Nuvista Tubes - General

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