[Amps] B- questions

[Carl]

----- Original Message ----- 
From: "Bill, W6WRT" <dezrat1242 at yahoo.com>
To: <amps at contesting.com>
Sent: Sunday, November 08, 2009 10:01 PM
Subject: Re: [Amps] B- questions


> ORIGINAL MESSAGE:
>
> On Sun, 8 Nov 2009 18:43:18 -0500, "Carl" <km1h at jeremy.mv.com> wrote:
>
>>
>>I'll repeat one more time. The dinky diode doesnt prevent tube and circuit
>>damage, it shorts after the damage is done.
>
> REPLY:
>
> You are half right.
>
> I agree it won't prevent tube damage, but it will indeed prevent damage 
> caused
> by the B- line being driven to the full HV negative with respect to 
> ground.
>
> Think about a negative 3000 volt pulse being coupled back through the 
> input
> circuit to the output of your transceiver. Think about a negative 3000 
> volt
> pulse appearing on the cathode of your tube if the filament is DC 
> grounded. If
> the filament is connected to the cathode, think about a 3000 volt pulse 
> applied
> to the secondary winding of the filament transformer. Think about the tube
> socket, the tuning caps in the input circuit, etc, etc...   Few if any of 
> those
> components will withstand the full HV.  Get the idea?
>
> Thank goodness for the dinky diode!     :-)
>
> 73, Bill W6WRT

In amps that werent designed correctly in the first place it might work 
depending upon the speed to short. The better amps have a 3kV or higher 
coupling cap to keep HV from going to the xcvr. Even the Heathkits have that 
protection. Tube failures in them do not take out filament transformers.

My own GG HB amps use 100-115W wirewound resistors and 10 kV coupling caps 
or are using VHF/UHF input circuits with inductive coupling with sufficient 
spacing. They also use control circuitry to dump all voltages when a short 
occurs. In 25+ years Ive only had one bad tube (3CX1000A7) and that was one 
I was testing and half expected a problem; the amp performed as 
designed.....nothing else was damaged.

When working on store bought ham amps its usually a crap shoot, especially 
when they arrive here fried. For not much more money the damage could have 
been minimized to just the tube.

Carl
KM1H

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