[Amps] AMP INT LK500ZC

[Bee Walton]

Serial number 4017.

Symptom:  When A/C power switch is turned on, plate current is 325 MA and
does not go to zero.

Plate voltage is normal at 3200 volts DC.  Grid current meter goes
negative; pinned to left side of meter.  Do not use QSK.

Troubleshooting thus far:  Disconnected B+ line at power supply board. 
Power supply is OK.  Substituted two other known good 3-500Z tubes; tubes
are OK.

Measures 5.79 rather than 5.0 filament volts (T1).  Caps C24, C25, C22,
C23 and C28 check OK.

Should D2 the 1N3308 Zener Diode be suspect?  This component seems
difficult to check.

Request help to troubleshoot the above described symptom.

Thanks,

Bee, K4JLD