[Amps] setting the grid adrift

[Tom W8JI]

> I read this over, and decided it need some explanation.
> I'm not making some kind of challenge... I am honestly
> happy that some kind of controlled test can lay the issue
> to rest.

Chris,

In the manufacturing process, great effort is made to 
understand why and how things fail and what to do to prevent 
failure. When you have several thousands of something in the 
field you don't want to start buying radios or having other 
major problems.

The AL572 was a good platform for tests like this, since 
more than 50% of brand new Svetlana tubes at the time of 
initial production arced. It was a horrible situation since 
the amplifier was really engineered around a single tube 
source that turned out to be very unreliable. (Cetron was 
priced out of reason, and there were no Chinese at the 
time.) That meant virtually 100% of amplifiers would have 
hard faults during use. Because it was impossible to match 
tubes closer than a certain amount, the tubes required a 
little self bias for optimum IM performance.

With grids hard grounded, fault voltage at the cathode was 
in the area of a few hundred volts maximum during a hard 
fault. With the grids grounded through small resistors, 
fault voltages commonly reached 1500 volts and higher. 
Because the grids had to be lifted off ground to minimize IM 
products and prevent unequal dive levels, a safe way around 
the higher cathode fault voltages had to be found.

I can tell you with absolute certainty there were multiple 
design problems caused by using the unavoidable large 2 watt 
resistors in the grids. It was like being between a rock and 
a hard place. If one or two tubes wouldn't have hogged 
power, those grid resistors would have been out and the 
grids grounded in a heartbeat.

73 Tom


73 Tom