Reliability issues with VTL amps/pre-amps

I have a VTL ST-85 which I use mostly, in the triode mode. The left channel tubes have blown 5 times in a two year period. I had repairs and the tube compliment replaced by VTL during this time. After all the shipping costs, VTL replaced the last set of tubes for free and all is fine for now, but I'm worried it will blow again. No one at VTL has ever told me why they keep blowing. Has anyone had similar experiences with VTL? Is this type of problem common with their products?
i have owned the same amp for 2 years with no problems, how old is your amp? Also does it have both triode and tetrode switch?What tubes do you use. I use NOS fatboy tubes from the 60's.Tell what tubes you use so i never use them in this amp, thanks. Ps How often do you bias the tubes...mine hols bias for at least 3 months or longer....
I have two pair of VTL 750's, meaning I have 48 6550 tubes to contend with.

I have lost tubes and with that the large fuse behind the screw plug on the rear of the amp. No damage to the unit other than fuse and I'm into my 5th year with these.

I think VTL amps are among the most reliable and best sounding of all the big amps. If you want to keep them reliable be sure bias is right and tubes are suitable. I don't know about your ST-85, but the VTL 450, 750 and 1250 all run very reliably with Wing C 6550.
Hi. I am in my 7th or 8th year using my VTL MB450s (and I bought them used). They continue to be fine. Because I do not get enough hours on mine, I may not have the problems others have had, but so far, other than a couple of blown tubes after 4-5yrs, I have not had any issues. I use mine in triode almost exclusively.
I own VTL 125 mono block amps that are 11 years old never a blown tube . I had a on / off switch go bad a few years ago sent to VTL for repair and has worked great ever since . My amps get use just about every day for the past 11 years I like em and they sound great . Marc
I've owned a pair of older VTL 300s for at least a dozen years. While they have had little use in the past couple of years they have been very reliable. They had one part failure which VTL replaced for free even though I was not the original owner.

I agree they are among the best sounding and most reliable of the big tube amps.
If you look at VTL schematics you will see they are very simple and intrinsically reliable, operating the output tubes at low current. However they use a high anode voltage and the choice of output tubes is critical - if you take Albert Porter recommendation of Wing C 6550 you will have no problems!
Many cheap 6550 can not withstand these high voltages. Also, when tubes became tired replace them - do not wait until they break!
Thanks for all your input. I do love the sound of this amp. I have used only tubes supplied by VTL and Bea Lam has been very helpful in this regard. However, I still have no answer as to why, other than the tubes were bad. Perhaps quarterly biasing will help (as ryriken has suggested).

It does have a triode/tetrode switch and really sounds better, in my system, in the triode mode. I am using a Melos MA-110 pre-amp, B & W Matrix I speakers, a Meridian 200 transport and 203 processor, A Rega P3 TT w/a Project Tubebox SE. I've recently added a Paradigm sub (but this has been added after I got the amp back from VTL, not during the problematic period).
Until recently I've been a VTL MB-125 owner with tremendous satisfaction. It uses EL-34's (4 per mono bloc)and I've used both JJ's and Electro Harmonics with no troubles at all. I used them for over 6 years and averaged about 30-50 hours a week. Never a reliability issue. I would check the bias every month or so and they almost never drifted or needed much adjustment at all. Be sure you never switch between tetrode/triode with it on, of course, but other than that they are just outstanding products with awesome sound [and superb] reliability. I hope your ST-85 has finally "settled down". Best of luck.