Some rough ballpark numbers: If there is a 25 ohm glitch resistor and an
instantaneous short from B+ to ground, there could be a maximum current of
3000/25 or 120 amps. If the meter circuit was 1 ohm, then a pulse of 120vdc
would appear across it. The length of the pulse depends on the amount of
capacitance in the power supply filter circuit and how long it takes a fuse or
breaker to interrupt the AC supply.
Most relays need 8 to 20 milliseconds to operate. Would a 1A meter be
damaged by a 120v spike for a few milliseconds? I don't know but maybe someone
here knows.
The reverse connected diodes would absorb this energy and hold the voltage
to maybe 1v or so until the surge rating of the diode is reached and then
it would fail shorted, providing further meter protection.
Perhaps a really hefty diode would withstand this kind of abuse but I
wonder about how quickly it would begin conduction considering the high
capacitance of the junction.
73,
Gerald K5GW
In a message dated 11/9/2009 9:59:03 A.M. Central Standard Time,
km1h@jeremy.mv.com writes:
----- 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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