[Central-Electronics] 20A 6AG7 screen grid resistors

[Jon Kannegaard]

It's a world gone mad: paper caps good;  mica caps bad.

As Colin points out the screens draw almost nothing so 180 ohms won't drop the voltage more than a smidgen.  This is another strike against the mica bypass caps since something was drawing enough current to fry the resistors.  So I checked a few more or those big micas.  They are all very leaky except, interestingly, the ones used to bypass the AC mains which I replaced anyway with X1/Y2 rated caps.

Meanwhile several of the Sangamo paper caps have tested good!  That was the last thing I expected after my earlier experiences with them.

Thanks again.

Jon

On Mar 20, 2011, at 1:52 PM, Colin Lamb wrote:

> Hi Jon:
> 
> At 180 ohms, it is not a dropping resistor.  Calculate the voltage drop of the very small screen current through 180 ohms.  It will probably be no more than a couple of volts.  180 ohms would be an isolation resistor.
> 
> Also, unless the 6AG7 is operating class A (it probably is), then a dropping resistor would not be appropriate as it would cause the screen voltge to change with power level, increasing distortion.  Screen voltage in a linear amp (unless class A) should be regulated.
> 
> 73,  Colin  K7FM
>