VTL MB-450 "Generations"

I believe the first significant upgrade was the "Signature" version output transformers (although I'm not completely sure this basic amp actually went by the "450" designation and used 6550C's before those transformers were incorporated). In any case, taking that version (introduced around '96 I think, supposedly concurrent with a leap in build quality) as the baseline, the next significant changes, in addition to the resistor package mentioned above, also upgraded the B+ fusing to 600v ceramic types. Coupling cap upgrades may have been optional during this time. Likewise the external 'soft-start' modules, which I think became standard somewhere along the way (with a price increase), maybe at the same time the 'super-balanced' input circuit was introduced.

This last significant change might also incorporate upgraded caps as standard, but I'm not sure about that either. This version cannot be upgraded to from the previous versions, the circuit board is too different. I can't comment on Albert's info that this version may not sound as good as the previous Signature versions, but Bea has told me that the new input is not as big a sonic upgrade in her view as are the better coupling caps.

Personally, if I were in the market for these amps I would try to go with '98 production or newer (this is what my 185's are, and I think they're more nicely turned-out than the '96 example I've seen -- don't know about '97), and either stick with ones that came originally with the higher-rated fusing or were upgraded at the factory, or else be prepared to do the same if they predate this improvement (as I did). If you can find a pair with either the MultiCap or InfiniCap upgrade that's great, but if not you'll probably want to figure on upgrading these yourself (I'm planning on doing this, but not through the factory). The soft-start modules are probably very optional unless you have a chronic problem with tripping your line's circuit-breaker upon turn-on of one monoblock.

I agree that the new faceplate design, assuming it's similar to the new ST-150S, certainly looks nice with the glass and curved lines, but personally I wouldn't trade the utilitarian handles of the old faceplate -- these amps are heavy. What I didn't realize is that (provided your dealer is correct) the newest versions of the Signature amps will automatically adjust bias...

Albert wrote: "I'm pretty sure the cap upgrade path removes the MIT's and replaces them with Infinicaps." The Sig amps like mine, with no cap upgrades, had VTL-branded caps, made I believe by Reliable but not as premium as their Multi's or MIT's. This can be confusing, because there are several different kinds/grades of MultiCaps, and VTL never specified which one they used, claiming they were proprietary. They do the same thing now with their TRT InfiniCaps. It's something I asked Peter Moncrieff at TRT about, but he didn't or wouldn't answer on that topic. I wound up buying his DynamiCaps instead anyway, which he indicates have a slightly more euphonically tailored sound, whereas the Infini's are supposed to be more "direct". I sort of made my choice based on complementing the swap to my preferred EH KT88 output tubes, and also Michael Percy's advice.

Hi Bruce: You've done more on your own with your old tube guitar amps than I've done myself with mine. The VTL circuit board seems pretty spacious and easy to access as far as modern hi-fi tube amps go. This is a job that I personally want to enlist a tech with skills and knowledge I don't have for, but you may be able to take care of it yourself. In either case, the cost should be far lower than what VTL wants, not even figuring in shipping. Just don't expect to get any help or advice (including parts or schematics) from VTL under any circumstances other than sending your amps to their factory, regardless of whether you buy new and are in warranty (in which case you'd be told to go through your dealer) or whether you buy used with no warranty expectations and are willing to pay.

My experiences with tube swaps are detailed on Albert Porter's system thread and some other archived threads, in addition to the experiences of others, so there's plenty to read on that around here. I haven't tried everything, but I can tell you that it's not just a 6550 vs. KT88 question; different brands of each sound different from each other.

"I checked with VTL today and they never put InfiniCaps in the stnadard MB-450's. They only do it as a special mod. The standard is the MIT MultiCaps."

That sounds right, however, I believe if you go back far enough, even the MultiCaps were an optional upgrade, but I don't know the time frame on this.

Bruce: Michael Percy is a well-known parts-seller in the industry. He is not a tech for hire as far as I know, but I suppose he may be able to recommend someone fairly local to you.

Your question about remote turn-on, whether via the soft-start modules or not, is one you'll probably have to ask VTL.

Bmcleod: That's interesting, Bea Lam told me (albeit a couple of years ago) that the "Super Balanced" input couldn't be upgraded to because it involved replacing the monolithic circuit board of the older amps completely, and wouldn't be worth the cost for what she described as a fairly minor change sonically speaking. I think you might be right that less than enthusiastic reports could be due to connection to single-ended preamps. The "Tube Saver" upgrade for $500 has to be the external soft-start modules, that's what they cost per pair.