[Amps] GS35B questions

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Fri, 10 Sep 2010 00:03:26 +0200, Marius Hauki
<rezycle.bin at gmail.com> wrote:

> The minus lead should be isolated from the
>chassis since I plan on using a high wattage drop resistor from
>chassis ground to B- to be able to measure grid current.
>
>-Should I place the grid drop resistor in the PSU cabinet or in the PA
>deck? I was thinking that placing it in the PA deck would be safer
>because in case only B+ is connected before B-, the grid drop resistor
>would connect B- to chassis ground and not keep the PA deck at B+
>potential (No I am not going to hot plug the B+ connector, but perhaps
>some other operator will attempt just that. You never know what can
>happen in the mid of a contest battle)

REPLY:

You are correct to leave the B- lead isolated from chassis, but you
don't need the high wattage resistor you mentioned. Instead, you
should connect a pair of diodes back-to-back from the B- lead to
chassis ground inside the PS chassis and another pair across the grid
meter inside the amp chassis as described below. 

Use high surge current rated diodes such as the 6A10 or equivalent.
With the diodes present, you do not need the high wattage resistor you
mentioned above. For GS-35b I would use a 500 mA meter. You should
protect the meter with a low value resistor in series with the meter.
The value of the resistor should be chosen so the diodes begin to
conduct when the drop across the meter + resistor equals about .7
volts.  This combination of series resistor and the back-to-back
diodes mentioned above will protect the grid meter against even
massive arcs. The negative side of the grid current meter is connected
to ground and the positive side is connected to the positive side of
the cathode current meter, below. 

The cathode current meter is placed in the B- lead and should also
have a series resistor on it's negative side and protective diodes
back-to-back across the combination of meter and series resistor. The
value of the series resistor is chosen by the same means as above for
the grid current meter. 

If any of this is not clear, please ask questions. Good luck and keep
us posted on your project. 

Bill, W6WRT