Adjusting to a bias voltage isn't really very accurate because you also need to take into account what the plate voltage is, and how the particular tubes want to idle. Not to mention but since the AC wall voltage is higher than when the amp was made, that -55v reading isn't very accurate anymore. A better way to set the bias is by the cathode current method. This does require either putting 1 ohm resistors in between pin 8 and ground on the power tubes, and measuring across this resistor and measuring the plate voltage, or using something like a Weber Bias Rite meter. I recommend the Weber Bias Rite as then you can use it with any amp with the same socket arrangement. You should first adjust to maximum negative voltage out of the bias circuit with the tubes out. Then with the Bias Rite, you plug it into the socket, then plug the tubes into it. You turn the amp on and let it warm up. If you spring for the better of the Bias Rites, then it will read the plate voltage, and the plate current with the flip of a switch. So you read the plate voltage, then flip your switch and read the plate current. You look up the spec for the KT88 and it says what the maximum dissipation is for that tube, which is 42 watts. So you take 60% and 70% of that number, and that is the range you want to adjust within. I'll give an example....say your plate voltage is 560v. Say you measure the cathode current with the meter and it says that you have 35. This is in voltage, but it gets converted to milliamps across the 1 ohm resistor that is in the meter. So you take 560 * .035, which gives you 19.6. This is the watts of plate + screen dissipation that you have right now, though you can ignore the screen dissipation in most cases since it is an error in a good direction, and is minimal too. So 60% and 70% of 42 are 25.2 and 29.4 respectively. So the 19.6 is biased cold. You readjust, but you have to measure your plate voltage again and recalculate, because the reading will change since the current draw changes as you adjust the bias pot. So lets say now it is 550v and 45 for your two readings. This gives you 24.75 watts dissipation, (550 * .045) which is closer to the range, but anywhere in there is fine if it sounds good, and the colder it is, the more longevity you will get out of the power tubes. Colder also tends to sound better in a bass amp within reason. Hope that helps!
Incidentally, the 6550's would have been fine too and were also original tubes in Sunns depending on the vintage.
Greg
