ARC recommends that Sovtek be replaced no
Later than 2000. The 6H30's go 4000. Trust me by around this point that Sovtek isn't probably full on any more. You've used about 80% of its recommended life in the circuit. Test it to confirm. Most guys start hearing issues with them by the number of hours you cite you have on it. It could very well be skewing your sonic comparisons somewhat. ARC runs those 6550 tubes hard in the power supply which is why they say replace it no later than 2000 hrs. Even with dual power supply channels in my Ref 40 so each tube isn't run probably as hard handling only one channel not both through the tube they say to replace no later than 2000 hrs.
As Harlequin notes, vintage GE, Sylvania or Tung-Sol 6550 tubes would last longer and sound unbelievably better but I'd still test them on a Maxi-Matcher around 2000 to confirm status of the tube.
ARC has to use and recommend the Sovtek as its the only current production 6550 that is robust enough really on a current tube production basis for them to be able to use at the current time with the demise of the Winged C which was a much better tube sonically.
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