To your more information:
Grid-bias pot can damage the circuit if not properly adjusted or turned to the extreams. In my vintage model MB100 it's accessible from the top pannel. I don't know what's in your case but I assume that this pot might be hidden on-purpose. I'm sure that with help of VTL(call or e-mail) you'll find out fast how to locate the master-bias pot and will be able to do a simple steps that(not in either VTL website or manuals) I'll share with you.
In your case(under-bias) which is quite normal, you simply set all tubes to max possible equal bias voltage and than adjust master-bias to the nominal manufacturer recommended level taking measurements from only one of tubes. Repeat measurements on the rest of tubes to ensure their equal bias voltage. Please note that under-bias cases are not distructive unless you have a largely exceeding negative grid voltage. Your bias-voltage readings will be somewhere bellow 100mV!!! You than have a few seconds to turn your amp off. Negative grid voltage is checked between pin5 of any tube socket and ground that should be in value ranges specified by manufacturer(find that out too) with amplifier on with no tubes. Readjust the bias voltage if neccessary to normal using master-bias pot.
If the case is opposite, i.e. you're having bias voltage too high at minimum pot setting you should turn off the amp immediately, remove the tubes turn the amp back on and measure between ground and pin5(grid) of one of the tube sockets. Increase the negative voltage(using master-bias pot) by 4...5V making it for instace instead of -45 to -50V and re-measure results with tubes and amp on. Turn the amp off before installing new tubes. If after number of measurements you end-up with negative grid-bias voltage too high than either the tube is probably bad or deffected or you've got added technical issues with unit that needs to be serviced.
As you might know that aging tubes tend to change their characteristics and even if you know you're setting bias pots at extreams in order to reach normal you'd better readjust the master-bias to have your local ones in the middle area for larger bias flexibility.
Grid-bias pot can damage the circuit if not properly adjusted or turned to the extreams. In my vintage model MB100 it's accessible from the top pannel. I don't know what's in your case but I assume that this pot might be hidden on-purpose. I'm sure that with help of VTL(call or e-mail) you'll find out fast how to locate the master-bias pot and will be able to do a simple steps that(not in either VTL website or manuals) I'll share with you.
In your case(under-bias) which is quite normal, you simply set all tubes to max possible equal bias voltage and than adjust master-bias to the nominal manufacturer recommended level taking measurements from only one of tubes. Repeat measurements on the rest of tubes to ensure their equal bias voltage. Please note that under-bias cases are not distructive unless you have a largely exceeding negative grid voltage. Your bias-voltage readings will be somewhere bellow 100mV!!! You than have a few seconds to turn your amp off. Negative grid voltage is checked between pin5 of any tube socket and ground that should be in value ranges specified by manufacturer(find that out too) with amplifier on with no tubes. Readjust the bias voltage if neccessary to normal using master-bias pot.
If the case is opposite, i.e. you're having bias voltage too high at minimum pot setting you should turn off the amp immediately, remove the tubes turn the amp back on and measure between ground and pin5(grid) of one of the tube sockets. Increase the negative voltage(using master-bias pot) by 4...5V making it for instace instead of -45 to -50V and re-measure results with tubes and amp on. Turn the amp off before installing new tubes. If after number of measurements you end-up with negative grid-bias voltage too high than either the tube is probably bad or deffected or you've got added technical issues with unit that needs to be serviced.
As you might know that aging tubes tend to change their characteristics and even if you know you're setting bias pots at extreams in order to reach normal you'd better readjust the master-bias to have your local ones in the middle area for larger bias flexibility.