----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: <amps@contesting.com>
Sent: Sunday, November 08, 2009 5:46 PM
Subject: Re: [Amps] B- questions
> ORIGINAL MESSAGE:
>
> On Sun, 8 Nov 2009 12:41:38 -0500, "Carl" <km1h@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
I'll repeat one more time. The dinky diode doesnt prevent tube and circuit
damage, it shorts after the damage is done. All it provides is a false sense
of security and a cheap way to do metering. As seen in the examples Ive had
in for repair over the decades.
Carl
KM1H
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