Update: Factory 8FQ7 tubes in SRM-007tII
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Originally Posted by Downrange  Good possibility. I was surprised to see the 8V filament tubes in there. I may measure pin voltages later if I get time to see if they changed the filament supply voltage from 6-8V. I may also call Yamas and see if they know what's up with the change. If they changed the supply, that will mean you really can't tube swap with 6's. (OTOH, If they're putting 6V on those 8V filaments, would the tubes even work right? Seems unlikely, to me, as they wouldn't reach normal operating temperatures.)
It's kind of weird. Maybe when they revamped the PS (for DC on the filaments), they also upped the supply voltage to take advantage of stocks of 8V tubes, as Spritzer suggested. (Mine came from Stax with GEs) The only other difference I've found in the tube construction is a more squared-off shape to the plate structures on the 8s. Not much info out there on them. Be interesting to know the story behind the change. Anyone else who has a 7tII or 6tII should check what tubes came with it, and post here.
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I called Tats at Yamas yesterday and asked about the change in tubes in the 7tII. He said he would check with Stax and let me know, and, I just heard back from Tats. Apparently, Stax found a supply of the 8V tubes, and, upon testing, found them to perform marginally better than the 6FQ7s. As Tats said, I was "lucky" in getting one of these specimens, as they've now gone back to using the 6s. They apparently had a limited supply of the GE 8s, carefully matched them up as sets to go in amplifiers, and now just have a small stock kept back for service replacements. He confirmed that they did make a heater power supply adjustment so the tubes are being run on 8v, and cautioned that if I wanted to use 6V tubes, to send it to them first, so they could set the voltage accordingly.
Sooo..... I guess I'm OK with all this, as I've been pretty happy with the sound of the 7tII I have, but, for those thinking of buying one, you might want to confirm which tubes (and heater voltage setting) the particular specimen you are buying contains. That way you can be sure you're getting a 6V model so as to preserve the option to tube roll with 6V tubes without having to change over the heater PS. Or, you can look around for one of the group to have the 8s. But Tats cautioned there are only so many out there with 8s. Frankly, I doubt if there's much of a performance difference, but Tats said the 8s did measure a little better.
When I get a chance, I'll take a closer look inside and see if I can figure out if there are any simple adjustments for the 6 vs. 8 V. heater settings - might just be an extra tap off the transformer windings....
Anyway, mystery solved. Anyone who's bought a 7tII or 7tA, check to see what tubes it came with. I'm curious to see how many there are out there.
