Thanks for the reply. I am using a digital meter. I put the old 6550 back in the amp and they could be biased easily to 250-300 millivolts. Some have said the new tubes were too "cold" and to look for tubes with higher plate current. I did try some SovtekWE with known current of 44 and these could be appropriately biased so I think I just need the tubes to be matched but select those with higher plate current.
Brand new tubes MB-450 and bias adjustment
I replaced all 6550 with Apex Matched JJ Electronics 6550 tubes. Got all tubes installed and went to adjust bias and found that I could not reach the VTL suggested 250-300 millivolts, with many of the JJ tubes maxing out in bias adjustment at or a little above 200 millivolts. I set them all to 200 millivolts and gave a listen and they sounded really good, but for many of the tubes they are at or near the maximum bias adjustment on the amp meaning no way to further increase bias if things change. Has anyone seen this before on MB450 amps? Maybe the JJ's need less bias adjustment than typical Russian tubes, or Apex matching was not consistent with real world conditions?
Do I return the JJ's and get Svetlana tubes and get bias to VTL recommended millivolt?
Do I return the JJ's and get Svetlana tubes and get bias to VTL recommended millivolt?
2 responses Add your response