When you find out, let me know. I am ready to send my PH7 in for the upgrade, some positive feedback would be nice, but would ARC offer an upgrade that did not show an obvious improvement?
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I talked to Dave Gordon of Audio Research at an event in Seattle and asked him about the change. He said there were two reasons you should upgrade: first, he thinks it improves the sound (he had just told one of his friends to send his back for the upgrade) and second, not only is the 5881 tube less expensive than the 2 6H30's but they are hard to keep matched during longterm use (don't have to keep 1 tube matched :)). I am going to send mine back in the nest several weeks.
I had severe problems with the original power regulation scheme of the PH7 which uses three 6H30 valves. Faint noise that varied in frequency was audible while playing music. Roger Hug put it on the bench and the oscilloscope showed a very strange waveform at the preamp output at around 430 kHz. With the cover removed it could be seen that one of the 6H30 was burning much more brightly. I bought three more 6H30 and Roger installed them. It might have been nice to be able to test the new tubes but try to find a tester that accommodates the oddball 6H30. Again one of the tubes would run away. It is my opinion that the PH7 suffered from a flawed power regulation design, and the offer of a 5881 circuit from ARC was not so much an upgrade as it was a bug fix. I called Leonard at ARC to express this view. His reply to the assertion that the PH7 had a design flaw was to tell me the upgrade cost $250 plus shipping. So I had the 5881 circuit installed and the PH7 is well behaved now.