Your friends experience bespeaks volumes. Surprisingly you`re still considering them. Good luck.
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I have VPAs, but they are on dealer loan, so I can not really speak about reliability issues from my expirience. However, a close friend of mine is running VPAs for more than 2 years now, first with chinese tubes, and now with KR tubes and only once had a tube failure (the KR one) which was replaced by Nagra for free. Apart from that tube - no issues.
i have had my VPA now for 3 yrs they have balanced and single ended inputs so they are the newer type, i have tried the valve art tubes and found them too coarse. Iam now using the very latest 845m with superb results, shying away from the KR tubes, i have had no problems at all with the amps, these are for keeps they are absolutely superb with my nagra plp preamp.There has been some issues using the KR tubes and i believe Nagra have stopped using them as they have shorted out various VPAs. I guess no matter what make of amps etc you go for someone would report some kind of reliability issues.
I'm not sure about the reliability but I can talk about the sound. I had ARC VT100 MKIII and recently upgraded to the VPA's. NO COMPARISON! The VPA's will breath life to your recordings....recreates the recording space.
I bought used on the ...GON, was so impressed that I exchanged for new at full price! I'm still using the stock tubes but have ordered the 845M. Tangram what differences did you notice with the 845M please?
in term of 845 tubes, I've tried 3 different pair in my system (Mastersound Reference 845/Italian). I tried the new 845M and was totally disapointed by the sound. Then I switched to Fullmusic 845 ($820 for quad) and got disapointed as well.
My current setup is using Valve Art, which I'm very satisfy with the sound. However, these Valve Art are getting to old so time for replacement. I just recently ordered 4 845tube from KR, let's see what's the result!
Jjwa. The difference with the 845m is lower noise floor, you can hear into the music much easier picking out what is in the background, they are faster, more dynamic with better defined bass, the top end has much more air making cymbals more real also there is less grain making them more listenable and more enjoyable. In a word more Natural.You only have to turn the bias pots very little and your in the green.You will be impressed! also iam using au24 cables with superb results.
With respect to the original question, Nagra has been making bulletproof gear for a long time and I don't think the VPA is any exception...unless you mess with them.
I found the KR 845's to sound exceptional. Subjectively more powerful, with a darker tone and a richly saturated, more vivid sound. Very nice in my system anyways. Only problem is that one of the tubes "took out" one of my amps after some 20 hours. Said tube was said to test fine afterwards and last I heard Nagra was said to be working on a fix for this recognized problem with no resolution as yet.
I'm currently burning in the new 845M's and while they sound fine, the KR's sounded better. If I were a VPA owner, however, I'd probably stick to RCA spec'ed 845's rather than the KR's which are not as, for example, they use a different filament that draws more current. Therein may lie the problem. The VPA could probably be modified to use safely with the KR's.
One word about the new 845M's...one of my quad was misshapen such that I could not fit the tight VPA tube cage over it. This was replaced only after some hassle so when ordering make a point to mention your amp and these cages.
I have had a pair of VPA's for about 3 years and a year ago 1 tube went bad and I replaced with tubes from Sophia Electric. They have tubes from Shuguang in China and a matched pair for $210 is great. I installed the tubes and had to adjust the bias somewhat and realized then that they were better and run cooler than the orginals. I would do it again. My VPA's were made before the KR tube came out.
Does anybody know the scales of meter on VPA? Recently I found the anode voltage of one of my VPA reach the right edge of the "green zone" (both front and rear 845 tubes). I just want to make sure that 845 is running at a reasonable operation point. I have the new version VPA and run with original tubes.