Blackstar HT100 redplates

[wksonamps]

I just bought this amp and plugged in a speaker cab and powered it up and the power tubes red plated within 15 seconds. So I swapped power tubes to some JJ's that I had in stock and fired it up and have the same problem. So I opened it up to look inside for anything obvious and I noticed that I am missing the component PR2. (don't know why anyone would want to remove it) Now I have read online that sets the bias for the power tubes. So I know thats very important. Does anyone know where I can get the little variable pot or know what resistance it is?
Much thanks for all help.