Marshall 1959SLP Low Bias readings. Help !

[nyeviking]

Hello from Italy, I have a problem with my 1959SLP Head
All seem OK but I can't reach the correct bias values with VR1
I can reach max 20mA on each EL34, and this is too low.
I checked the tubes, filter caps, bias supply, grid resistences, catode to ground. All seems OK.
The palte voltage is 455 Volts and the bias on the grid is -45 Volts
I hope that you can give me some suggestion.
Thanks,

Nye

[Amp Kat]

See if R28 is a 56k resistor and try changing that to 33k I think that will drop the current but it may be the other way but I think that is the resistor.