How to increase Harmonic distortion for clean sounds

[Tahoebrian5]

I'm sneaking up on doing a custom build and the clean channel is first on my agenda. My understanding is that the more THD the more even harmonics, the sweeter the tone. I like a swirly tone that almost sounds like it has a chorus on it bone dry. I'm talking about unclipped distortion due to wave form warping in triode stages. Please correct me if my initial assumption are incorrect or incomplete. So, I'm considering using something other than the 12ax7. Can someone recommend which triodes have more THD? My concept is to take a fender twin type circuit and use lower gain tubes with higher THD, and maybe use more stages to make up for the lower gain of these tubes. I'm thinking the first stage would be the only 12Ax7. Thoughts?

[Jazz P Bass]

I do not think the two terms can be used in the same phrase.

A 'clean' sound is not distorted.

If the signal is linear, there is no distortion.

But if you want to add it, avoid a push pull output section.

Maybe single ended would be better.

[Tahoebrian5]

Not true!!! Harmonic distortion is introduced with warping of the output wave compared to the input wave and is present even without clipping.

[Jazz P Bass]

Actually, i was not speaking of 'clipping' as distortion.

You said a clean signal.

That implies 'not distorted'.

Any change of the output when compared to the input is technically distortion.

So maybe you could explain this 'warping of the output wave' a little better.

There are a gazillion kinds of distortion, but only one clean.

[eschertron]

Tahoebrian, you are correct. Distortion is technically any nonlinearity in the signal. Since THD = total (or third) harmonic distortion, you can't have any THD if you have no distortion. However, many guitar players equate clipping with distortion.

Nonlinearities in class A stages - pretty much all guitar preamps are this - produce amounts of 2HD that add the even harmonics. Careful design, to get the sound you want without clipping, is probably the ticket to harmonic nirvana. You can use tubes other than 12AX7, but that choice will be informed by your design needs for the gain structure of the amp. The amount of harmonic distortion that you get from a gain stage depends on how the tube's curves interact with the input signal (volts peak-peak is how I like to think of it when looking at tube characteristic charts). So you need to determine the B+ voltage available to that stage, and the anode/cathode resistor values. If you see that you have 'too much gain' in a stage, one design change you can make is to swap the tube type out for a lower-mu tube. That's not the only way to soften the distortion. There are other ways to do that, and many schems that show those solutions.

Use whatever tools you can get your hands on to cogitate/calculate/simulate a design (then of course you'll have to build, test, and modify!). This forum, available schematics, software programs, and books, articles and internet resources can help you make your design choices. My only concrete suggestion is to start from a preamp design in an existing amp that you think comes close to your ideal. There are many schematics accessible through the forum.

edit: @ JazzP - Tahoebrian reffered to "unclipped distortion" and equated it with clean. I understood him to mean the kind of distortion you get when you push the clean channel. He never used the term 'clean signal'.

[NateS]

Technically true, but we don't call pitch shifters distortion, even if they add gobs of 2nd harmonic. I think it's a valid request, even not phrased in a technically accurate way. (Aphex Aural Exciter anyone?) Check out the various doubler circuits as a good starting point.

[Alan0354]

To me, clean sound is just not clipping sound.....fuzzy sound. Clean sound can contain plenty of distortion, it's the distortion in the clean sound that make the tube amp sounds alive!!! But let's not get into definition. I think everyone know what OP is referring to.

[Alan0354]

This forum, available schematics, software programs, and books, articles and internet resources can help you make your design choices. My only concrete suggestion is to start from a preamp design in an existing amp that you think comes close to your ideal. There are many schematics accessible through the forum.

Any free simulation for tubes? Does LT spice simulate preamp tubes and it's distortion?

[Tahoebrian5]

Since this is a technical forum I believe I'm not out of line speaking in technical terms

For reference, see section 1.11 of the following link to Merlin's site
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
Lets not confuse overdrive clipping with distortion. Pitch shifters are not called distortion because that is the intended effect. Harmonic distortion was a byproduct (or error) of tube amplification that turned out to be pleasing to the ear in the right context

I am indeed planning to use the fender twin as a rough guideline. The input stage to tone stack will be the same, at that point I plan to use tubes with more THD then the 12ax7 as defined in their anode characteristic charts.. I'm just not sure which ones yet. I was hoping someone could direct me to which ones typically have more THD. These will be lower gain so am expecting to have to add an additional stage or two to get up to pre PI signal level.

[Tahoebrian5]

Also, I'm well aware I can figure it out myself if I draw load lines and compare the out swing to the input voltage swing from each tube.. Someone out there has got to already know the answer though. I'm also interested in other ways THD is added.

[Leo_Gnardo]

Wasn't this how Aphex got started - with their "Aural Exciter" - which at first they did not sell, only leased at a high $ per hour rate, accompanied by an "engineer" whose job was to make sure nobody opened up the box to try and reverse-engineer it? Tons of hit songs, mid 70's, mostly recorded in the LA area, had the Aural Exciter on vocals, but no doubt it was also used on guitar tracks to make 'em "pop out of the mix." Since then Aphex has become a very successful company, and includes that exciter technology in their excellent "Vocal Stressor", "Distressor" and other compressor products. Heck, Aphex even made a series of guitar pedals about 10 years ago. Not long after Aphex made their splash, BBE Barcus Berry electronics released their version. I have the 2-rack space original, and it's a treat to use on mixes. These are solid-state products, but the idea is to emulate certain mic pre circuits that grab the ear just right, some of those are tube. It's not exactly a new idea BUT don't let that slow you down. If I understand the original Aphex correctly, a band of high frequencies from the original signal was filtered, sent into a side-chain circuit, slightly delayed on the order of a couple milliseconds at most and (gently) distorted, then re-added to the original circuit. Hey if it worked for Linda Ronstadt, the Eagles, Warren Zevon, Jackson Browne and dozens more, it could work for you. Just gotta do it with tubes...

[Gingertube]

Tahoebrian5,
I will introduce you to a way of looking at the triode gain stage which allows you to think about the distortion level.
You put an input signal voltage on the grid.
You take an output signal voltage from the anode

What you have inbetween is a signal voltage generetor of Vg1k x mu. For a 12AX7 that is the grid 1 to cathhode voltage (Vg1k) x 100.
The output of this signal voltage generator feeds 2 resistances in series connecting to AC 0V forming a voltage divider
The first resistance is the internal tube ra, the second resistance is the anode load resistor Ra.
The output is take from the centre of these 2 resistors.
As the current in the tube changes with signal (swings up and down either side of the bias current) then the internal ra value changes. The Vg1k x mu generator signal does not.
It is that change in ra with signal which produces your distortion.

If you want to keep the harmonic Distortion content up then using a tube with high ra compared to the Ra value is what you want. The high ra is the most likely to give you high ra change with signal.
That means you do NOT want to use something other than a 12AX7.
The 12AX7 has an ra of 62K at usual operating current and is usually used with a Ra load resistor of 100K. That is why it gives a reasonable amount of distortion.
ASIDE: When you see 12AX7 used in HiFI circuits it is operated with Ra of 470K so that the delta ra vs Ra is small hence small distortion. (delta means "change of")

For more distortion:
Increase the delta ra vs Ra by
1) reduce the operating current by increasing the cathode bias resistor, that increases ra and therefore also increases the change in ra with signal
2) reduce the Anode Load resistor Ra, try say 82K 68K or even 47K instead of the 100K, That makes the ra change more significant compared to Ra and so you get more distortion.

Hope this make sense to you.

Check out the page labelled 345 here:
http://tdsl.duncanamps.com/pdf/vm345.pdf

This is the data for a 12AX7A / ECC83 Tube.

That shows you the change in Ri with operating current, Ri is what Merlin calls ra and so I used ra as well (sometimes this is also called rp).
For maximum distortion you want to operate on that part of teh Ri curve which shows maximum slope, that is at the low current end.

Cheers,
Ian

[Alan0354]

How is the ra change with current, ra goes up as current goes down so lowering the plate current increase ra?

How is ra change with plate voltage ( voltage coef of ra)?

thanks

[jazbo8]

How is the ra change with current, ra goes up as current goes down so lowering the plate current increase ra?

How is ra change with plate voltage ( voltage coef of ra)?

thanks

Ohm's Law reigns supreme

ra = delta Ep / delta Ip

see (this one from 6SN7):

[Tahoebrian5]

Thank you GT, that was actually quite enlightening and a different way of thinking about the circuit in general. So it sounds like my assumption that there are other triodes out there with more inherent THD , due to changing internal Ra, is not true. I had come across the idea reading thru random build threads and lost it. I guess you can't believe everything you read online! So lower B+ and lower anode resistors will get me more THD, thank you!