Nagra VPA 845 retubing

my dealer said the VPA need tight specs tubes so to work properly and flavorly with their amps. he come across one of his customers whom send back VPA for repair and was inform by Nagra that his customer had used third party tubes and cause certain part of tube rectifing circuits to burn out gradually.

Philipwu, I believe that was true with the first generation VPA and especially with KR Tubes. Nagra later fix this problem in the second gerneration VPA which allowed for a wider variance of 845 to be used.

Thanks for all of your replies. Very useful. Considering your inputs I think that the wise choice would be to buy tubes from Nagra.

My VPAs are old, but they're not the first generation: they have on the front panel the button that you can press to set bias. I suspect that they are second generation (end of the 1990 or early 2000 I think). They only have XLR connectors. I sent a note tonight to Nagra to check if there was a way to date them.

Interestingly one of my friends has a newer generation (with both RCA and XLR connections). We never compared them side by side but everyone that has heard those two pairs of amps seem to think that the older ones (mine) sound better.

BTW, do you have any opinion with regards to tube? You replace them after how many hours of use?

Obrennan1963,
according to my dealer, he said tubes start deteriorating from 3000hrs onwards if you are using Nagra tubes. You would hear noticeable differences from 5000hrs onwards. Best is to use their own tubes because they(Nagra) will buy and burn-in all tubes for 24hrs then throw away those which dies off. Tubes that survive the burn-stress test will go through tube tester to discard those out of specs.. and further match & pair individually. That's the labor & quality involve.