[TenTec] SB220-Tubes and QSK

[Ken Brown]

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?

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.

DE N6KB
>  I  
> had seen the reviews saying that the SB-220 ran cooler after  
> installing the new tubes. Reading this casually, I sort of  
> disregarded it. But when I observed it for myself, I was impressed.  
> It is beginnng to make sense. The input power was about the same  
> before the new tubes. If the amp is 60% efficient now, it must be  
> dissipating about 670 watts. If the additional power I am now getting  
> out was previously being dissipated, the amp was dissipating 300 -400  
> watts more before or about a KW. That explains why it is running so  
> much cooler. If all this logic is sound, the efficiency with the old  
> tubes was only about 40%. Now that is a good reason to buy new tubes  
> even though only a dB or two power gain is realized.
>
>