Trying a set of KT-120 in my premier 11-A

[Bugs762]

Okay, now I'm rethinking what I know (which is a little bit) and what I don't know (which is a lot). I was thinking the bias point was a function of the tube, meaning that when you set the bias you where the tube wants to operate. But what I tink I'm hearing is that the bias point is a function of the amp, meaning when you set the bias you are setting the amp to run where it want to operate.... So, the amp will have the same bias point regardless of which tube is being used? However, all tubes may not function at the amp's bias point. I'm I close here?

Does anyone know if the MV 60SE was voiced for the GE 6550A or the Winged C 6550C? I've read the Stereophile article and the different poosts on this forum and I really want to try the KT-120s, but I don't want to damage the amp either. I haven't received an answer back from CJ when I emailed them and asked.

Many many thanks,
Doug

[ronenash]

Regarding your first point, both are true. The bias point is to bring the tube to its linear operating conditions. A 6550/KT88/KT120 operate in their linear range when the tube dissipation is above 24w and below its maximum dissipation value (35w for the 6550, 42w for the KT88 and 60w! for the KT120). Operating the tube at its maximum dissipation will provide more Class A power but will shorten tube life.
The tube power dissipation is a multiplication of the B+ voltage and the bias current through the tube. CJ Operate their power amplifiers at a B+ voltage of 525v and a typical bias current of 60mA. 525*0.06 = ~31W.
The bias current has nothing to do with amplifier power. In order to achieve more power from the same number of output tubes you need to increase the B+ voltage. with two output tubes you will get 100w at a B+ voltage of ~580v, this will be above the safe operating range of most 6550s and you will need a KT88 to achieve that much power from a pair of output tubes. The KT120 can go even higher and achieve 150w from a pair of tubes. The operating point that CJ uses with the KT120 is very conservative and ensures the tubes will last for a long time.

As for the MV60SE, it was designed with the 6550C as the GE 6550A were no longer available in mass at the time. I think the last amplifiers that were supplied with GE 6550A were the Premier 11A,12,8. With that said the MV60SE will work with KT120 as well.

Hope this clarifies things for you.

[Bugs762]

Originally Posted by ronenash

Regarding your first point, both are true. The bias point is to bring the tube to its linear operating conditions. A 6550/KT88/KT120 operate in their linear range when the tube dissipation is above 24w and below its maximum dissipation value (35w for the 6550, 42w for the KT88 and 60w! for the KT120). Operating the tube at its maximum dissipation will provide more Class A power but will shorten tube life.
The tube power dissipation is a multiplication of the B+ voltage and the bias current through the tube. CJ Operate their power amplifiers at a B+ voltage of 525v and a typical bias current of 60mA. 525*0.06 = ~31W.
The bias current has nothing to do with amplifier power. In order to achieve more power from the same number of output tubes you need to increase the B+ voltage. with two output tubes you will get 100w at a B+ voltage of ~580v, this will be above the safe operating range of most 6550s and you will need a KT88 to achieve that much power from a pair of output tubes. The KT120 can go even higher and achieve 150w from a pair of tubes. The operating point that CJ uses with the KT120 is very conservative and ensures the tubes will last for a long time.

As for the MV60SE, it was designed with the 6550C as the GE 6550A were no longer available in mass at the time. I think the last amplifiers that were supplied with GE 6550A were the Premier 11A,12,8. With that said the MV60SE will work with KT120 as well.

Hope this clarifies things for you.

ronenash -

Thank you for taking the time to explain amp/tube biasing to me. Still a little over my head, but I get the general idea.

Best,
Doug

[ronenash]

Originally Posted by Bugs762

ronenash -

Thank you for taking the time to explain amp/tube biasing to me. Still a little over my head, but I get the general idea.

Best,
Doug

All you basically need to know is that KT120 are safe in your Pr11A and that you do not need to change anything except setting the bias with using the LEDs once you replace output tubes.

Enjoy and let us know how you like the KT120s in your 11A if you get to replace them.