[Amps] Triode grounded grid amp strange failure

[Guido Tedeschi]

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