VTA Dyna Mk III vis a vis ST-120

Hello All,
There's a lot more traffic here regarding the ST-120 and M-125s than the VTA Mk IIIs.  In part, I suppose because the Mk IIIs are not a Bob L. product, but sourced from Dynaparts along with Roy's boards.  I'm curious however as to the similarities and differences in terms of basic configuration, e.g. power and output trannies, etc.  (-I have Roy's octal version boards.)
I am curious whether what goes for the Latino 120s (kind of a stereo version of the Mk III) in terms of power tubes, rectification, etc. generally applies as well to the Mk IIIs.  

Any feedback, comment appreciated.

Thanks,

K.

A good local friend purchased a VTA 120 a few months before I decided to build a pair of Mark III's w/ the VTA octal driver board. We use similar B&W speakers, although his are a recently purchased  current model where mine are about 10 years old. He's using a SP 12 pre and I have a vintage conrad johnson PV7 pre. His may be a little brighter sounding overall where I think mine sounds a tad warmer but that's more likely speaker and or pre-amp differences. We both tend to occasionally listen at fairly loud volumes and I think my system is generally louder at any given volume setting, for example I might listen at 12:30 on the volume pot where he might push it to 2:00 but that could be down to the speakers and the room. The good thing about hi fi is each person can honestly think their system sounds better!

In my opinion, both systems are a fantastic value and are about as good as either one of us would want and sound very similar.  I think my listening space may be better, but he may have a different opinion on that. I choose the Mark III option partially so we wouldn't have the same amps but mostly I liked the idea of having two independent PS and two rectifier tubes. I have never had trouble w/ failing rectifier tubes (knock on wood) where he's had numerous rectifier tube failures, bias control problems and many blown bias resistors. After about a year, he eventually moved to a Weber SS rectifier and KT120 tubes as is as happy as a clam. That brings up another difference, the VTA 120 can use a set of KT 120's while the Mark III's will not.

I do think the Mark III's are easier to build and troubleshoot and I like the option of positioning the amps near the speakers. I also think they look cool and no nonsense under their covers.

as with many things in life, audio, sound, likes etc are so subjective.
But yes, both the updated VTA MKIII and ST120 are awesome amps, there is no doubt about that!
I would also say that the ST120 is basically two MKIII's in one chassis, especially a MKIII using the VTA driver board.
A lot of times it comes down to the tubes used, which will make the differences in sound. Even a batch of the same model tubes can sound so very different.
Since I've had my tube tester, I am really surprised at the differences between same model tubes, especially new production tubes. I guess that is the nature of tubes and the materials used in their production, unlike solid state, where tolerances are usually much closer from one piece to another.
So tube rolling is a lot of fun, but can also be somewhat frustrating at times   
Since I've switched to Weber rectifiers on all my amps, they just seem to be more 'stable' for one of the better words, I also use KT120's in my ST120's, KT88's in my ST70's and I will be using 4x KT88's in my M125's.

Good info., thanks.  No sour grapes about topics here, just curiosity... i.e. post above indicates KT120s won't go in the Mk III, which speaks to different construction, either in capacitance or trannies (per Bob's comment above).  In spite of all the similarities, what do the ST 120s have that give this extra capability which the Mk IIIs lack?

Thanks again,

K

PS- Montana, you seem to be going for the full boat on VTA products / amps.  -Sounds like a happy customer to me!

Dingojazz wrote:Good info., thanks.  No sour grapes about topics here, just curiosity... i.e. post above indicates KT120s won't go in the Mk III, which speaks to different construction, either in capacitance or trannies (per Bob's comment above).  In spite of all the similarities, what do the ST 120s have that give this extra capability which the Mk IIIs lack?

Thanks again,

K

PS- Montana, you seem to be going for the full boat on VTA products / amps.  -Sounds like a happy customer to me!

well I think the MKIII's are of course an older design, with probably marginal transformers, as the original Dynaco ST70's were, I'm sure that would have an influence to the overall performance & sound, maybe Bob and Roy could chime in as well.

Okay, thanks. These are "New" amps sourced from Dynaparts as kits, not the "original" Dynaco gear. I think these are upgraded transformers, though I don't know for sure... something to check on with the Dynaparts site.

No heartburn if I can't use KT120's, though I suppose everyone fancies more "horses under the hood" from time to time.

K.

I just checked my Mark III P-782 power transformer data sheet I received w/ the kit. It shows that the 6.3VAC winding is rated at 5A, so that's 10 amps for 2 ea vs 12 amp for the VTA120.   Remember if you run the octal boards that will use a little more filament current than the small driver tubes in the VTA 120's.

Also shows the 5 VAC rectifier section is rated at 3 A, so perhaps the the VTA 120 is rated about double that if I'm reading things correctly.

Another possible difference between the two models could be in the size of the output transformers. The A-431 transformers on the Mark III appear to be larger, measuring 4.5" deep by 4.625" high and 3.812" wide.  I don't have the dimensions on the ones in the VTA 120 however.

Do you get more power using KT120 tubes with an ST-120 amp? If not is there a benefit? Or if not really a benefit what is the difference?

you might get a bit more out of the KT120's by increasing the bias somewhat, bit caution here, too high a bias will shorten tube live.
One if the benefits of using KT120's is longevity, if bias is set to recommended level, the KT120's are just going along with for the ride, as they are not worked very hard, plus of course you will more than likely get a different sound/tonal range as well. Part of the fun of tube rolling.

I was actually thinking there would be a sonic change as well...since the tube isn't pushed as hard is the sound from a KT120 less bright or smoother or softer than the sound from a KT88?

Also, the KT120's do benefit from a higher than "normal" bias ... they just get a lot more pop and sizzle ... the good kind ... running at 60mV per tube. Few of us here have gone that route with good results. I remember Bob running his up to 75mV with no repercussions.

Bonus ... the new VTA amps have iron that can handle it. I also run my ST120 through a bucker that drops the line voltage to a more comfortable 117vac, and that does make an audible difference for the better.

And as mentioned before, you do see a decent 10% bump on output power just plugging them in.

Impact on longevity? Ida know ... let me get back to you on that in a couple three years ...

Given the competitive price if the KT120's, can't imagine why I'd want to go with anything else at this time.

Per our own Roy M.;

Dynaparts Mk III power tranny limit = 5 amp
KT 88 1.6 amp per tube; x2 = 3.2 amps per monoblock
KT 120 1.8 amp per tube; x2 = 3.6 amps per monoblock
Mk III with Octal drivers; 1.2 amps per monobloc
Total per monoblock with 120s = 4.8 amps  (or .2 amp under the max of 5 amp)

According to Roy, KT 120's in the Mk III  "should be O.K."

K.

Somebody try it and let me know....