I want to put 6550 tubes in my Marshall 2555

[marshall2555]

Which resistor do I need to change in order to bring the bias adjustment pot into range? What ohm value should that resistor be? What wattage rating?

[marshall2555]

I am looking at the schematic, it looks like r21 and r22 are the last line of defense between the variable resistor and the tubes. Is this right? Are those the right ones to swap?

[stokes]

R21 and R22 should not be changed.If the pot wont get you into the bias range,R23 should be adjusted,depending on which way you want to go.If your pot is adjusted for the most resistance and you are not in range,you need more resistance at R23 and vice-versa.

[marshall2555]

so should R23 either be a 47K or a 56K depending on 6550 or el34? what wattage rating of a resistor should be used in this application?

[stokes]

You have to put the tubes in and see where you stand first.The bias pot may have enough adjustment as is.Like I said,if you install the tubes and the pot wont get you where you want to be you then adjust the resistor between the pot and ground accordingly.There is no hard and fast value,that I know of.If you dial in the pot to its lowest resistance and you have to go lower then you lower the value of the resistor to ground,if the reverse is the case and you have the pot at its max resistance and you need more,you increase the value of the resistor between the pot and ground.A 1/2 watt resistor is fine,you really have no current in a bias supply.First install the tubes and see where you are,its possible the pot will have enough range as is.

[marshall2555]

R23 is empty, does this mean R22 is the one? R22 has a resistor piggy backed off of it, probably what the amp tech used to change the range of the trim pot if this is correct