Hi Mike I think you only have to bias the output stage tubes that are in pairs, so on a SVT3 no problem just change like for like.I have just changed mine in newly aquired SVTPCL that had the same as Marks Pro 3xsovtek 1xgroove tube,though the PRO has 5, so Ithink that must be Ampegs shipping norm, the difference is amazing, harma and philips. Rich