----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: <amps@contesting.com>
Sent: Monday, November 09, 2009 4:08 AM
Subject: Re: [Amps] B- questions
> ORIGINAL MESSAGE:
>
> On Sun, 08 Nov 2009 22:35:36 -0500, Roger <sub1@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
I fail to see where you obtain a -3000V, perhaps its a terminology
confusion.
Even a grid driven amp can use the elevated B- and is nothing new used that
way.
In any circuit, come off the B- rail with a fast acting plate overcurrent
relay, adjusted properly with its own shunt resistor and then thru the meter
to ground. The meter is shunted with a very low value high wattage resistor.
Reverse parallel diodes may protect the meter but long term experience with
an almost 50 year old commercial amp still in service worldwide says the
power is dumped with sufficient speed that it becomes a frill. Since the
meter is now unobtanium the cost of 2 diodes cant hurt.
Carl
KM1H
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