[TenTec] SB220-Tubes and QSK

[Dave Heil]

Ken Brown wrote:
> The logic seems sort of sound, except:
> 
> If an amplifier is designed to work with 3-500Z tubes, that is it has 
> the bias set to the proper voltage for clean linear and efficient 
> operation with 3-500Z, and it has a output network designed to match the 
> plate impedance of 3-500Z operating at the plate voltage and bias 
> voltage the amplifier provides to the tubes, how can a tube that is also 
> a 3-500Z operate differently and be more efficient than a 3-500Z?

3-500Z's are zero bias tubes, Ken.  There are variations in the 3-500Z's 
made by various manufacturers.  The Amperex bottles drew less grid 
current at idle for example, than the Eimac tubes.

> Seems like it is not exactly a 3-500Z, but rather a variant of a 3-500Z, 
> and if it is dissipating less power than a 3-500Z, I would be suspicious 
> that it actually needs a slightly different bias voltage to operate 
> properly. If it is dissipating less power, I would suspect that it is 
> operating more on the Class C side with current flowing in a smaller 
> portion of the RF cycle. I would wonder about the linearity.

I draw a different conclusion:  His old tired bottles were worn out. 
They weren't capable of producing the same output as the new tubes with 
the same amount of drive.

Dave Heil K8MN