[Amps] Triode grounded grid amp strange failure

[Guido Tedeschi]

Thanks to everybody who answered.
I think that what Dave wrote could be very probable: next weekend I will 
open the PA and let you know what I will find.
The HV comes with a coaxial cable, but I think that cable and connector 
have two concentric paths (B+ and B-) and screen as safety protection 
connected to ground, otherwise the B- would be connected directly to 
ground and it would be impossible to measure grid current.
Thanks again and 73
Guido, k2bcp


Il 15/07/2015 02:21, Dave Hachadorian ha scritto:
> This type of thing can happen if B- and ground are tied together. 
> There should be a beefy diode between B- and ground.  Sometimes this 
> diode can short if there is a high current glitch event. Also, the 
> plate current meter may have blown open during such an event.
>
> Dave Hachadorian, K6LL
> Yuma, AZ
>
>
>
>
> -----Original Message----- From: Guido Tedeschi
> Sent: Tuesday, July 14, 2015 1:47 PM
> To: amps at contesting.com
> Subject: [Amps] Triode grounded grid amp strange failure
>
> Hello All,
>     a homebrewed two chassis 3CX3000A7 GG amp of a friend has this
> strange failure when switched on TX:
> - no more current on plate meter
> - 200 mA of grid current with no signal
> - some power out (few hundred Watts) when driven with 40-50 W, to limit
> the grid current to less than 500 mA, but still no current on plate meter
>
> We don't have any schematic diagram so, before opening the amp and
> troubleshooting, some suggestions to begin would be very appreciated
> (hoping that the tube will not be the culprit...).
> Thanks and 73
> Guido
> K2BCP
>
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