Power Tubes for ARC VT100 MkI. Genalex or SED?

In addition, the KT-120 can not be used as a regulator tube in ARC amps. That job still goes to the Winged C 6550.

According to Audio Research the KT 120 is a drop-in replacement for their current amps. The Ref series amps are now shipping with the KT 120 tubes as standard. There were no other changes made to the Ref series amps. I know there are differences between the 6550 and the KT 120 tubes including that they draw about .3 amps more heater current per tube. This is not a problem in current production ARC amps. In the older amps there are concerns with the actual size of the tube and possible cooling problems. There is also a question of range on the bias pots of some older amps. My guess is that the KT 120 will work fine in a VT 100 but I would call ARC to verify.

The KT 120 tubes are available. They are just not available currently from ARC. This will change soon as they build up an inventory. When they are available they will cost approx $20 more per tube than the SED 6550.

The KT-120 is not a drop in for the 6550. In addition to the issues cited by Hifigeek1, the KT-120 specifies a maximum grid 1 resisitance of 50k, whereas many amps using 6550 tubes use a grid resistance of 100K.

Your amp was voiced with Winged C 6550's. Not a big fan of any KT-88's in an Audio Research Amp. As I said in another thread, I don't believe your going to get 2000 hrs. out of that tube. My suggestion would be to wait, if possible, until the KT-120's are available. A far more robust tube for the VT-100MKI. I will need to check with ARC and find out if that amp can run on the KT-120 and if there's enough room in the amp for them as the KT-120 is taller then the 6550. KT-120's draw more heater current but you would get more power out of the amp. When was the last time the front end tubes were replaced and re calibrated? That will make a big difference in distortion products.

The SED 6550's have been out of production for a while now from what I've heard. That being said I have the SED 6550's and Genalex Gold Lion KT88's but in an Octave integrated. Personally I think the GL KT88 has better deeper bass with a slightly clearer mid range and the highs are about the same. I prefer the GL KT88's over the SED 6550's in my system