Nuvista Tubes - General

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