# Do all vacuum tube electronics distort?



## rongon (Aug 23, 2012)

Yes, I am trolling. 

I've been trying to learn as much as I can about audio electronics these last few years, and have built some tube audio circuits that I've decided are keepers. I started back in the 1980s with mainstream 'good' solid-state gear, worked my way up to a complete Audio by Van Alstine (AVA) setup, with one of their SuperFET preamps, a MOSFET 100C amplifier, B&W speakers and a harman kardon T60C turntable with Grado cartrdige ("Longhorn" version from AVA). Sometime around 1990, I was introduced to a little shop called "Fi" on Watts St. in NYC. That blew my mind. 

A few years ago, I was introduced to "Valve Amplifiers" by Morgan Jones. (http://www.amazon.com/Valve-Amplifiers-Fourth-Edition-Morgan/dp/0080966403) Mr. Jones methodically tests vacuum tubes for distortion and noise, and shows he can easily get less than 0.1% harmonic distortion at line level from several common triodes. He also demonstrates that this distortion is practically all composed of 2nd harmonic distortion, at least for the better performing triodes. This is all open loop, without negative feedback wrapped from output to input to linearize the stage. You can't add quite as much negative feedback (NFB) to a vacuum tube as you can to an opamp or FET stage, but -10dB of NFB should be attainable. Mr. Jones also shows that if the open loop distortion of the stage is already low, and is of predominantly 2nd order harmonic, NFB will not generate noticeable higher order harmonics when applied. In other words, adding NFB to an already clean circuit will only make it cleaner. Therefore, a vacuum tube stage can be made to have less than 0.05% total harmonic distortion, with not much trouble. 0.01% THD should be attainable, with care. 

So my question is, does an opamp stage (such as is common now) have orders of magnitude lower distortion than this, in actual practice, at line level (let's say 100mV rms input to 2V rms output)?

--


----------



## fmw (Aug 11, 2013)

All amplifiers distort. Most modern ones, though, have their distortion level low enough to be below audibility. There are some tube amps that also have inaudible distortion but not many and those that do are generally quite expensive and have no characteristic sound.

As a rule of thumb, harmonic distortion above 1% is considered audible and most tube amps have harmonic distortion above that level. We tested one once that had 5%. Some people find it very pleasing.


----------



## rongon (Aug 23, 2012)

Was that amp with > 5% THD measured at full power, or at one watt output? Was it a single-ended triode (SET) amp? 

Remember that 2nd harmonic distortion is far less noticeable than 3rd, which is less noticeable than 5th or 7th, and so on. 5% 2nd HD might not be subjectively objectionable, but 5% 3rd HD absolutely would be. 

That said, I'm thinking push-pull, where the amp should cancel most of the 2nd HD, leaving only odd harmonics. If the odd harmonics are low, then even a tube PP amp should have usefully low distortion. 

I'm not worrying too much about expense. I know that it's far more expensive to make a proper tube amp. No question. This is more a theoretical question. I'm trying to find out if there is something other than harmonic distortion that distinguishes a tube amp's "sound" from that of a comparable solid state amp. 

When you tested a tube amp that yielded low distortion, did you all find its sound was indistinguishable from that of a solid state amp of similar quality and (low) power output? 

--


----------



## fmw (Aug 11, 2013)

Yes the 5% amp was an SET. Low power and full power were about the same.  If I remember right the thing only put out about 8 watts per channel.

Besides harmonic distortion, some tube amps have some peaks and dips in frequency response. I can't find the manual to my old Audio Research amp. It was pretty expensive and had measurements that were better than most other tube amps but there were still some areas in the frequency response curve that went over 3 db from flat in the audible range. The measurements would say it was bloaty in the mid bass but, if it was, it wasn't serious.

I wouldn't spend any time worrying about measurements with tube amps. Their measurements are always inferior to those of solid state amps. If you like that sound, that is what matters. Personally, I don't mind the sound I just mind it being built into the component and not defeatable. I'd rather EQ a clean, straight amp to get the sound. It is all a matter of preference.


----------



## rongon (Aug 23, 2012)

SET amps are a special case. They have way too high of an output resistance, which *will* interact with the loudspeaker's impedance curve and cause frequency response errors. That's the major problem with SET amps. They absolutely need high-sensitivity speakers that are designed to present a flat impedance load within the audible band. That makes it really hard to make a good-sounding system around an SET amp. 

I have friends who absolutely prefer SET's, but I prefer push-pull. First off, the output transformers are a LOT less money. Since the OPT is the weakest link in any reasonable tube amp, I figure I need to maximize the quality-per-dollar right there in the OPT. 

[ Wait... The ARC amp's *manual* shows frequency response dips or peaks of 3dB? You might want to double-check that. I can imagine that happening if you measure the amp in the real world with a very difficult load (see below), but I cannot imagine ARC actually publishing that, and that being better than most other tube amps. Really, modern push-pull tube amps are NOT that bad. Honestly. ]

Audio Research amps were designed with 6550A's or similar output tubes that ran with pretty high odd-order HD. They would use quite a bit of NFB around them, including the OPT. The NFB lowers the distortion and the output impedance, making the amp cleaner and more able to drive a difficult load. If the OPT was of less than stellar quality (which almost all vintage OPT's are) then frequency response problems could result. However, I'm very surprised errors this large would happen in the audio band (20Hz - 20kHz) from that kind of an amp. A higher-powered tube amp that caused a 3dB deviation from flat response must have either been broken or just a really bad design. Maybe the OPT's were just cheap. Or... It could be that you had a speaker on it that presented a widely varying impedance within the audible range, which lower-powered tube amps will have a problem with. But an ARC amp? Those things were pretty beefy. Really, I'm surprised at a 3dB freq resp error from that well-regarded an amp. 

I bought a pair or Klipsch speakers to use with my weenie little 6Wpc tube amp, but found that they didn't sound as good as I expected. Since Klipsch crossovers were designed without concern for presenting a flat impedance load to the amp, it doesn't surprise me that my weenie little tube amp had a problem with them. A big pair of Snell Type A's is working fabulously well, though. 

I'd expect large errors from my little amp into a difficult speaker load, but not from a big ARC with lots of NFB and 30+ watts per channel (if not more). Maybe those amps aren't all they're cracked up to be?


----------



## rongon (Aug 23, 2012)

Which model ARC amp?


----------



## fmw (Aug 11, 2013)

I don't remember the model name. I went looking for it on the internet. It looked like the Reference 110 but was only a 50 watt amp. The speakers were B&W Matrix 802. It was about 15 years ago. I used to sell vacuum tubes on the internet and I lost interest in tube components when I moved on to other things. Sorry my memory is so fuzzy.


----------



## rongon (Aug 23, 2012)

Yeah, a B&W 802 would be a really tough load for a typical 50 watt per ch tube amp. Has a wicked crossover.


----------



## fmw (Aug 11, 2013)

Whatever you say.


----------

