SS recipe for tube sound

[Mick Bailey]

When I implemented mixed-mode feedback in my Sessionette 75 the results were pretty impressive, and also in the first instance, too extreme. I had to fine tune the feedback to get the sound I wanted, but the results are chalk-and-cheese compared to the stock amp. It has a few other changes as well. I love to demo this amp against expensive or boutique tube amps, just to see the look of disbelief.

[J M Fahey]

Sessionette 75 is an impressive amp, excellent sound and of course proof that Mr Award Sessions (is that his real name?) did his homework thoroughly.
Using Strats or Teles is indistinguishable from a good tube amp; can be told apart with humbuckers because they are harder to amplify, since they have more lows and low mids and less shimmer than Fendery singles, but even so ..........

He also chose an excellent speaker, which certainly helps-

[loudthud]

At the beginning I made single rail tests, recently a build a new dual rail power section.

what I did not right?

In a technical sense, tubes are imperfect amplifying devices. They amplify, but add distortion. Because of their cost, size and power consumption, circuits are usually chosen that are rather simple. The distortion is not eliminated, it is accepted as an inevitable byproduct. In guitar amps, we like the distortion. Transistors also add distortion, but because they can be made much smaller at low cost and don't consume as much power, the circuits are many times more complex and distortion is reduced greatly. Integrated Circuits employ such circuits internally. But, and it's a big butt, without the distortion, transistor circuits sound inferior to tube circuits as guitar amplifiers unless steps are taken to make the transistor circuits behave more like tubes. The transistor circuits are too close to perfect. Mixed mode feedback is one of the ways solid state circuits are made to act more like tubes.

Mixed mode feedback is not a cure-all to instantly make a solid state guitar amp sound like a tube amp, but it is a step in the right direction. Mixed mode feedback does not address the distortion aspect of tube sound and how an amp behaves when overdriven. If you overdrive a sine wave into a tube circuit, most of the time you will not get a perfectly symmetrical square wave on the output. (It contains even and odd harmonics.) Most solid state circuits however will produce a perfectly symmetrical square wave. The symmetrical square wave sounds hollow and is not desirable for a guitar amp. It is mostly composed of odd harmonics.

Many things have been tried to make solid state guitar amps sound more like tube amps. Much of it is documented in teemuk's book, you can download it here: Teemuk's Book about solid-state guitar amplifiers I think you have to joint the forum and login to see it. I saw an old post from teemuk where he was asking about mixed mode feedback because he didn't understand it. Now he's an expert. A huge thanks to teemuk for his book and all his work.

[benito_red]

@ Loudthud

In a technical sense, tubes are imperfect amplifying devices. .... .

I know well the book of Teemuk but it does not go much in the deep about CD.

- you say that CD does not affect distortion.
that is absolutely correct. In fact I choosed a bad title for this thread, because there is not a single factor in the recipe of tube sound, I just wanted to talk instead, at least for now, just about how much CD can be considered essential factor in tube sound simulation. I do not agree instead that SS produces simmetrical (odd armonics) distortion and tube asimmetrical (even ar). this is a myth that has been completely debunked. Even/odd armonics only depend on the circuit topology.
But let's go back talking of CD, there is no dubt that tube power amp exhibit hight output impedance (even if that is not real CD) but anyway the question is:
How much, in what degree, hight output impedence is a essential for tube simulation?
I think that the best answer was so far provided by J M FAHEY : it is a matter of tone balancing and at the end of tastes. And I add to that the fact that increasing the output impedence improve for sure reliability.

But note that all info you can find in the web about CD (you can refer to the sites of Lenard Audio, Rod Elliot, Peavey etc...) they all state the CD is one of the essential factor of tube sound.
And that is misleading

[R.G.]

There's only one way to get real tube sound with solid state equipment.

[uneumann]

But note that all info you can find in the web about CD (you can refer to the sites of Lenard Audio, Rod Elliot, Peavey etc...) they all state the CD is one of the essential factor of tube sound.
And that is misleading

Oh my ... misleading info on the web? How is that possible?

Well - I happen to think there are elements of "truth" in many of these points. You can do spice sims to test the impacts of high vs low impedance drive of speakers. Your ears will likely confirm the sim results. Even though the sims may only be approximate - they confirm what's already been said. The impedance curve of a speaker is all you need to look at to see why the LF and HF boosts occur for HiZ drive.

The effect is a scoop'd power response curve for current drive (tubes) and a humped one for voltage drive (SS). The difference is not small - as much as 8-10 db at the extremes. That's pretty significant and it's no surprise that people can hear it.

What's also clear (to me at least) is that there is nothing magic about getting that scoop or hump from the output stage. You can also get it in the preamp and we see lots of scoop'd response curves in preamps - so they seem to be "liked" by players. The combination of Fender tone stack scoop and HiZ speaker drive scoop have to a large degree "defined" a key elemental tube tone - at least as far as "clean" tone goes. Distortion is another matter and should be treated as such. How a circuit distorts and how a circuit passes frequencies are often inter-related, but they don't need to be.

Back to scoops - the HiZ drive scoop has its major impact at very low freq's - I note most speakers produce almost all their bass boost between ~80 and 200 Hz. That's way below the tone stack scoop - 3-4 octaves in many tone stacks. With a SS output (LoZ drive) you're tempted to turn up the bass in these stacks to recover the 100-200Hz bass a HiZ drive gives you. However, while you will boost 100-200Hz, you'll also boost 200-500 Hz and that's where things start sounding very different. Human perception in that region is very sensitive and the effect is (to me at least) the "boomy - cardboard box - cheapo amp" sound. Boosting those freq's has a huge impact that is often bad.

You can play with this effect yourself. Get a hifi SS power amp and drive it from a scoop response guitar preamp and you will not get the same tone as you get from a tube power amp. For example, I recently did a lot of experiments like this with a tweed bassman. The bassman 6L6 tube power amp produces the classic "tube sound", but the same preamp with a SS power amp (hifi-grade w no current feedback) doesn't sound anything like it (same tone settings, spkrs, cab). Frankly, I like the SS tone too - but it's clearly not the same. I then add a 1/3 octave EQ in the signal path for the SS amp - and voila - with an additional EQ response curve - the two sound amazingly similar. In fact, I bet they're indistinguishable (for clean tone at least). The EQ curves I wind up with are set to ~12db boost between 80-220Hz and then a gradual 6db boost from ~1k-6k. YMMV due to different speakers, cabs, etc - but the general effect is clear. Switching the EQ in/out, the effect is striking.

So, for me at least, the experiments were convincing - a separate EQ in the preamp can/does compensate for voltage drive. Current feedback in the output stage may do the same thing, but it is not the only way to do it. In fact my new project includes a SS power amp a second tone stack (switchable in/out) that is tuned to compensate for these effects.

Getting back to my earlier point that EQ and overdrive are separate issues. Preamp EQ compensation means that a simple SS power amp - with no feedback at all - can drive the speaker with a "tube" power response curve. That also means the overdrive behavior in the tube preamp or in a SS output stage can be tweaked without NFB of any sort getting in the way.

[nickb]

Another idea why you might not be getting what you expect when using CD, is that at resonance and higher frequencies you run out of voltage to drive the speaker. Thus you could run into clipping which will significantly alter the sound from the ideal.

[R.G.]

Then there's the truism that if it was easy, someone would already have done it long ago, and every amp would be like that now.

[loudthud]

There's only one way to get real tube sound with solid state equipment.

[ATTACH=CONFIG]41440[/ATTACH]

You haven't been to my shop yet, I use a bigger hammer.

[nickb]

There's only one way to get real tube sound with solid state equipment.

[ATTACH=CONFIG]41440[/ATTACH]

I hear that works great for a percussive effect....
...once.

[Mike Sulzer]

What's also clear (to me at least) is that there is nothing magic about getting that scoop or hump from the output stage.

Well, there sort of is. If you do it with the output stage, then the response changes with different speakers in the same way it does with a tube amp with the same output impedance.

Something that Benito said needed checking, that is, that the cabinet so modifies the driver impedance that this effect is less significant. I checked a Red Fang in a Line Six open back cabinet. Of course the bass is different: the peak impedance is a bit under 15 ohms rather than over 100 as the plot on the web site shows. But from about 300 Hz up, my measurements track the graph on the Eminence web site very closely, closer than I would have expected based on reasonable tolerances. Reasonable agreement is expected because there is not much coupling to the cabinet outside of the bass.

[teemuk]

How much, in what degree, hight output impedence is a essential for tube simulation?

Depends.

Tube power amps tend to have high output impedance due to following reasons:

- Output tube type: Triodes have higher internal feedback and lower internal impedance than pentode / beam tetrode tubes. Yet most "power" tubes are pentode / beam tetrode -type.

- Feedback. Transformer coupling introduces a stability limit to magnitude of FB, too much NFB and the amp turns instable. If output Z is to be decreased further, one needs alternative methods than plain global voltage NFB.

Of course, all these issues can be compensated to a point and we once again come to the fact that THERE IS NO SINGLE ARCHETYPAL TUBE AMP DESIGN. If a designer wishes to mimic "tubes" he first has to choose from thousands of different sounding tube-based units what he exactly even wants to mimic. Rest assured that damping factor of an open loop power amp of Vox AC30 is something entirely different than damping factor of an Audio Research reference tube amp. Heck, one essential difference between Fender Bassman and early Marshall power amp was how much NFB they applied. "Tighter" Marshall tone was due to higher damping factor.

So, you will not find THE exact amount of current feedback to apply in order to acquire "tube sound". How much and in what degree are largely preference-related issues and based on what each individual subjectively considers as archetypal tube sound. ...But undisputable fact is that one can "mimic" high output impedance (and its effects) by introducing high output impedance.

[benito_red]

Well, there sort of is. If you do it with the output stage, then the response changes with different speakers in the same way it does with a tube amp with the same output impedance.

Something that Benito said needed checking, that is, that the cabinet so modifies the driver impedance that this effect is less significant. I checked a Red Fang in a Line Six open back cabinet. Of course the bass is different: the peak impedance is a bit under 15 ohms rather than over 100 as the plot on the web site shows. But from about 300 Hz up, my measurements track the graph on the Eminence web site very closely, closer than I would have expected based on reasonable tolerances. Reasonable agreement is expected because there is not much coupling to the cabinet outside of the bass.

I repeated just yesterday the tests with a Jensen P8R-4ohm:
the cabinet size was 35x33x19cm, I think adeguate for 8" drive.
the open window at the bottom of the rear side is of 30x7=213square-cm = diaphram area of P8R, that is a method to
lower the resonance frequency of the speaker.
On P8R Thiele&Small data, the Res is 16ohm, this is the impedance at resonance (pure resistence).
That means a power amp output impedence of 150ohm can work closely to real CurrentDrive.
That means also we must expect 12dB difference between resonance and nominal impedence output [20xlog(16/4)].
In fact I measured a difference of 13dB (the ratio was 4.5) with the speaker out of the cabinet.
With the speaker inside the cabinet the difference was 8dB (ratio 2.5)
In order to simulate a tube power amp with negative feed-back I assumed an output impedence of 6.8ohm
RESULTs:
Speaker out of cab, 6dB (ratio=2).
Speaker in the cab, 4dB (ratio 1,6).
Evidently different figures will be yield with different speaker, cabinet, outZ.

[benito_red]

Another idea why you might not be getting what you expect when using CD, is that at resonance and higher frequencies you run out of voltage to drive the speaker. Thus you could run into clipping which will significantly alter the sound from the ideal.

Yes I think this is a good observation, and need to ensure that it is not the case

[benito_red]

Oh my ... misleading info on the web? How is that possible?
..........
.

agree that proper preamp EQ can be an alternative to hi-out-Z.

You mentioned SS without feedback. AFAIK nobody never built such amp, but it would provide not only hi-out-Z but likely soft-clipping as well. The only way I know to do that would be
to use the same tube topology: power transistors coupled to the load thru out-transformer. Do you know a different way and eventually share your findings (if it is not a secret)?