I second Don c55 here. Over the years, i have purchased tubes like KT91 & KT88 from ARC and 6550C Winged C from The Tube Store and Parts Connexion for my Classic 120 monos. Biasing is very straight forward (unlike other ARC models). One biasing pot per amp. The only problem i have had when i changed the KT91 to 6550C, i did not have enough range on the pot to adjust down to .65ma. I had to replace one of the resistor beside the pot. ARC sent me so resistors of different values at no charge. Another problem i have experienced about 5 years ago, the 6550c Winged C were produced with smaller pins and fit loosely in the sockets. That issue was wide spread worldwide and an outcry ensued. Fortunately, mine are mounted vertically but those that are mounted horizontally, many people had reported their tubes sagging from their sockets. ARC use much better sockets today.
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