwa,

Originally posted by jogas
I just have a question referred to the 300B bias at 400V and 80mA per tube.
This is 32W against the max 40W plate dissipation.
What is the expected tube life under these (extreme?) biasing conditions?

The operation is still well within spec. For old WE Valves WE would estimate a service life of 5,000-10,000 for their 300B under such conditions.

Sayonara