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Old 12th February 2009, 08:16 AM   #1
hkibbe is offline hkibbe  United States
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Default Technical Survey Paper -- Need Some Help!

Hey everyone,

I have just recently taken the (possibly ill-advised ) dive into tube-amp design. As a guitar-player studying electrical engineering, I find this stuff incredibly interesting, and would like to note that this forum has proved an indispensable resource to me (and presumably many others) in the quest for the perfect tone.

Moving on,

I am writing a technical survey for a college course, specifically on both the auditory and electrical characteristics of vacuum tubes (the "tube sound"), , and also, (much less importantly ), solid-state and digital methods of emulating the aforementioned "tube sound" (please don't kill me... I'm just trying to write objectively).

Although it is not required, I feel as though it would be a great addition to this paper to have some insight from primary sources, preferably with experience in the field of vacuum tube audio. As much as I'd like to be able to cite my own experience, it is probably not appropriate for this paper.

If anyone here would like to help me out, I would greatly appreciate any insight you could provide into any of the following:

-Characteristics of the vacuum tube sound
-How these characteristics relate to the solid-state sound
-Desireable/undesirable qualities of either
-Design considerations and the differences in designing solid-state and tube circuits
-Solid-state tube emulation, in hardware, DSP, or anywhere in between
-Anything you deem important that I seem to have left out or that you feel would add to this paper

If you choose to help me out here, It would be much appreciated if you could provide a little background into who you are, your credentials, design experience, anything like that so that I may cite you.

Thanks in advance
Hamilton Kibbe
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Old 12th February 2009, 11:59 AM   #2
Gordy is offline Gordy  United Kingdom
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I hope not to sound too negative, however please understand that you are working from the false premise that there is something called 'tube sound'.

Here is the news: there is no 'tube sound'. Indeed, if there was 'tube sound' then by inference every tube would sound the same. I.e. every tube would have 'tube sound'.

So, take a tube in one hand and a transistor in the other hand and hold them to you ears... what do you hear? Nothing, obviously. So clearly tubes and transistors have 'no sound'. (I know that sounds patronising, but you have to get a grip on this fundamental fact if you are to succeed).

Next, either measure the transistor and tube or study their datasheets, and what do you find? You find that they have electrical characteristics. It is these characteristics that may be applied in circuits. It is those circuits that can be applied in systems, and it is those systems that can reproduce sound with a certain level of performance often noted as 'sound quality'.

Next, let us consider that history has produced tube based equipment that has had good, mediocre, and poor performance. Therefore we can say clearly that the inclusion of a tube in a circuit does not automatically equate to good sound.

OK, so now we are getting somewhere. So my advice to you is to try and understand which electrical characteristics of vacuum tubes, and which characteristics of their supporting circuits, influence the performance of the system.

Along the way you may wish to consider two important factors:
1) the different way a distortion meter measures signals compared to the way your ear / mind / intellect interprets received sound.
2) the way equipment works under real application conditions compared with standardised test-bench conditions.

You have a lot of reading ahead of you and I wish you well in your studies.

Finally, to reiterate, no matter what drivel you may read in magazines and the on the internet, and no matter how appealing the sentiment of drama-queen audio gurus, please understand that there is no such thing as universal 'tube sound'.
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Old 12th February 2009, 12:02 PM   #3
SY is offline SY  United States
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Is your concern hifi amps or guitar amps? The answers will be quite different. For the latter, emulation will not be trivial as it involves not just electrical transfer characteristics, but microphonics, overload characteristics, and overload recovery.
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Old 13th February 2009, 01:53 AM   #4
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A few thoughts on this.

Transistor amps got a bad name in the late 1950s/early 1960s, when the first commercial models were launched. The unavailability of good complementary power transistors at that time gave rise to the development of quasi-complementary designs, the sound of which was poor. The two halves of the push-pull driver-power transistor combination had different transfer characteristics, causing a discontinuity at the crossover point, a form of crossover distortion.

Later years saw the arrival of good complementary power BJTs and MOSFETS, allowing the quasi-comp approach to be dropped in favor of fully-complementary topology. However, output stages were still almost universally run in Class B, which gave rise to another form of crossover distortion. This crossover distortion, not being proportional to the volume level, sounds particularly nasty at low sound levels

Transistors, being low impedance devices, could be made to drive loudspeakers directly without using an OP transformer. This opened the door for circuits with a much higher degree of negative feedback than could be applied to tube circuits, because an OP transformer gives rise to phase shift and places limitations on the amount of NFB that can be applied without incurring instability.

The ability to apply heavy NFB helped the designer to mask the non-linearities and crossover diastortion of SS and, indeed, to attain very low levels of THD, but it also had its disadvantages. One of these was what happened when an amp with heavy NFB was overdriven and started clipping. The NFB lost its grip suddenly and to drastic effect, and the resulting harsh distortion was extremely unpleasant. This happened whether the overdrive condition was due to a momentary transient or was deliberately caused for musical instrument effect.

Tube amps, on the other hand, can be designed to behave in a far more genteel fashion when overdriven, mainly because they sound OK with much less negative feedback than transistors. This gives tube amps two advantages over SS amps: (a) they can be designed to handle transients without excessive distortion on occasional overdrive; and (b) they can be run countiuously overdriven, as musical instrument amps, to achieve interesting and attractive effects.

Tubes are big, hot, short-lived, fragile and expensive devices compared with transistors, thus, there is an incentive to simulate their behavior using SS.
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Old 13th February 2009, 02:32 AM   #5
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Definitely read up on the work Bob Carver did in duplicating the sound of any given amp by matching the transfer function of his amps, to the amp in question. He did a remarkably good job, supporting what you've read above. You'd also be missing a lot by not getting a copy of Doug Self's book on audio power amp design.
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Old 14th February 2009, 02:10 AM   #6
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Default Re: Technical Survey Paper -- Need Some Help!

Hamilton,

I think a very necessary survey, as these differences are often poorly undestood, not least so because of the utter drivel - er, well - uttered in magazines and on the internet as Gordy warned.

Briefly, just for the record, I have done audio design since school (!), i.e. for more than a half century (!!), both in the hey-day of tubes and later semiconductors. I am a graduated electronic engineer, retired, and still designing amplifiers!

I will try to contribute without duplicating previous posts:

Characteristics of the vacuum tube sound-
How these characteristics relate to the solid-state sound

I accept that you have some background in tube/semiconductor basics. Tubes come as triodes or pentodes; semiconductors only as 'equivalent' to pentodes as far as characterisitcs are concerned. Triodes have largely square law characteristics, thus they give mainly even-order harmonic distortion and mainly low order (in this post one assumes proper designs!). Thus they can give very low distortion amplifiers in push-pull, where even harmonic distortion cancels out. They also feature relatively low internal resistance (called plate resistance). This is to my mind, where one can use the term 'tube sound', though obviously much depends on the circuit design as Gordy pointed out.

Semiconductors and pentode tubes are alike - they are like constant current 'generators'. This gives higher available stage gain, but also needs more critical design. They can generate copious amounts of higher order harmonic distortion especially under variable loads, e.g. feeding a loudspeaker in output stages.

Desireable/undesirable qualities of either
Apart from the above: As said by others, specifically for guitar amplifiers, certain desirable overload/second order effects can be easier generated with tube circuits, especially triodes. Tubes have a limited life, semi-conductors not (again, in proper designs). Tubes require both heating (filament/heater) power as well as a high voltage supply (typically >200V - 600V for high-power amplifiers); semiconductors from as low as 12V to 200V or more.

Design considerations and the differences in designing solid-state and tube circuits
Whew - that will be a long answer. Briefly, both need to be fitted into their characteristics according to design requirements. Triodes are low internal impedance devices; pentodes and semiconductors have high internal impedances.

But the main difference is that in practice tubes have infinite input impedances, thus are voltage driven (output is equivalent to input voltage waveform) - while transistors have low input impedances, and are thus current-driven (output is equivalent to input current waveform). This can make the design of transistor circuits more troublesome/demanding. Field-effect transistors (fets) are far more like pentodes (high input impedance), but have a relatively high input capacitance.

Solid-state tube emulation, in hardware, DSP, or anywhere in between
Mmmm - Solid state devices are dramatically smaller, far cheaper to mass-produce, far greater diversity exists, and have thus largely replaced tubes. Manufacture of tubes are dwindling. For this reason accurate computer models exist, while that of tubes are sometimes 'contrived' though they can be accurate enough for design purposes.

But semiconductors come in far greater spread of characteristics (typically 30% - 500%) comapred to tubes where +/- 10% is (or at least was ) deemed to be proper.

Anything you deem important that I seem to have left out or that you feel would add to this paper
There are different proper design approaches. The main thing to perhaps point out here is the nonsense that is sometimes piously disseminated, and even found in so-called high-end designs! (Not trying to be derogatory or 'holier-than-thou' - it is simply and sadly the truth.) This would mainly pertain to semiconductor designs since they have become (and rightly so) modern amplifier practice. In summary I can perhaps generalise and say that tubes are easier to make a good design with; semiconductor designers all to often dolly up mediocre designs with lots of negative feedback with disasterous results (thus, incidentally, the totally undeserved bad reputation given to nfb).

I am seriously exceeding my welcome and perhaps message length; this in a (very small) nutshell routes along which I would approach such a survey.
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Old 16th February 2009, 01:56 AM   #7
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Be sure to read and cite the following article from IEEE Spectrum:

The Cool Sound of Tubes
www.spectrum.ieee.org/print/1640

Enjoy,
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Old 16th February 2009, 03:37 AM   #8
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Maybe was mentioned already, tubes generally will survive exceeding their specs for a while (long enough to hit the off switch) while SS devices will often fail immediately when specs are violated.

The large (relative) size of traditional tubes makes DIY projects much more practical than SS, which is going to all surface mount devices. Tubes don't require heatsinking usually, so they can actually weigh less than SS (without a Low Freq. steel XFMR at least).

The VT pentode can also be seen as a triode with separate output and feedback nodes. Nothing like that is really available in SS. But that feature hasn't been utilized much in tube circuits to date either.

SS devices have had 50 years of development beyond the cutoff in major tube development, particularly regarding miniaturization. (VT related microwave tube tech and particle accelerator tech have proceeded well though.) But micro-miniaturized tubes do exist in university labs, made with silicon lithography, just like nanotechnology. (and some flat panel displays using micro tubes exist there too)

The historical requirement for a heater for the cathode's operation has been solved in several ways now too. So the power efficiency of SS can now be equalled by vacuum tubes in theory. The current density of these filament free cathodes is close to SS too. The electron transit time in vacuum is much faster than SS materials, so when size is equalized, micro tubes can be very fast devices.

So if SS didn't already exist, vacuum tubes could be brought up to equivalent development levels for current electronic technology (after considerable costly process development that isn't getting much funding now).

The just emerging development of graphene technology may well supercede SS technology altogether, but will probably be made compatible with it initially. Graphene also would make an outstanding grid material for micro tubes as well as for FETs, so the tide may be turning there. The high speed and very low resistance of electrons in graphene, comparable to vacuum, should produce microlithic microwave devices, possibly with hybrid graphene/VTs. Likely, a collection of all these technologies will eventually merge into a very broadly capable process technology in the future.

One might also notice the urgency for low dielectric constant insulating materials for present LSI devices, and that vacuum is the ultimate low dielectric constant "material".

Don
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Old 19th February 2009, 01:40 PM   #9
hkibbe is offline hkibbe  United States
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Thank you everyone for your responses, I have been taking alot of time to do research based on information all of you provided, and am very grateful for your input! I'll let you know what I come up with if anyone is interested.
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Old 19th February 2009, 01:40 PM   #10
hkibbe is offline hkibbe  United States
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It may also help to clarify what I am asking if I define the "tube sound" i referenced in my original post as something like:

A set of characteristics within a system's properties (eg. transfer function, impulse and frequency response, etc.) that differs between solid state and vacuum tube devices.

if that makes any sense. it's late.
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