Tube amplifiers and interstage transformers

I've not found that transformers contribute to transparency. IME they always detract from it. Bandwidth aside that has been the main reason I avoid them if possible.

Distortion is however a main reason people will often prefer transformers, as these days of inter-stage transformer use it is mostly in single-ended circuits. The type of distortion I am referring to is the lower ordered harmonics, the 2nd, 3rd and 4th.

We have built a number of OTL guitar amps, and one of the problems we ran into was the fact that the amp was transparent and not a lot of distortion. This is great for a high end audio amp but bad for guitar. So we built an OTL that used a single-ended driver circuit and an inter-stage transformer to go from single-ended to push-pull. This got us a lot more 2nd ordered harmonics, which made the amp sound richer.

Sunn used this technique in their early transistor guitar amps, which have a reputation amongst guitar players as one of the few transistor amps you can use with electric guitar, due to the richer sound of their single-ended circuits (which were also mostly zero feedback).

When you are using a transformer for audio purposes, loading of the transformer is a big deal. If not loaded at all, the transformer will merely express the inter-winding capacitances rather than the turns ratio. This will result in poor bandwidth and distortion. As you load the transformer more and more, approaching a value known as 'critical damping' that bandwidth will widen and the distortion will drop. Because there is not a lot of power involved, the bandwidth of the transformer can be quite decent, easily exceeding 100KHz, so the real issue is how well it plays bass.

In SET circuits this is usually not much of a problem because once you get above about 7 watts or so you aren't going to see much low frequency bandwidth out of the amplifier anyway. Plus, to take advantage of the SET's ability to have unmeasurable distortion (in a lot of SETs distortion decreases linearly to unmeasurable as power is decreased towards zero) you need a really efficient speaker (103 db or more) and you are not going to get much bass out of most of those speakers either.

In order to get a condition of no phase shift in the audio passband (20-20KHz), the amplifier must have bandwidth from 2Hz to 200KHz, IOW 1/10th to 10X of the frequencies to be amplified. There are very few tube amps with bandwidth to 2Hz, and not many speakers that go to 20Hz, so this bass thing is where most of the fudging occurs in tube amplifier design, which is exacerbated when interstage transformers are employed.

Phase shift in the bass region translates to a loss of impact to a frequency up to 10X the frequency of the cutoff. So if the amp rolls off at 15Hz its a problem up to about 150Hz or so. This is the other main objection I have, as the speakers I play at home go to 20Hz. I played bass in the orchestra for a long time and as a consequence I have found that I am hard to please in this regard, hence I stay away from transformers in general.

Based on theory,measurements and the viewpoint of designers of solid state components,tubes should`nt even be a consideration for best sound aspirations.They`re too colored,too much distortion,outdated and measure poorly. Heaven forbid those comptemplating the SET amplifiers(worse of the tube lot).

Charles1dad, I can't agree with that statement, unless you exclude the 'theory,measurements' part of the first sentence.

The human ear is more sensitive to odd-ordered harmonics than just about anything else, as it uses them to determine the volume or loudness of a sound. This is why transistor amps sound bright even though the odd ordered distortion coming out of them is very low. Its just that the odd ordered harmonic distortion coming out of a tube amp is usually a lot lower, which is why they sound smoother.

SETs in particular have an additional quality which is that all forms of distortion they make decreases to unmeasurable at lower power levels (our amps do that too FWIW), something that transistors don't do (below a certain power level, distortion rises with them) and also something that most push-pull tube amps don't do (the traditional thinking being that this is caused by the phase splitter circuit).

IOW, the **theory** says that an SET (or other amplifier with similar abilities) should sound better to the ear. It is unfortunate that so much weight is given to specs on paper that don't mean anything to the human ear, which is the final arbiter. It is possible to create and measure specs that *do* mean something to the ear (IOW have a direct correlation to what we hear) but the industry has zero motivation to do anything like that. So for now, SETs will have to 'measure poorly' as you put it :)

Charles1dad, something that has puzzled me for a long time is the also 'common argument' that triodes are the most linear form of amplification known to man, yet they have more distortion??

The fact is that they don't. Try putting a transistor in a single-ended zero feedback configuration and see what happens :)

The 'common argument' is apparently false, and should be addressed where ever it appears.

We found that by adding feedback to our OTLs we could get the distortion down well below that of conventional transistor amps, but in doing so the amp also took on some of the sterile qualities (read: brighter, less musical) that are hallmarks of amps with feedback. We chose at that point (about 1980) to eschew feedback; if the speaker thus did not allow the amp to sound right on that speaker, the conclusion is that the speaker would never sound right with *any* amplifier! This has limited our market, but the upside is that if a person sets up the amp right its hard to beat.

SETs have a similar 'weakness' (although I regard it as a musical strength) on the same account.

Transistors 'measure better' on account of the fact that the 'measurement' has almost nothing to do with human hearing rules and are simple a method of making the amp look good on paper.

I don't know why the laws of human hearing are'nt appreciated more and respected.

A lot of what we know about human hearing rules has only been worked out in the last 35- 40 years. But by 50 years ago, most of the understanding of how to analyze audio products was already in place. To get the really low THD specs you have to use feedback; when it was discovered that the human ear uses odd orders to detect volume (and also that trace amounts of odd ordered harmonic distortion is really irritating to the human ear), the fact that negative feedback was contributing to that became an 'inconvenient truth'; essentially the audio industry paid no attention and may have even swept the discovery under the carpet.

So for the last 45 years or so the spec sheet has told us nothing about how an amp sounds- in fact as you say may even show an inverse relationship to the sound of the product. But changing the way the audio industry does things is hard- there are a lot of companies that simply will not accept human hearing rules as important, even though they pay lip service.

The result is that audiophiles, no matter what they have read or heard from others (and no matter who those others are) have to audition things for themselves. You would think that the spec sheet would help in that purpose- what else could it be for??

Of course the issue is pursuit of the almighty dollar. Tube amplifier power is more expensive than transistor power and high efficiency speakers are more expensive than low efficiency speakers. But the hype is that the cheaper transistors and lower efficiency speakers are somehow better because of the 'specs'! Whatta world!