Marshall 6100 / 6101 ECC803S V1 Option

These are all adjustable bias amps and all you need to check and adjust the bias is a bias probe and a Multimeter, or our all-in-one Eurotubes "Pro One" bias probe that reads both the plate current and the plate voltage simultaneously. You can find all these under the Related Products Tab. 

You can use the "Generic Bias Video" we made using a Marshall 800 for reference.

We've had a lot of experience with both the LE and LM versions, Marshall does recommend using 1K Screen grid resistors and a bias resistor swap to run EL34 type tubes but since the LM 5881 amp uses hefty 5 watt 470 ohm screen grid resistors we have had no issues running EL34 type tubes including the JJ E34L's, KT77's 6CA7's and EL34's. As long as warmer grades are used biasing has not been an issue. 

The video below explains the difference between bias probe types.