[Amps] B- questions

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Sun, 08 Nov 2009 22:35:36 -0500, Roger <sub1 at rogerhalstead.com> wrote:

>Rephrasing what others have said, the diodes are purely a safety device 
>with the protection of the operator and meter (if lucky).  They play no 
>part in protecting the tube or amplifier circuitry.
>
>As I believe Carl has said, they are a short cut. 

REPLY:

Sorry to disagree, but the B- clamping diode does indeed protect some of the
amplifier circuitry. In the case where the B+ is shorted to ground, the B- is
instantly driven to the full HV negative with respect to ground. The clamping
diode prevents this. Without the diode, the full B- appears in the cathode
circuit (of a GG amp) and can readily destroy the grid meter, the input tuning
caps, can arc the tube from cathode to heater if they are not already connected,
can damage the heater transformer and can send an HV pulse back into the
transceiver. Think about that last one for a moment.  $$$  :-)

I don't see why Carl thinks a clamping diode is a short cut. A diode can do the
clamping much faster than any relay or fuse can remove the HV. It is not a short
cut, it is an absolute necessity, IMO.  Of course you still need the other HV
protection circuitry, but that is in addition to the clamping diode, not instead
of it. 

73, Bill W6WRT