This video is of a Marshall DSL100 and the bias procedure is the same on TSL100's and 122's. For the DSL50 the procedure is the same but you would be setting the bias between 40 to 45 millivolts. The middle prong is your ground. The test points or prongs are shown in the video and in the pic's below.

The bias for the TSL60 is NOT the same! It's actually even easier to adjust than the DSL/TSL50's and 100's which have two trim pots as explained above and shown in the video. The center tap on the TSL60 is ground and the left or right tap measures both the left tube and the right tube which is why there is only one trim pot. A setting between 70 to 80 millivolts is good because you are measuring both tubes at once. Just set your multimeter to DC millivolts and dial in the tubes.

 

The bias in the new JVM four channel Marshall's is the same as the DSL/TSL amps but Marshall inconveniently moved the test points and trim pots to the inside of the amp so you have to remove the amp from the chassis where you will find the same three prong test point setup with two trim pots.

I get a lot of email and phone calls asking "where are the test points on my 900? Or 800?" Sorry to say that there are none...  For these models and all other models including the Vintage / modern you must use a bias probe.