I'm in the mood to make mods.

I've realized that keeping a soldering iron in your hand is a kind of stress release...

I want to find out what kind of mods is possible to do with VTL MB100. Rail caps? Coupling Capacitors? Feedback reduction or implementing a feedback selection switch?

For this I pretty-much need circuitry but the manufacturer do not provide it.

Please advice.
OK, LOTS you can do!

My 100 monos use EL34's. I replaced them with EI (Yugoslavia) 6CA7 BIG BULB. Not expensive and more neutral. Best EL34 type for my taste - towards the KT88/6550 sound, but still a bit sweeter than that.
Much more detail and slam then the 34's and I've used MANY types, including Mullard, Bugle Boy and Seimens.
Not so keen on Russian ones (a bit boring to me), most of the Chinese ones I tried didn't like 500V up 'em (some gave a fairly good techno lighting display), appart from Golden Dragons, which I found had only average life and inferiour sonics - a bit "glassy" to me, but trust your own ears.
NOS small signal tubes can be good, but my money goes on better passive components first - they last longer!

Next, rectifier diodes. The originals are not nice, replace with an IXYS bridge, the 2000V/20A one is lovely overkill. Don't use seperate diodes, they may not be matched - it all matters!
Do the same for the bias supply, I know the 600V/27A is using a Mack truck to carry a teaspoon, but matched quiet diodes help the power supplies a lot. Remember, the bias supply is mixed with the signal before the power tubes, it is VERY MUCH in the signal path and often overlooked. Replace the smoothing capacators for the bias supply with suitable Black Gate caps.
These bridges give the caps an easier time as they put less noise into them. It can also be worth putting a 0.1uF/1000V paper in oil or military surplus teflon caps across the the output winding of the power transformer (one seperate one across each winding) and another from + to ground after each rectifier, before the filter cap to short out more transformer and diode noise. Yes, transformers do put out electrical noise, sadly!

Oh yes, I find it worthwhile to use audiophile type solder but I havn't established any real preference.

Next up is coupling caps. I'd go for Relcap RTX, but many prefer the softer sound of paper in oils, or perhaps Hovlands. To me RTX are the most detailed without any harshness, simply accurate. Get 'em before they run out of polystyrene film and there is no more....
There is a 47uF electro bypassing the 2K2 cathode resistor of the input tube. Replace it with a 47uF/50V bi-polar black gate, replace the resistor too, as per the next para.

Then I'd go for front end and feedback resistors. I'm a great fan of Caddocks. To me the TF020 are top of the heap, better even than Vishay (and cheaper!) which to me seem a bit clinical. MK132 Caddocks are not as good, but not that far behind either and will take a bit more power.
The values to look for are 270K, 10K (input) 2 x 1meg, 1k and 12K (in front of the splitter/driver tube and all joined at one end to the same point) and 2 x 10K in parallel, with a 180pF cap also in parallell (feedback - very imortant, use good resistors here, the stock cap is OK).

Next, power dropping resistors and small power supply caps. There is a 7K5, 51K and 100K resistor assosciated with a 47uF/500V cap. Also a 100K and a 68K assosciated with a 400uF/400V cap.
My solution was to replace the resistors with the large size Tantalum resistors from Angela Instruments. They sound bright and horrible at first, but burn in SLOWLY to something wonderfull.
The caps I replaced with 630V Solen or Aeon, the largest I could fit next to (not on) the board, plus the largest I could fit in place of the old one. From memory, that is about 60 - 70uF total. Sounds too small? NO! We are only dealing with a couple of milliamps here and metalised caps have FAR higher ripple rating (hence current ability) than sleepy old elecrolytics. It will make the amp quicker, but not harsher. Film and foil caps would be even better, but then we are into new chasis territory.....

Iso sockets for the small tubes and Pearl tube coolers everywhere worked for me and I reccomend 'em.

Last, the main filter cap. I am looking at either making the transformer holes oval to squeeze an extra few millimetres between the two transformers so I can use a Ulytic 260uF/600V film and foil cap - or more likely, throw the original chassis away and build one with the room for two Ulytics with a large choke between. If the resistance of the choke is kept low and the inductance is at least a couple of Henries, then I have NEVER heard a choke in a tube amp power supply do other than VERY good things to the sound, but you need a cap either side of the choke and the choke will be as big as the power transformer......
Oh yes, 260uF of FOIL cap is enough, they put out 8-10 times the current of an electro, i.e. more than the stock cap in there and they are QUICK.
No power amplifier can be quicker than the power supply which feeds it.....

Last up. you could change the 12AT7 driver splitter (second tube) for a 12BH7A like the larger VTL's use. I did and it put the sound on steriods - in a nice way!
To do that, the 2 x Imeg resistors and the 12K resistor which are all joined to a 1K resistor whos other end is on the cathode of the splitter driver need a change of values. The 1megs become 470K and the 12K becomes 8K2. Hey, worked for me, for the last 7 years or so....

You could also convert the small tubes to DC heater supply, they are on Manley gear...... I havn't, but the idea of current regulators rather than voltage regs makes sense to me, soft start the heaters and compensate for aging.....

If you don't make a new chasis, look at damping the old one, it rings like a bell! That lets every capacator function as a microphone.....
Microsorb and EAR product are my choice, I use both. I have found that despite seeming very sticky, it is worth using an adhesive to glue the microsorb on with. The glue on the EAR products is fine.
I also use the EAR feet everywhere. They caused the Black Diamond cones to be put under my speakers and 50lb external passive crossovers, the rest are in a draw somewhere! Not bad for a fraction the price!
Don't underestimate the power of hot-melt glueing components down either - try make a lump of hotmelt resonate! Even better, it's cheap.

Anyway, remember your safety and discharge the caps of the power supply well (I use a very well insulated 10K wirewound with aligator clips on each end), remember they will re-form some voltage if not discharged long enough. Don't rely on leaving the amp off for a while! You may make only one mistake.... then it's celestial hi-fi!

I tend to get my stuff mostly from Michael Percy (www.percyaudio.com). Consolidate into the largest order you can, mark it urgent if you are in a hurry and I have always found him very good. I have used him for years, I consolidate my orders up with a friend who orders lots more than me. I think he has been out of stock once and I can't remember him getting an order wrong.
Usual disclaimers of course and as I live in New Zealand - obviously I can have no connection with a firm in California, apart from being a very satisfied customer.
Likewise Angela, a very good guy, VERY knowledgable and it's worth looking at his site if you have a sense of humour - he does!

Last thought - tube amps are more mains sensitive than solid state. I have found the XLO reference power cord from Percy excellent, a sod to terminate, but worth it!
The other thing I have done is rescued a 2 phase power filter from an old mainframe at the electronics scrap yard, about 60 amps a phase. Refer to your local wiring regs as they store power internally when unplugged, so over here they are supposed to be wired in. I use one for power amps, one for front end and one for digital.

Have you done simmilar for your preamp? Does it used a stepped attenuator????
Black Gates are wonderfull in CD/DAC power supplies (especially after regulators, BG bi-polars best of all for digital). Muse are also very good for the raw supply, but not the Muse bi-polar. - but use static safe gear, a cheap field kit will do, pick one up surplus. TI sheild works great inside digital electronics! With microsorb it made quite a difference to my AA dac3, dti32pro and Monolithic power supply.
If you like LP, microsorb is GREAT on turntable chassis and sub-chassis.
Mmmmmmmmm, film and foil caps for the speaker crossover? Air core foil inductors for the same? I could go on - and have for years with my own gear.

Hope this all hasn't been boring, welcome to the Order of the Blue Striped Apron (a.k.a "butcher"), trust your own ears and enjoy the music!
...i'm still overlooking your post with open amp bottom...
I believe that chasis is the last thing to do...
I have stepped attenuators in my passive preamp...
I believe that once your warranty is ended you can modify instead of upgrade with better results.

Thanks for great info!
So glad to be of some help.

I'm totaly hooked on mods myself, nothing in my system is stock, or even near! You can do as you can afford it, you learn a lot and I agree, it is relaxing.
It also keeps the upgrade bug at bay because, if you choose something with a sound basic design (like VTL power amps), then you end up with a piece of gear better than anything you could afford off the shelf. The long chain of price markups sees to that.

Ever thought of building your own cables? I'm tinkering with Cardas coppies, 6 runs of fine cardas wire (3+ and 3- alternating) around a teflon tube, with another ring of teflon tubes outside of that, then flexible heatshrink, copper screening braid and then woven polyester expanding tubing to make it look pretty. Cheaper than the finished cardas - which I just can't afford as I have about 75 feet of interconnect alone in my system!
The construction is illustrated quite well on the cardas site.
Building this keeps me out of trouble for hours!!!

Enjoy the moding and take pride in a hi-fi that is very much your own.
If you consider cables this might be even more time consuming that anything else if you don't have proper tools and a doesen of soldering irons for different purpouses.
In addition to that you need to buy component materials.
I've ended up with VanDenHul D102H un-terminated bulk cable that is actually used in Type1 interconnects and terminated them with Neutrik Pro-Fi RCA plugs. Neutrik Pro-Fi have better conductivity and even firmer connection than Cardas or AudioQuest.
All of this stuff is time consuming and presents many challanges to be overcome!

The more I've done, the more ambitious I get.

I'm presently extending amplifier cases so the power supply I think is good enough can fit - its a 250W mosfet monoblock that is having a 600VA (3" x 8") torrodial transformer, with 2 x 10,000 uF of Nichicon Great Supply caps per rail - untill I can afford another 2 per rail plus a 2,200 uF directly onto each of the 6 output devices - per amp of course!

Said amp already has high speed bridges, caddocks, black gates (for voltage gain stage) and better regulators also for the voltage gain stage - not to forget lots of damping materials on the chasis that is already there!
Of course, the chassis extension will need damping materials added as funds allow.....

Oh yes, as is my want they are fed mains by XLO ref. power cord, home terminated.

See? Hours of work!

The tool selection slowly grows - absorbing less than 10% of the money I would have spent on top end gear. Now I want an oscilloscope and a signal generator, so I can fool round with designing tube stuff - just for "fun"!

You don't want to know:
1) How long it takes to build a 6 conductor interconnect.
2) The language it sometimes provokes.
3) The ammount of tea coffee and occasionaly (after an ooopsie) beer involved.
4) The funny crude jigs I use as the extra seven pairs of hands!

But at the end of the day, somehow I call it "fun" - even my friends think it is fun, or did they say "funny"? How come they pat me on the head and look sad?