Want a more vintage tone from your Vintage / Modern? This is the ECC803S V1 Option Retube Kit for the Marshall Vintage / Modern 100 Watt 2466 series amps. The preamp section uses a JJ Long Plate ECC803S for V1 for a little lower gain with big thick mids and a little more sparkle in the highs, two standard ECC83S's for V2 and V3, and a balanced ECC83S for the phase inverter in V4. The power tubes are a matched quad of the JJ Electronic KT66's.
If your amp is a combo version or you want to use the correct tube retainers then the "Spring and Cap" style retainers are available. These mount right on the amp using the stock screws and allow you to correctly retain the stock tubes or use any full base KT66.
These are all adjustable bias amps and all you need to check and adjust the bias is a Multimeter and you can find all these under the Related Products Tab. These amps have internal test points. They look like three pins, the middle pin is ground, the right pin is for the right pair of tubes and left pin is for the left pair of tubes. Right next to the test points is the trim pot, it controls the bias for all four tubes. Set it so that each side reads around 72mV to 80mV. We usually like to see each side within 2mV but it appears that there is some error between the two test points so as long as they are within 5mV you will be fine.