[Amps] Bill Orr comment

[Steve Katz]

::I agree completely with the comment from Ueter.  You need to operate a 3-500Z at its maximum rated anode voltage in order to derive 2*Pd (1kW) output power in AB2, otherwise Ic will be beyond safe ratings.

AB2 eff cannot possibly be 75% unless you're at 180 degree conduction angle, where the maximum theoretical efficiency is 78.5% for Class B.  AB2 should be between 50% and 60%, usually.

If you pick the more optimistic value (60%), and use a 3-500Z as an example, Pin needs to be 1667W to achieve 1kW output.  That leaves 667W Pdiss for a 500W rated tube.  At 4kV anode, this is 417mA, which should be safe enough.   But rarely do amplifiers using 3-500Zs operate at 4kV.  The SB-220, which was the product in question prior to this thread, runs about 3kV nominal.  At 3kV, Ip needs to be 556mA for 1667W Pin.  That not only exceeds the dissipation rating for the tube, but also exceeds the Ic rating by about 39%.

Take a look at IMD when it's running that way.

I don't have any tubes that can run 2.5x Pdiss (rated) as output power.

Orr made some very quackish remarks in his day, wish he were here to partake in this.  One was that an 8877 would produce harmful X-radiation when operated above 4kV Ep.  I challenged him on this, and used an MRI machine in a lab with X-ray facilities to operate the 8877 at 7500V Ep, and not a trace of X-radiation could be found, anywhere.  (This was at PPPL in NJ, back in 1984, about the time of Orr's comment.)  So I questioned him how he reasoned the X-ray theory, and as it turns out, it was just that: A theory.  Eimac had no data to indicate such radiation occurs at any particular voltage, or that it occurs at all.  

That was the basis of my "a bit of a quack" remark.  I still have his Radio Handbooks and many Eimac app notes and use them as guidelines like anyone else.

-WB2WIK/6



The 3-500Z does it when the anode gets 4KV, otherwise its current is the
limit.
You are right with the 8877.
It all depends on the type of cathode.

73
Üeter 

____