Problems w/biasing a lC50

[gtrtech]

A customer brought in a Laney LC50 combo, It's not the Ser. 2 but it should be close enough for me to use the schematics from one. The problem I’m having is trying to bias it. After installing 1 ohm res. to the cathode and setting the volume to where the wave on the o’scope is just under the breaking point. On one side I get a reading of 0.70 and I get a reading from the plate at 390v which after multiplying them I get 27.3 which is way to hot. I’ve reversed the tubes, tried different tubes and still the same. The customer has 2 JJ EC 803s preamp tubes in it and I’ve heard because of the long plate structure you shouldn’t use these in a combo, am I heading in the right direction or should I look else ware. BTW curious at what freq. & amplitude does everyone use when checking the wave w/ a O’scope and what are the best setting for the scope in dual mode for the setting
Thanks
Tony