[TenTec] SB220-Tubes and QSK

[Ken Brown]

Hi Dave
> 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.
>   
Yes, that is what the Z stands for. Nevertheless most amplifiers using 
3-500Z will have some kind of bias circuitry to switch between standby 
mode with cutoff bias and operating mode with operating bias. The bias 
voltage for cutoff is typically determined by a Zener diode and there 
may be another to keep the operating bias just right, even if it is zero.
> 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.
>   
Your are probably correct. However when a directly heated cathode tube 
gets old and tired, it is because the cathode emission has dropped off, 
right? The power output drops off too, and so should the overall average 
plate current, shouldn't it? So, yes you're gonna get more power out of 
the new tubes, and require less drive to get a given amount of power. 
How is it that the new tubes are going to run cooler? The difference in 
drive power, say 100 watts with the old tubes and maybe only 60 watts 
with the new tubes, would be difficult to notice as a difference in 
temperature of the system, and I would expect higher plate current and 
higher dissipation with the newer tubes. That is hotter not cooler.

DE N6KB