Tubes in Audio Research Equipment: 6550

I have experimented with the current ARC 6550-C's and the Upscale Audio supplied Type 3 KT-90's in my VT100MKII,and before that,a VT100. As I understand it, the current 6550-C's are an upgraded tube from the ones provided in the earlier VT100's. In my system, the ARC 6550-C's sound quite nice: transparent, good bottom, good top-end extension. In the end, though, I prefered the Type 3 KT90's. These KT90's have more mid-range bloom and present images with a much more 3-D presentation. If you do try the Type 3 KT90's in a VT100, the auto-biasing circuitry in the amp requires matched quartets. Also, I was advised to bias the KT90's to the low 40's verses the 65ma with the factory 6550-C's. The tubes run much cooler and do, in fact, sound better at this lower bias setting. Upgrading the input/driver tubes was also a very effective enhancement.

J_k. Thanks for your helpful feedback regarding the requirement for matched quads in the VT100 and VT100MKII, as it related to my post. In my post, I was relating cautionary information I was told so that anyone (not experienced with ARC gear) reading my post would not confuse this recent ARC design with earlier designs which had individual tube bias points. Thus, they may not know of the need for matched quartets. Also,I may be wrong on this, but I understood that the factory output bias setting for the VT100 6550C's is 65mA. However, for adjusting across the test points for each channel of the VT100, ARC recommends a reading of 130mVDC (0.13 Volt DC) for the 6550C's. Best Regards!