Purchased a used Alpha this past summer which came with a pair of GE 7025's. Decided to try a pair of Sovtek 12AX7 lps's. Noticed the right channel went out after about 11 hrs of use. Assumed tubes were bad and rather than replace went ahead and purchased a pair of EAT cool valves ECC803s. Probably one of the finest 12AX's available today. Love the sound of these tubes and was greatly disappointed when after 16 hrs. of use the left channel went out. Instantly thought this DAC was blowing these tubes out. Fortunately for me, I contacted Bob and Gary Backert of RHB Design http://www.rhbsounddezign.com. After many detailed questions Bob felt that the tubes may not have failed and may be a filament voltage problem. After putting the DAC into his very capable hands, it turned out that the new tubes run at a slightly higher filament voltage. So without this extra voltage for the tubes the circuit simply drops out. Bob modified the circuit and I am now enjoying this very musical DAC. In closing to all you Alpha owners if you have experienced this problem don't get rid of your DAC.
Just a correction, It turns out that the new tubes run at a slightly higher current causing a voltage drop at the tube filaments. So without the correct voltage for the tubes the circuit simply drops out. Bob Backert, RHB Sound Dezign