Pre-amp tubes normaly don't need to wear in as far as I know,maybe you had a bad contact?
Ok,I found out more of the issue of the Bias.
Above you see a part of a data sheet from a 12BH7 tube.
The part between the yellow lines is the max. -dc bias voltage for the whole tube = -50 Vdc,since the tube is a double triode,each side of the tube may have a maximum of - 25 Vdc .
So the bias has to be adjusted with this value measured on bove sides,as you can see on my drawing posted earlier on this thread.
I did also some mods on the amp,I put a accutronics reverb unit in it.
Left you see the digital reverb unit from accutronics,there are 3 types of reverb,short,medium and long reverb time.I took the medium one.Along with the unit you see also a voltage regulator,wich gets the 15 Vdc to the required 5 Vdc,also did I put the reverb pot on the pcb.
Right you see the connector of the blackstar pcb,where I connected the wires from the reverb unit to.
Left you see the reverb unit in place in the amp,the large piece of metal is for cooling of the voltage regulator.
Right you see the part of the schematic that shows the function of the connector pins.On the Fx send comes the input of the reverb,on the Fx return comes the output of the reverb,and the Cd input,0 V is the earth connection of the reverb,the +15 volts delivers the power for the reverb unit.
Above you see the back side of the chassis with the knob for the reverb level and the CD input.
Left you see the backside panel,where I mounted a little computer fan onto it,also you see a voltage regulator that regulates the voltage from Dc12 to Dc 9 volt,in this way the fan makes less rpm,and makes less noise.
Right you see the panel with the fan in place,I did put the fan just befor the power transformer,so this gets the first cool air,then the air flows into the box,cools the tubes and leaves the box after that.
Also you see here the seperate power supply for the fan.