Dynaco ST-35, A First
I keep a Dynaco system in my office consisting of a PAS 3x, FM 3 and an ST 35, and includes a Sony 5-disk CD changer. As it happens the 35 is a first-issue device with the US-made output transformers, and multi-voltage power transformer. The system runs for 9 hours per day, 5 days per week, driving an AR Athena speaker system. Until this morning, all tubes in all three Dynaco components were original Dynaco-marked products, being mostly Mullard and Telefunken.
Well, yesterday, the 35 popped one of the 7247/12DW7 drivers - a fade-to-black in the left channel that followed the driver when I swapped them during the diagnostic process.
After a bit of research, and after learning that this driver consisted of half-a-12AX and half-a-12AU7, and not having the time to search through my tube stash, where I am sure I have several, I decided to try a pair of clear-top RCA 12AU7s from the couple of sleeves I snagged at Kutztown last Saturday ($4 each, NIB). I was warned from the research that the gain would be lower, but that there were no specific hazards in using this option.
I dropped them in this morning, they are singing sweetly as I type. And, yes, I need to increase the volume from the preamp some to get the same volume, but not excessively much. From about 9:00 to about 10:15. Still lots to go.
Note that this is the first failure-in-use of an audio tube I have had in 40 years and thousands of hours in the hobby. Failures at turn-on, sure. But not while under use. Interesting!
Melrose Park, PA