VTL, MIT caps and how close is the 450?

I own the 185's with the standard caps (and agree with your assessment of the amps' sonic quality). From what I understand, talking to Luke Manley and some other Audiogon members, the improvements can be worth the upgrade. VTL actually offers a choice between a couple of different cap brands for the upgrade, each having its own characteristics, but both apparently being an improvement on the original caps (we're talking about the coupling caps only here). I've also talked about this kind of upgrade with local techs, and at this point don't recall exactly which caps are the ones VTL is dealing with - I seem to remember hearing the names MIT, Musicaps, Multicaps, and Auricaps (one of the latter may actually be the same thing as the MIT's). I may have this upgrade done in the month of June, and so could have more to report later...

Multicaps RTX for coupling capacitors are always one of the best options. MB185 to MB450 is only the matter with output power and so is with room you're going to have the system. In vast majority of cases MB185 is plenty-enough for even large size room and toughest speakers.
I also noticed that in VTL they use one coupling cap for EACH power tube. I opted having only one expencive for each phase when made mods on my VTL MB100. I.e. it's sufficient enough to have two coupling caps in general. They made a pre-cautions that if one coupling cap goes bad(certainly within the time) than only one tube and possibly power-dropping resistors will. In my amp(4 power tubz) I realy risk with one extra tube if coupling capacitor goes bad but in reality distortions are reduced by default(by removing extra caps)

That's an interesting proposition, Marakanetz, about reducing the number of these caps in each of my monoblocks from 6 to 2. I'll have to bounce this idea off the tech I'm considering hiring. BTW, I suspect that the sonic difference between the 185 and the 450 is more than just one of sheer power - after all, the 450 only has one extra pair of output tubes, but uses a much beefier power supply and output transformer, which probably account for why it's nearly twice the price (VTL recommends the 185 for speakers with woofers up to 8", vs. I believe 10" or 12" for the 450). While I think the 185's are wonderful and all I need for my medium-sized room and speakers, if I had large speakers in a large room and could afford it, I would go for the 450's or 750's (keeping in mind that the quoted power figures for all the VTL amps are likely over-optimistic in reality).

Z..,,
I need to rephraze myself:
In case if one power tube has a shortage between grid and plate which is very typical for bad tube it may result drastical increase of bias current and the voltage applied to the coupling caps will be +520(maybe different in MB185 especially for different tubes) and -50 that results to total of 570V in my case. You than have only 20...30 seconds to shut down the amp and than not to turn it on untill checked by tech.

Normally from the splitter stage's plate the voltage applied to the coupling cap is arround 200V.

VTL coupling caps are rated with 400V maximum and if one will be blown or enough to say become bad the rest of tubes will be left untouched. Otherwise the "part" of positive plate voltage voltage will be applied to the rest of tubes with more or less degree and certainly blow them in the best case scenario!

Please note that very often you cannot determin bad capacitor just simply measuring its resistance or shortage and on certain voltage levels as very obvious example with the tube amps with 450...600V on the plate! bad coupling cap may do a very serious damage

Becides introducing high-voltage rated RTX(650V in particular) to protect the bias circuit from the bad tubes, I also introduced regulated STABLE power supply modifications that realy will not supply negative 50V to the bad tube and it's opposite phase "partner" which is pretty-complicated design taken from basically one of the circuit published at Berning website so the blown tube is recognized by decreased power of the monoblock compared to the other one. Simple, convenient, no worries, do play music even if the tube is blown or even without one at all in case you've no spare one!

Please note that RTX capacitors in comparison to electrolythics have linear operation accross the whole voltage bandwidth thus needn't to be matched 50 or 25% above the normal operated voltage.

So you should realy wage the trade-offs as extra coupling caps realy serve as fuses that prevent the rest of tubes to be blown again due to the UNSTABLE power supply.

What means unstable that if there will be surge the bias currents will rise exactly proportionally as the voltage in the wall that will definitely result blown tube and further the coupling capacitor(s). To recover VTL amplifier after the surge might realy be complicated especially if you need to repair more than one output tube circuit. I always curse when I take this amp appart! In surge cases I don't want anyone rely on power conditioner or any surge protector. Surge Is Faster period! After the implemented mods I've now no worries whatsoever to leave the amp on, don't have to dip screw to bias tubes anymore and sounds great but to tell you it's not VTL anymore! Furthermore, it's now realy easy to modify it to use different types of tubes in switchable operation if and only if I won't be lazy to hold hot iron again(man, that's an addiction worse than listening to tell you!:-))

Well, it's an addiction you needn't worry about me contracting, but thanks for the elaboration...I wonder what your amps would sound like compared to mine...

Well,
They definitely sound faster. Although I'm not fan of heavy metal but have few to rock myself up some times and damn that was a day when I listened to everything heavy just enjoying my work effort! Ever heard a real slam? Not as much classical I have whre depth is the king but also sounds great but a-bit thin. Classical in my collection just a few compared to the thousands of rock, jazz and electronics.