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12ax7 perfect bias point
12ax7 perfect bias point
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Old 2nd August 2011, 03:52 AM   #11
jjman is offline jjman  United States
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Join Date: Jan 2009
I think 3V p-p, for any real period of time is only going to come from very strong HB pickups. It's hard to measure a guitar input since the signal is so nasty on a scope. I think I've used a compressor to try to measure my SD '59 pickups and I think I get around 2V p-p. Turn the compressor off and strum hard and it's hard to quantify. I was never able to find good documented figures on this.

Normally you don't want the 1st stage to clip since it normally has no attenuation on it's input (other than the guitar's volume knob.) Many guitars need to be turned up all the way to avoid a change in the EQ you hear. If the 1st stage clips under a "default" setting on your guitar you will never be able to play the amp "clean" at any volume from the speaker. This only happened to me with one guitar. A Gibson LP with Dimarzio Super Distortion pickups. The volume on that guitar has to be down somewhat to avoid overdriving the (#1) inputs on my '71 Deluxe Reverb. This is why they also have the #2 input of course.

However, a bias in the middle of cutoff and saturation helps allow for "more cleans" than otherwise, so that spec target seem fine to me. Based on your 205 plate volts at cutoff (PS voltage at that node,) your plate resistor and middle bias target I get this from my load line excel file. The red line is 1watt which is not being reached. I donít think guitar amp 12ax7s typically reach 1watt at any moment on the typical load line but I could be wrong of course.
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Old 29th April 2012, 08:54 AM   #12
nazaroo is offline nazaroo  Canada
Join Date: Feb 2012
Originally Posted by taj View Post
Hi Alex,

Someone here recently showed me this website for a guy who really studied distortion spectrum. Naturally, his perspective might not match yours or mine (he's after an Eric Clapton sound), but the information regarding distortion technique is really interesting. Read through his guitar amp projects.

Bob Richards Audio Index

Hmmm...this doesn't start off well:

"My understanding of the musical relationshipsbetween the fundamental and its harmonic distortion products led me to realize that although I definitely don't want distortion products to extend way out in frequency from the fundamental (which feedback causes), the other really important thing is to have a dominant 2nd harmonic (same note as fundamental but one octave higher). The 2nd harmonic is also the first "even" harmonic. The 2nd harmonic seems to enhance anything. It seems always desireable.

The third harmonic is 1.5 octaves above the fundamental - it forms a major chord with the fundamental and/or the 2nd harmonic. That's an enhancement when the song is in a major key, but a likely a "dissonance" when the song is in a minor key, or any minor chord is employed.

Harmonics beyond the third are hit/miss as to whether they are pleasantly musically related to the fundamental. I found this out by playing the notes on a piano that were of the same frequency as each of the harmonics, along with the fundamental."
Bob is a bit confused here.

The third harmonic is not a musical major 3rd interval, but actually a "musical 5th". That is, it forms an open two-note chord, left undefined as "major" or "minor".

This is a typical confusion by amateur musicians/electronic experimenters.

The "musical 5th" (an interval of 5 notes in a major/minor scale, counting both the starting and ending note) is a mathematical '3rd', that is a multiple of 3 times the ROOT frequency of a 2-dimensional vibrating object like a string or column of air.

The "musical 3rd" (major 3rd) is actually the FIFTH harmonic, or a frequency 5 times that of the Root or fundamental vibration. It happens to fall upon a 'major 3rd' interval on a pure (untempered) musical scale, because it is an interval of 3 notes (counting start and finish), the notes actually being that of the 4th and 5th harmonics, not the interval between the Root and the harmonic, which would be two whole octaves plus a musical major 3rd.

The interposition of '3rd' and '5th' between harmonics and musical scales is a random coincidence based on conventional naming, and completely irrelevant (unless you have a New Age religion to base on it).


Moving along, Bob's uncontrolled experiments based on Rickard Berglund's 1995 observations probably have some significance, although Bob's understanding of higher harmonics and their musical contribution to tone amounts to unusable anecdoting.

The final quotation from Russell 0. Hamm's observations cannot be taken and applied, since it takes no account of the various effects of musical tempering of scales, and the difference for instance between the musical meaning of 'equal temperament' musical major 3rds, and actual major 3rds as played in string quartets and horn sections (entirely different musical languages).

A few decades of music theory would help Bob immensely here.

Last edited by nazaroo; 29th April 2012 at 09:12 AM.
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Old 29th April 2012, 11:20 AM   #13
nazaroo is offline nazaroo  Canada
Join Date: Feb 2012
What you need really is my ideal Bias point chart:

This is the Bermuda Triangle of Tube Biasing:

(click to enlarge)

Click the image to open in full size.

There is really no single ideal bias setup,
however, you can intelligently select operating points and ranges based on intended use.

Here are some examples:

(1) Hi-Fi setup: High Voltage B+ (500v) and Large Load (420k), to minimize distortion and maximize voltage transfer to load (this is not a power circuit, so voltage is more important than power transfer efficiency). Although higher voltages raise risks, they give more horizontal loadlines, which means current remains stable (think CCS). Headroom is not paramount here, because typical input signals will be played at low to mid-level volumes, to further limit system distortion, and protection against high voltage swings can be built in via -db pads for input of stage.
A lower bias point is selected, to better center swing in zone of maximum linearity.

(2) Universal Soldier Setup: Slightly Higher Voltage B+ (400 v range) Here is the mid-zone. Reasonable voltage and current excursion is expected, and more current can give both stability and a current source for subsequent stages which might need draw. a bias-point of -2v gives a nice centering for a balance of headroom and linearity.

(3) Guitar Maniac Setup:
Here overloading is expected and harmonic distortion (non-linearity) is actually desired. Lower plate load gives a nice steep 'dive-bomb' loadline, while lower plate voltage (330 B+) protects tube from HV shorting, and allows pushing tube into cut-off safely. The higher bias point (-2.5 to even 3.5) is chosen to give maximum headroom for wild guitar antics and easy sliding into non-linear 'sweet-spots', without driving tube into grid-conduction, which is a non-musical type of distortion.

Last edited by nazaroo; 29th April 2012 at 11:29 AM.
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Old 29th April 2012, 11:06 PM   #14
nazaroo is offline nazaroo  Canada
Join Date: Feb 2012
One thing that many people will notice, is that a large number of circuits load and bias 12AX7s nowhere near the design-center Triangle depicted above.

It will be a good exercise in fact to take both your favorite circuits (and your notorious pet peeves) and put them on my chart, to see how and where they are screwing up.

As I maintained in another thread, few people know how to properly set up a tube for the intended purpose.

People should feel free to post their examples here, not for ridicule, but for analysis, and prediction of circuit behavior.
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Old 29th April 2012, 11:36 PM   #15
Chris Hornbeck is offline Chris Hornbeck  United States
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A pair of triodes cathode coupled can be set up to give symmetrical clipping, and the clipping level can be adjusted by varying the tail current.

All good fortune,
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Old 29th April 2012, 11:48 PM   #16
nazaroo is offline nazaroo  Canada
Join Date: Feb 2012
One thing to observe here is that:

(1) proper bias and setup is first dictated by intended use of the circuit. This must be nailed down first.

(2) Next, appropriate B+ voltages and loads are chosen, to fix the slope and position of the load line.

(3) Now, the bias-point is selected based on the balance desired between headroom/input range and non-linearity/harmonic distortion.

(4) To force the bias-point, the correct self-biasing cathode resistor is chosen, or better, several tubes are set up in a rig, and the resistor is selected by experiment to put the bias-point in the best compromise position between the acceptable range of tube samples.

(5) The performance of the circuit is tested under realistic conditions, including input signals, and output loads from following stages. Attenuation or amplification is adjusted at the input, and impedance matching is done at the output, to conform to expected conditions.

Finally, notice that the ideal bias-point slides along the load-line to the left, as the load-line tilts toward horizontal and slides to the right. The Bias Point traces the beautiful mathematical curve known as the sea-shell spiral:

Click the image to open in full size.
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