Nuvista Tubes - General


May I have opinions please on the pluses and minuses of Nuvista tubes versus vacuum tubes? Supposedly and from the basic homework I've done, Nuvista tubes purportedly offer high reliability, low microphony, low noise, consistency, and small size compared to their vacuum counterparts. So what's the down side? 
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I do not know what a Nuvista is so I hope someone hear can help you with that I do know what a Nuvistor is but it would not seem to be part of your original question it would appear because a Nuvistor is a vacuum tube.
I do not know what a Nuvista is so I hope someone hear can help you with that I do know what a Nuvistor is

Yes, excellent point. My bad. The essence of my question stems from Musical Fidelity amplifier products http://www.musicalfidelity.com/products/nu-vista/nu-vista-800 and how these types of tubes differ from the more commonly used (in audio) vacuum tubes.

I note they are using "nu-vista" in their own "marketing way", but the description does in fact indicate " Nuvistor tubes were invented in the 1950s to solve the many shortcomings of conventional tubes."

Therefore, given the titling of my post, please consider "nuvista" (and other permutations of nuvistor) to in fact mean "nuvistor", unless of course someone points out that in fact there is a functional difference on the basis of the label naming.
So what’s the down side?

They are a tube, and a tube needs to either have capacitor or transformer coupling, and the best transformer or tube coupling is no transformer or tube coupling.
Unless you go direct coupling with them then there are other pitfalls/compromises that have to be made.

BTW: I believe in the equipment I’ve worked on CDP and DAC, Musical Fidelity never used these tubes without having solid state transistor after them as proper output buffers.

Cheers George
Hi @georgehifi 

Your reply is a bit "over-my-head", but I glean the gist of what you're saying/writing is to avoid designs that use nuvistor tubes? 

To your point about needing to have capacitor or transformer coupling, is this something unique to nuvistors? (meaning vacuum tubes do not have capacitor or transformer coupling)

Nuvistors were the last vacuum tubes designed and are both highly linear and compact. In addition to Musical Fidelity, Conrad-johnson used them in a head amp, much earlier.

They are also found in the FM front ends of late stereo tube tuners, and television sets. The legendary Ampex MR70 was based, almost completely, on Nuvistors.

To your point about needing to have capacitor or transformer coupling, is this something unique to nuvistors?
No, directed at all tubes, unless direct output coupled.

I don't believe anyone has made a tube amp/preamp that is direct coupled (no caps or transformers in the signal path) from input to output, like you can with solid state.

Cheers George 
Of course there are direct coupled tube amps. The Loftin-White instrumentation amps, designed in 1929, and still used today, were direct coupled designs. The Acrosound Stereo 20-20, designed by the brilliant Ed Laurent, only has an input coupling cap, the triode driver/phase splitter is directly coupled to the output tube, and if the preamp is capacitor coupled, or uses a servo to null DC at the output, the input coupling cap can be deleted so that there are no caps in the amplifier signal path.
@viridian 

Do you have an opinion (and care to share) of the Musical Fidelity amplifier products/designs that use nuvistor tubes? I'm currently using an M6si which I'm very happy with, but the "nuvista" series is their top-of-the-line offering. Thanks.

If you like the MF sonic signature, there is no reason not to migrate to the Nuvistor products. Nuvistors are very long lived and quite reliable when used in a conservative circuit.

Me, I find the MF products way to soft sounding for my taste. YMMV and probably will.

You are most welcome. To clarify one other thing, Nuvistors are vacuum tubes. They just come in a smaller package than the glass tubes that we see in most gear, and relied on “modern” manufacturing techniques to miniaturize them ( “modern” circa 1959 that is).


To clarify one other thing, Nuvistors are vacuum tubes.
Appreciate the clarification. Then, given they are vacuum tubes, assuming all else being equal, should one expect to achieve the same level of "famed" tube sound by using nuvistors (as compared to some other vacuum tube)? 
No. Not necessarily. It depends on everything - circuit, quality of transformers, type of tube, configuration of tube, quality of passive components. Adding tubes is not a panacea for some "tube sound."
Of course there are direct coupled tube amps. The Loftin-White instrumentation amps, designed in 1929, and still used today, were direct coupled designs. The Acrosound Stereo 20-20, designed by the brilliant Ed Laurent, only has an input coupling cap

" The Acrosound Stereo 20-20, designed by the brilliant Ed Laurent, only has an input coupling cap"

This technically should not then be called a direct coupled amp. As there is a cap in the signal path.
It should be described as a capacitor coupled amp with direct coupled output stage.

PS: But wait it’s not even that, it has an output transformer as well, so it has transformer coupled output as well. So in no way can this be called a direct coupled tube amp.

http://www.cma4ch.org/chemo/image/hifi/acrosound-stereo-20-mod-r.jpg

http://2.bp.blogspot.com/-AuRjf-5iXM8/UEYcDrM6X8I/AAAAAAAAAew/RixShlEJghU/s1600/Acrosound_20_20_sche...

Cheers George
Wow, what a great question, and one that I can’t answer. I don’t really think that there is a vacuum tube sound, per se. Some tube circuits can sound very linear, and if blindfolded, you might say that they are very good solid-state. And others have that soft liquid sound with good sustain and slow attack.

And, I believe that George suggested that solid-state buffers follow the Nuvistors in the MF circuit - George, please correct me if I am misquoting you - in which case the circuit is no longer pure tube, but hybrid. Nothing wrong with that either.

So it’s really a matter of implementation and circuit topology. You know, I can put Pirelli tires on a Ferrari and I can put them on a Camry and those two cars will handle completely differently. Other design elements will swamp the contribution of the tires.

I will say that Michael Fremer has owned, and reviewed various MF (yes, I like the “MF” abbreviation) New-Vista products in Stereophile. He prefers a more direct sound and nowhere in any of his reviews has he said that any of these products evince the stereotypical soft and syrupy vintage tube sound. So I would imagine that they just represent an enhancement of the MF house sound rather than some radical departure. But that is speculation on my part.
George, please correct me if I am misquoting you - in which case the circuit is no longer pure tube, but hybrid.

Correct, as I was letting the OP know that the final sound signature of it was solid state and not tube.
If it didn’t have the solid state buffer, the sound would be very different and not be able to drive very well either due to high impedance, that why Anthony Michaelson (MF) put the solid state output buffer in it.

Here is a pic of the output section.
https://ibb.co/nvT2sR

Cheers George
George, completely agree that the Acro is not direct coupled, and I use the common parlance here, in tube design speak that the output transformer is not included; a direct coupled tube design generally referring to the circuit being direct coupled up to, but not including the output transformer as this is used primarily an impedance matching device as opposed to blocking DC, which it does quite nicely as well.

And, as I pointed out, one would need to delete the input coupling cap in the Acro for it to be so, and I have done this. And, as I said, it requires a DC servo or output coupling cap at the output of the preamp. At that point it is what tube designers refer to as a direct coupled amp. But yes, this would not be inclusive of the output transformer.

I don’t believe that there are any output transformerless tube amps that are direct coupled, as this is really playing with fire.

I also don’t really see DC coupling as a benefit in an amp, as DC can only serve to weld the voice coils of ones speakers and provided that the cutoff is low enough the phase shift in the audible band from AC coupling should be negligible.

And thanks for your link showing the Nuvistors and the SS finals. I have lots of Nuvistors and none of them look like that. Those appear to be glass cased tubes. My understanding was always that the miniaturized metal case of the Nuvistors was a part of the technology and whereas in a glass tube the air is evacuated from the envelope as the glass is sealed, Nuvistors were actually built in a vacuum chamber.




I also don’t really see DC coupling as a benefit in an amp

That's your opinion, listen to a good direct coupled amp, say Gryphon/Krell ect, then listen to it with the best cap or transformer you can get inserted somewhere in the signal path.

For the most transparent sound, the best cap is no cap, and the best transformer is no transformer.

Cheers George
George, we will agree to disagree on cap coupling. I don’t see how taking a circuit that is optimized for direct coupling and hanging a cap on it is a test of anything.

I do listen to a friends Krell Monos and have owned Krell amps in the past, so I am familiar with them. I am just not as bright as you and don’t know if the excellent sound can be ascribed to the DC coupling or some other aspect of the circuit design.

I do build tube amps and will certainly agree that all caps, and all transformers have unique sounds, and a straight piece of wire does not. But if one wants to work with tubes, there are going to be caps somewhere in the chain. And I don’t diagree that the best cap is no cap, but sometimes the additional DC management circuitry involved makes it a bit more than no cap.

Hey, on a more important note, I was editing my post above as you were posting. Can you have a look at my question about the Nuvistors in your link. I would appreciate your guidance.
George, we will agree to disagree on cap coupling.

That’s why there’s so much rolling goes on with caps as they all have their own coloured sound, the best cap is no cap for the ultimate transparency..

DC management circuitry involved makes it a bit more than no cap.
It's a know fact, that DC servos are much more transparent than cap coupling.


Cheers George
George, I didn’t know that fact apparently.

Can you give me some insight into the glass cased Nuvistors in your link regarding my question above?
Can you have a look at my question about the Nuvistors in your link
I maybe wrong but I think Anthony Michaelson went to the Nuvsitor when the Trivistor became hard to get, in bulk, or the other way around.

From what I was told by a distributor, he’s known for loitering around military surplus auctions, where he can buy a pallet load of a part for next to nothing, then goes home and designs a specific number of editions using that part.

Cheers George
No. Not necessarily. It depends on everything - circuit, quality of transformers, type of tube, configuration of tube, quality of passive components. Adding tubes is not a panacea for some "tube sound."
Agreed. That's why I prefaced the question by stating "all else being equal".
@georgehifi and @viridian

Thank you both very much. A lot to read through and digest.

From what I gather from your input this far the use of nuvistors in the MF design does not necessarily in and among itself produce the famed tubed sound, and instead and more broadly where other tube amps are concerned, is dependent on the circuit in its entirety.

That’s why there’s so much rolling goes on with caps as they all have their own coloured sound, the best cap is no cap for the ultimate transparency..

Right. But frankly this is something I’d much prefer to avoid. I’m not looking to "tweak" the sound of a particular product, although I can understand the desire and benefit in doing so. Instead, I prefer to "set it and forget it". 

I see that. And I hope I wasn't patronizing! It's just that they are such different beasts. And what is a tube sound anyway? One would never believe my 1960s push-pull amps were tube if one thought tubes were warm and lush. I have had sand amps that were like that.  

I see that. And I hope I wasn’t patronizing! It’s just that they are such different beasts. And what is a tube sound anyway? ....
All good. And your question is well taken.

George, thank so much. That's exactly it, the pictured tubes in your link are trivistors not Nuvistors. I was sure that you would have the answer.

Interesting bit of info as well that MF went to the trivistor when the Nuvistors dried up.

Interesting bit of info as well that MF went to the trivistor when the Nuvistors dried up.
Musical Fidelity time-line http://www.musicalfidelity.com/about-us/timeline confirms that nuvista 1997 and trivista 2002, but why then (seeking speculation) would their current amplifier offerings utilize nuvistor tubes instead of trivistor? 
This is old stuff gd. As far as production quantities go, you take what you can get. So if you want to make 300 preamps you need to find the appropriate amount of tubes. As George pointed out, they get their tubes at military surplus auctions. You may find a load of Nuvistors one day and a load of trivistors another. When you are buying old stuff you are at the mercy of the market place.
I have owned the MF Nu- Vista 3D CD player for about 16 years. It is really a wonderful piece of gear. The sound is very "natural," almost analog in nature and is not at all "syrupy."  Pretty good for a metal encased micro-tube.

As for the Nu-Vista vs Tri- Vista issue, my understanding is that there were a lot of problems with overheating in the Tri- Vista components. I have read a number of posts on various audio forums attesting to that. The Nu-Vistas were designed to stay cooler than the average tube, so that may explain MF's return to its use.
and is not at all "syrupy." Pretty good for a metal encased micro-tube.
That's because it's got a transistor buffer after the Nuvista to drive the outside world with, without it it'd be "syrupy" if the Nuvista had to do the job.

Cheers George   
George ... then what's the point of a hybrid design if the transistor buffer controls sound quality?
what's the point of a hybrid design


Sales pitch, to make one believe there's a a romantic/euphonic side to it. as the tubes are in between the s/s I/V stage and output buffer and serves no purpose, but a sales pitch.

Cheers George 
Sales pitch, to make one believe there’s a a romantic/euphonic side to it. as the tubes are in between the s/s I/V stage and output buffer and serves no purpose, but a sales pitch.

Then perhaps I should stay with my M6si which I’m happy and content with, and when/if the time comes to "upgrade" look/think elsewhere than the MF line. Not to mention the fact that the Nuvista 600 is another five grand more (MSRP) than the M6si.

EDIT:

George, is your statement about a hybrid being a "sales pitch" in your opinion specific to MF, or in general and applicable to any hybrid amp manufacturer?




This is old stuff gd.
Right. I'm pretty sure there was a recent thread though here on the forum quoting Nelson Pass as to his thoughts on amplifier evolution. If I'm not mistaken it is somewhat at an impasse. To that end a fellow (Roger) from HCAT chimed in with a new (purportedly) design that many folks here poo-pooed for various reasons. The posts got rather argumentative and the thread was subsequently deleted.

I'm not all that interested in class D. If anything I'd go class A, tubes or a hybrid. Anything "new" in that realm?
Gd, I’m sorry, I was not clear. I meant that the tubes used in the MF gear were old stuff, not the design, which I am not familiar with. Just making the point that the designer must source sufficient quantities of a product that hasn’t been made in more than 50 years. Nothing wrong with that IMHO.

You seem to be focused on picking your gear based on the technology involved. I think that’s a slippery slope, and because of system interactions and personal tastes, I generally recommend purchasing based on sound quality, long term reliablility and serviceability. YMMV.
@viridian

No apology necessary. I’m very appreciative of your input (and clarifications). And I get what you’re stating (and agree) about purchasing based on sound quality, long term reliability and serviceability. That writes and sounds much easier than it is. Not always so easy to make a choice, given the hundreds if not thousands of available choices. Also, it's rather impractical to attempt to listen/audition all options.

Back to nuvistor for a moment, I received a message from Musical Fidelity in response to my inquiry as to where/how the improvement is between the M6si and their nuvista lineup. I don’t know what they mean by "psu", so perhaps you could translate that acronym. Thanks.

The following is from MF:
----------
The Nu-Vista 600 has the benefit of the Nu-Vista valve preamp stage. The preamp is directly coupled to the input stage so you have shorter signal paths, we have an improved psu over the M6si.
You hear the beauty, warmth and transparency of the valve stage and the transients and power of the transistor power amps.

The Nu-Vista 600 has 4 transistors per channel and outputs 200 wpc, the N800 has 10 transistors per channel and outputs 300w.
-------------
“That writes and sounds much easier than it is.” I’m certainly with you on that one!

PSU is power supply, arguably the heart of every amplifier. 

If you are looking for a rich sound, excellent reliability and serviceability you might have a look at Van Alstine. As you said there are tons of excellent choices out there. I am pre-disposed to gear made in the USA that is very reliable and easy to service. Some really like the marque and some find it lacking in clarity. Rogue also makes a couple of nice hybrid amps but these are class D and I think that you said that you had a technological aversion to that. 
No, directed at all tubes, unless direct output coupled.

I don't believe anyone has made a tube amp/preamp that is direct coupled (no caps or transformers in the signal path) from input to output, like you can with solid state.
The KSS OTL was direct coupled from input to output. You can do it, you just have to jump through some hoops.

I don't think direct coupling is a good idea for either solid state or tubes, because the timing constant of the amplifier goes lower than its power supply. At that point, its possible to modulate the supply, which in turn introduces IM distortion.

We limit the bandwidth of our amps to 1 or 2 Hz for this reason, but we do employ direct coupled inputs and outputs.

George, is your statement about a hybrid being a "sales pitch" in your opinion specific to MF, or in general and applicable to any hybrid amp manufacturer?

I just look at where he used them in the Trivista sacd, that they could be bypassed and probably get better sound/transparency without any circuit design negatives. And that they were maybe from army surplus, and just used to "maybe" give a product more cred/appeal to the "tube set". 

Same what I saw in the MF A3-24 dac a good dac, but two massive power supply chokes (army surplus?) on already very nice super low noise, very stiff low impedance solid state power supplies, what for?? just to make them not stiff again??? Again appeal to some that love chokes in tube equipment?

Cheers George   
I just look at where he used them in the Trivista sacd.......
.....Same what I saw in the MF A3-24 dac...........

@georgehifi

I take this to mean your statement about a hybrid being a sales pitch ** is ** specific to MF.

I take this to mean your statement about a hybrid being a sales pitch ** is ** specific to MF.

No there are others that do it, just to get the Tube guys interested as well.
Just like McIntosh throw an output transformer on the end of what "could be" perfect good solid state amp, absolutely no need for it if the solid state amp is "done right". Try putting an output transformer on the rear of a Gryphon, Krell, ect, and see what happens to the sound. 
 
Cheers George
Heard the Nu-Vista amp back in the early 2000s when it was new.

Opinion?  Good but not something I'd lust after. It was demo'd on unfamiliar speakers and source, however.   Plenty of "air," if that's your thing.
Per OPs question: another (older) use for nuvistors was for the front end of VHF receivers/preamplifiers and were employed in my (early 60s) ham radio equipment. The smaller size meant lower inter-electrode capacitance and therefor higher efficiency. Reliable low noise transistors for higher frequencies were rare and expensive then. Other than that (as already posted) they are just vacuum tubes. Thanks.
@fraterperdurabo and @andy8400

Thanks for the candid feedback. My curiosity with the Nuvista (nuvistor) is that within the realm of what I already have (MF M6si) it is Musical Fidelity’s "upgrade" offering (with respect to where I’m at now). But I just can’t wrap my head around how much improvement there would be, at more than twice the MSRP mind you. Sure, auditioning is worth something too and if it comes to being a more serious consideration I would do that. Typically though I’ve found auditioning components (speakers and basically anything) is only so efficient, viable, meaningful, etc. so I honestly give more credence to opinions of owners, manufacturer marketing hype, technical specifications and overall product reputation.

Given what has been stated in this thread it would seem nuvistors are worthy of consideration, but certainly not the be-all-end-all. I was perhaps hopeful to learn or field opinions that it could offer traditional vacuum tube sound without the negatives, whatever the traditional sound (if any) and negatives (if any) happen to be. And its already been pointed out (here and/or elsewhere) that there is no tube sound per se and everything has negatives.

In all likelihood if and when I do "upgrade", I would probably be inclined to get something along the lines of a class A Pass labs amp just "because". Class A (seemingly) always gets the most up votes as far as sound quality and Pass always the same kind of thumbs up. But for now, as the Rolling Stones might say, "I’m just sitting on a fence".

Here’s a write up of the MF Tri-Vista. I own this integrated, and it’s quite nice. I have upgraded the fuses and interconnecting cables for nice improvements; more delicacy and ’air’.

https://hometheaterreview.com/musical-fidelity-tri-vista-300-integrated-amp/?page=2

It seems like there’s always a Tri-Vista, Nu-Vista, or kW integrated for sale at TMR.
https://www.audiogon.com/listings/solid-state-musical-fidelity-tri-vista-300-integrated-amplifier-js...

I would agree that while maybe not the be-all, end all, they are a worthy consideration in the $3K range. Mine has bested the Joule Electra LA-100 preamp/Bryston 7B ST monos setup in my system (also nice separate components from yesteryear).