Phil Clements wrote:
>The grid-trip circuits I have used for years on 8877's
>and others look at the voltage drop across a resistor
>hung from the cathode to ground. This voltage goes through
>a pot to set the desired trip current. The pot gives another
>C. 1000 ohms or so "glitch" protection for the base of the
>transistor that opens the PTT line by removing coil voltage
>from the VOX/QSK relay. I have had many "big bangs" over
>the years, but have never lost a grid-trip transistor. (knock on wood)
>Perhaps just good luck, or over-engineering.
>(((73)))
>Phil, K5PC
That's the one I'm familiar with, but it doesn't seem like the one Rich
is describing (which is why I'm asking for more details).
>
>
>
>> >For instance, typical grid-current
>> >"protection" circuits for 8877s use a 1A to 2A transistor to interrupt
>> >the flow of grid-current. However, during a parasite, grid-current goes
>> >out of sight. An 8877 cathode is capable of >50A-peak emission. This
>> >amount of current will C-E short a 2A transistor in short order. Thus,
>> >there is essentially no grid protection during a major parasitic
>> >oscillation.
>>
>> Please can you give an example or reference to such a deficient circuit?
>> I've never seen one like you describe (but maybe I've led a sheltered
>> life). Where exactly is this 2A transistor?
>
>
>
>
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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