[Amps] B- questions

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Sun, 8 Nov 2009 12:41:38 -0500, "Carl" <km1h at jeremy.mv.com> wrote:

>
>It adds another failure point. A well designed amp should have only the 
>voltages safely removed during an event and once the fault is cleared a 
>reset will have it back on line with no other damage. Hams seem to be more 
>interested in dinky designs and shortcuts. Look at any of the larger 
>Ameritron amps. The dinky diode shorts after the tube short takes out 
>everything in its path....some protection!

REPLY:

The purpose of the diode from B- to chassis is to prevent the B- lead from being
driven more than about one volt above ground. Nothing "dinky" about it, just
common sense design. 

Without the diode, a short from B+ to ground will drive the B- lead to the full
HV negative with respect to ground. Since none of the components in the cathode
circuit are designed to ever see this much voltage, much damage would likely
follow. For less than a dollar, cheap protection.

73, Bill W6WRT