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Preamp Bias

FractalAudio

Administrator
Fractal Audio Systems
Moderator
The Preamp Bias control in the Amp block controls the operating point of the last virtual triode stage in the preamp. This is the most important stage wrt to the feel and texture of distortion. The earlier stages are important but generally not nearly as much and the bias points are not exposed to the user.

The operating point of a tube determines the symmetry of the clipping. If the tube is biased exactly halfway between the supply voltage and ground then it will clip symmetrically (greatly simplified). If the quiescent current is reduced the tube will biased more towards cutoff. If it is increased it will be biased more towards saturation. In general cutoff is smoother than saturation but it depends on the external circuitry. Negative values of Preamp Bias bias the virtual tube towards cutoff and positive values toward saturation.

Symmetrical distortion has lots of odd harmonics and very little even harmonics. The more asymmetrical the distortion the more even harmonics are introduced. Odd harmonics give clarity and a more aggressive, open tone but this can be cold and harsh. Adding even harmonics gives a warmer sound but too much and things can get muddy. Getting the right balance of even and odd harmonics is one of the keys to achieving "edge of breakup" tones. Experiment with the bias point to find your optimum tone.

Things get especially interesting when a cathode follower is involved. You can tell if an amp has a cathode follower if the Preamp Comp parameter is not zero. The cathode follower interacts with the last stage and slight adjustments to the bias point can cause major changes in the distortion characteristics. For example, the Dizzy Blue models are biased near zero (0.08 IIRC). If you play lightly you'll hear the bass is kind of stuffy and tubby. Reduce the Preamp Bias a bit and you'll hear the bass clean up. Too negative, however, and the sound can get indistinct.

The good amp designers understand the interaction between the last stage and the cathode follower and tune the bias point for the desired distortion characteristics. The cathode follower is a bit of an imperfect design though. It's great for vintage Plexi and other high gain sounds but its clipping behavior is not ideally suited to certain tones. Therefore the Comp Type parameter allows you to choose an idealized cathode follower with different distortion characteristics (Comp Type == Ideal). Note that the behavior of this type is similar to the algorithms used in profiling modelers and other products. Try using the Ideal mode. You will likely need to reduce the amount of Preamp Comp as this mode has much more compression.

Even amps that rely mostly on power amp distortion can benefit from fine tweaks to the Preamp Bias point. Shifting the bias point changes the harmonics into the power amp which changes the distortion character of the power amp (albeit less significantly).
 

Hyper Planet

Power User
Thank you Cliff I love this section of forum I'm always learning from your notes, I experimented the Preamp Bias control with Negative feed back in Mark IIC+, this parameter can change the character of the tone a lot from smooth to Saturate tone, bud I found default value has more musically character and it's kinda like a sweet spot !. could you please explain a little bit about this matter ? please correct me if I'm wrong, now It's my main parameter for having a really smooth warm and liquidy one note guitar solos

Thank you very much Sir :)
 

GibsonLesPaul

Experienced
This is really something I always wanted to know about bias.

" Oh yes, this amp has new tubes and the bias is checked against them......."
It's a word many use when it comes about amp design, but never new the details behind it.

Thanks Cliff
 

dpeterson

Fractal Fanatic
This is really something I always wanted to know about bias.

" Oh yes, this amp has new tubes and the bias is checked against them......."
It's a word many use when it comes about amp design, but never new the details behind it.

Thanks Cliff
I think most people there are referring to power tube bias.
 

soheil

Power User
Thanks for the great tips Cliff. your posts are always informative and I learn from every thing that you post. Thanks a million.
 

axel

Power User
I have one of the old Tube Works "Real Tube" preamps that has "Contour" and "Bias" controls on the front, which I remember having a very noticeable affect on the distortion character. Makes me wonder why more amp builders didn't put these type of preamp controls on their amps.
 

MaVo

Member
I think the reasons are mainly warranty issues, because with e.g. higher bias, meaning higher idle current, the life span of a triode can be significantly reduced, if set too high.
 

Rotti

Fractal Fanatic
Jesus, you weren't kidding about ideal pre comp being compressed!

I dropped it to about 50% (# picked at random) and it opened up again. Is there a ballpark value or range of values you gravitate towards when using the ideal setting?
 

shatteredsquare

Power User
if i wanted to completely defeat the preamp of an amp block to get JUST the power amp, as if i was plugging into the FX return, what would i do to the preamp settings in this majik black box to effectively do that? I tried putting the tonestack type to neutral with drive at zero, everything else at noon. didn't work out. i don't see a 'preamp off' button anywhere.

I'm assuming because the preamp settings for the second amp block are still set as if to receive the preamp signal, not just a flat line level like the input stage of a 9200 or 2:90...? I HAVE NO IDEA WHAT I'M DOING

i'm trying to take a Recto 2 Red Modern amp, turn sag to zero, and then take the new JMP-1 amp block and defeat the preamp completely, so basically like i was running a Rectifier Recording Preamp into my Marshall 9200. :grin:
 

iaresee

Moderator
Moderator
if i wanted to completely defeat the preamp of an amp block to get JUST the power amp, as if i was plugging into the FX return, what would i do to the preamp settings in this majik black box to effectively do that? I tried putting the tonestack type to neutral with drive at zero, everything else at noon. didn't work out. i don't see a 'preamp off' button anywhere.
This cannot be done. You can run without power amp modeling, but not without preamp modeling.
 

shatteredsquare

Power User
This cannot be done. You can run without power amp modeling, but not without preamp modeling.
why in the world not? that would basically double the functionality of this crazy thing. i tried playing with all the preamp settings to try and get it to be neutral, but everything i try still kills the bass and throws phase out the window and still has a lot of noticeable gain. sounds like trent reznor.

that sucks because the recto model with sag at zero sounds and behaves EXACTLY like my recto recording preamp, i just want to put it in the VHT power amp Cliff used for the triaxis models...or the FX return of the Mark IV model for some 90 watt simul-class action.

i guess the closest approximation you could do would be to set sag at 0.01 and change the power tube type and bias? more bias to get it to Simul-Class A+A/B territory, EL34s for 9200 delinquency and a 90% variac?
 

MaVo

Member
I have a question regarding the relatively new Harmonics parameter. Does this parameter alter the slope of the static transfer function of the last Triode while the Preamp Bias value sets the operating point of this Tube stage?
 

pitta

Inspired
As explained in the OP, CF comp on the preamp page dials in the effect of a cathode follower. Why is it set to zero on a Concorde? Also, on the Rocket variants (both Wrecker and Ruby), this parameter is 0, even though on a typical rocket schematic, there is one... SLO, Marshall, Top Boost, they have that parameter set at a value > 0. Just curious...
 

Wolfenstein98k

Power User
I
As explained in the OP, CF comp on the preamp page dials in the effect of a cathode follower. Why is it set to zero on a Concorde? Also, on the Rocket variants (both Wrecker and Ruby), this parameter is 0, even though on a typical rocket schematic, there is one... SLO, Marshall, Top Boost, they have that parameter set at a value > 0. Just curious...
If Preamp Comp is zero, there isn't a cathode follower
 
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