Best measuring tube amp

[Gorgonzola]

My current view is that looking for a "best measuring" tube power amp is, from practical POV, a pointless pursuit.

Best, rather, to look for a SOTA solid state amp with ultra-low distortion. If you want a little color, warmth, body, or "holographic" imaging, put a tube preamp upstream -- or maybe midstream because, IMO, your upstream DAC should also be ultra-low distortion

 

[DonH56]

Technically MOSFETs are transistors (that's what the T at the end stands for), and SS devices do age, though typically at a slower rate than a tube. Depends upon the circuit, of course.

SS amps usually have lower distortion due to higher feedback, which in turn is available because of their designs' higher open-loop gain (often because complementary devices are more readily available).

 

[Strumbringer]

It's the weirdest thing- I enjoy my tube amp and I'm listening to it daily, then I log on to ASR and I'm reading, and all of a sudden my tube amp doesn't sound as good to my ears. I then plug my headphones into my solid state amp and I'm happy. Strangest phenomenon.

 

[charleski]

McIntosh MC 3500. reviewed in Electronics World in the late 1960s. Measured THD under 0.1 % from 20-20kHz at full power, 350w.

McIntosh MC 275. My own unit measured under 0.1 % THD at 75 W, 20-20 kHz both channels driven when tested at one of Mac’s amplifier clinics in the 1970s.

Luxman 3045. Tested in the 1970s.

Marantz 9 and 8B also met the under 0.1% THD 20-20kHz at rated output ppower benchmark.

The EAR 509 also has very low distortion based on tests. Around 0.1% THD at full power. It is probably the only currently produced valve amp that is is the same class as the old Lux or Marantz or McIntosh amps.

0.1% = 60dB. This is pretty much the standard SINAD measure for commercial tube amps with a competent design, and has been for several decades now. It seems no-one really bothered with trying to drop distortion down below that, even though it is possible, as noted above. Getting this level of distortion @ 350W is impressive however.

Of course it seems that the majority of contemporary tube designs deliberately embrace much higher levels of THD to cater for those who want a blatantly muddy sound.

 

[al2002]

0.1% = 60dB. This is pretty much the standard SINAD measure for commercial tube amps with a competent design, and has been for several decades now. It seems no-one really bothered with trying to drop distortion down below that, even though it is possible, as noted above. Getting this level of distortion @ 350W is impressive however.

Of course it seems that the majority of contemporary tube designs deliberately embrace much higher levels of THD to cater for those who want a blatantly muddy sound.

You’ve missed the key point: any competent design right from the original Williamson of 70+ years ago, to the Quad Ii, the Leak Point One, various Dyancos, HKs, Radfords, to contemporary designs will meet the 0.1% THD criterion at 1 kHz, but not across the full spectrum. At 20 kHz distortion will be an order of magnitude greater because feedback needs to be rolled off in order to maintain stability. At the low end of the spectrum, 20 Hz, transformer cores start to saturate near full power, so distortion again rises. So only a very few valve amps had under 0.1% distortion across the full spectrum. Needless to say they were not inexpensive, but unlike today’s expensive gear, one got a genuinely high quality product.

IIRC, Kron Hite also made a very high quality 50-60 W power amp.

 

[Blumlein 88]

Wolcott tube amps were probably the best measuring tube amps. I seem to recall, depending upon the model, THD of .01% at rated power, and these were powerful amps. Easily drive Soundlabs big esl panels. The circuit topology was not your normal tube amp circuits. It had adjustable damping and with feedforward it could have zero or even what they called negative output impedance. There is a white paper describing what they do, but currently they charge $5 for it.

 

[charleski]

You’ve missed the key point: any competent design right from the original Williamson of 70+ years ago, to the Quad Ii, the Leak Point One, various Dyancos, HKs, Radfords, to contemporary designs will meet the 0.1% THD criterion at 1 kHz, but not across the full spectrum. At 20 kHz distortion will be an order of magnitude greater because feedback needs to be rolled off in order to maintain stability. At the low end of the spectrum, 20 Hz, transformer cores start to saturate near full power, so distortion again rises. So only a very few valve amps had under 0.1% distortion across the full spectrum. Needless to say they were not inexpensive, but unlike today’s expensive gear, one got a genuinely high quality product.

IIRC, Kron Hite also made a very high quality 50-60 W power amp.

The MC-3500 was indeed a beast (page 13) in terms of holding up with tremendous output. But it’s common to find fairly decent amps that can maintain 0.1% or less across the spectrum at low levels. The ARC VT150 is a reasonable example of a modern competent design, and I can’t see much difference between that and the venerable MC275.

 

[al2002]

The MC-3500 was indeed a beast (page 13) in terms of holding up with tremendous output. But it’s common to find fairly decent amps that can maintain 0.1% or less across the spectrum at low levels. The ARC VT150 is a reasonable example of a modern competent design, and I can’t see much difference between that and the venerable MC275.

Only a very few valve amps met the 0.1%, at rated power from 20-20kHz benchmark. can you provide a link for the ARC VT 150 measurements? I doubt it comes close to the old classics.

 

[al2002]

Wolcott tube amps were probably the best measuring tube amps. I seem to recall, depending upon the model, THD of .01% at rated power, and these were powerful amps. Easily drive Soundlabs big esl panels. The circuit topology was not your normal tube amp circuits. It had adjustable damping and with feedforward it could have zero or even what they called negative output impedance. There is a white paper describing what they do, but currently they charge $5 for it.

I’m not familiar with Wolcott amps. 0.1 % or even 0.1% at full power 20-20kHz? Sounds promising, where can one get info?

 

[Sokel]

I’m not familiar with Wolcott amps. 0.1 % or even 0.1% at full power 20-20kHz? Sounds promising, where can one get info?

Tube Amplifiers - Vacuum Tube Amplifiers - Loudspeakers - Wolcott Audio Research

Tube amplifiers, vacuum tube amplifiers, distortion-free speaker systems and more are the specialty of this boutique electronics firm renowned for its innovation in audio research.

www.wolcottaudio.com

 

[CMB]

How bad would you say compares this one Line Magnetic 845?
Stereophile measurements :
..."Measured at the 16 ohm taps and taken with the inputs shorted to ground and the volume control at its maximum, the amplifier's unweighted, wideband signal/noise ratio was 70.6dB (average of both channels) ref. 1W into 8 ohms. This ratio improved to 72.2dB when the measurement bandwidth was restricted to the audioband, and to 81.9dB when A-weighted. The S/N ratios from the 8 ohm tap were 2dB greater and another 2dB greater from the 4 ohm tap, these improvements correlating with the lower gain from these taps."...

 

[charleski]

Only a very few valve amps met the 0.1%, at rated power from 20-20kHz benchmark. can you provide a link for the ARC VT 150 measurements? I doubt it comes close to the old classics.

The links are in my post. 1W into 8ohms @ 20Hz produces 0.05% from the VT150 and 0.02% from the MC275. At 20kHz the figures are 0.08% and 0.06% respectively (I’m citing the good channel for the MC275, since the other was a lot worse).

The point is that commercial tube amp design hit a plateau 50 years ago and hasn’t moved on from there. I suppose those looking for low distortion simply moved on to solid state once it matured in the 70s and there wasn’t a market for commercial designs that adopted the heroic measures needed to reduce tube distortion below 0.1%.

 

[al2002]

Tube Amplifiers - Vacuum Tube Amplifiers - Loudspeakers - Wolcott Audio Research

Tube amplifiers, vacuum tube amplifiers, distortion-free speaker systems and more are the specialty of this boutique electronics firm renowned for its innovation in audio research.

www.wolcottaudio.com

Thanks for the link. 0.1-0.2 % at 20kHz, rated power, so in the same ball park as the classics.

Reading the description, they use a balanced front end (amplification and phase splitter) with positive feedback to increase gain, and then applyi overall feedback using a tertiary winding on the output transformer. The smaller Macs and the Marantzes also used tertiary windings, but as far as I know no positive feedback.

The mighty MC3500 doesn’t use overall feedback at all which makes its operformance all the more remarkable.

 

[westyjeff]

I almost bought a set of the Audiovalve Baldur 300 mono's a few years ago, decided I didn't need a second heating system in the home,,,,,,

Baldur 300+ sovereign – Audiovalve

www.audiovalve.info

 

[Blumlein 88]

 

[al2002]

The links are in my post. 1W into 8ohms @ 20Hz produces 0.05% from the VT150 and 0.02% from the MC275. At 20kHz the figures are 0.08% and 0.06% respectively (I’m citing the good channel for the MC275, since the other was a lot worse).

The point is that commercial tube amp design hit a plateau 50 years ago and hasn’t moved on from there. I suppose those looking for low distortion simply moved on to solid state once it matured in the 70s and there wasn’t a market for commercial designs that adopted the heroic measures needed to reduce tube distortion below 0.1%.

Got them. When I clicked on the links on my iPad they didn’t open, so I thought you had just underlined the model numbers, but I was able to open them on my laptop.

These are 1 W measurements, the ARC fallls far short of the old classics at higher powers. Re the MC 275, it does not perform anywhere close to my original. I wonder if the circuit and /or output transformers are up to original specs.

Agree that valve amp design peaked a long time ago. I’d say the 1960s. Is output transformer design now a Lost Art?

 

[al2002]

Interesting that no one has discussed the EAR 509. Measurement wise it is up there with best.

 

[Strumbringer]

I'm relatively new to understanding measurements. Can one of you take a look at this graph and let me know your impressions of this tube amp? This is the Linear Tube Audio MZ2. The test was a 1 kHz @ 0.550 Vrms into 330Ω capture.

 

[SIY]

I'm relatively new to understanding measurements. Can one of you take a look at this graph and let me know your impressions of this tube amp? This is the Linear Tube Audio MZ2. The test was a 1 kHz @ 0.550 Vrms into 330Ω capture.

The measurement resolution here is not good enough to show the weird sideband behavior.

 

[Gary_G]

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