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Article: Tube Biasing - How it contributes to Amp Tone, and other considerations.

Copyright webmaster@Fratus-Amplification.com 7/2010

Output tube biasing effects two major things: Clean Headroom and Breakover.
These can be balanced out so that half of the usable volume level is clean and the upper half is compressed and breaks over easily.
What contributes to this is the type of circuit (Class A or AB), the B+ voltage, output transformer resistance.


A Little Theory...

Class A operation makes both halves of the push-pull output section draw current constantly. This reduces crossover distortion, increases linearity, and makes the tubes stay close to the point where hard playing runs them into the rounded-top distortion that makes that "tube-y" sound tubes are famous for.

Class AB, on the other hand, is a good compromise when done right. One tube is always drawing current, and sometimes both, but always in a small amount. AB is more like an electronic see-saw, where when one tube is up, the other is down. Careful adjustment of distance from the ground (bias) and position of the balance of the see-saw board (bias balance) can make for a wonderful result.

In the world of amp tone seekers, though, there is a big difference in the Class A and Class AB structure when it comes to tone.
The Class A amp has a fuller sound (my opinion) and breaks over sooner.
The Class AB amp is cleaner for a wider range and breaks over with difficulty. You can bias a Class AB "Hot" and get more of a Class A sound, but usually the supporting circuit isn't able to carry the extra load.


The next consideration is the tone stack. That is where many amps get their distinctive sound. On amps that do analog modeling, the tone stack is one of the sections of the amp they switch out.

For a comparison of the response curves of different tone stacks, Duncan Amps has a Tone Stack simulator the is really nice. I've used it to experiment with responses BEFORE I even build the circuit. Its accurate, and FREE!
See Tone Stack Calculator for the Win version. You can run it in Linux if you are using Wine.

There are lots of sites that list great discussions and tutorials about tone. One that comes to mind is the Tone Lizard site Tales from the Tone Lounge. Be aware, it is not for the faint of heart. If you can't bear to hear a few well-placed cuss words about the stuff some people put out as truth about tone, do not click on their link... but I really enjoy the way the author presents his case for BS standing in for truth.
Warning, some of the links have expired. In fact, the home page itself announces that this site may soon expire, but that announcement was in 2009! All the same, get there quick and get the knowledge.

R. G. Keen has been on my list of references for a while. His site at www.geofex.com is full of electronic stuff, all good, but I want to point to one section in particular, the Tube Amp FAQ page. Lots of learning experiences there.

More to come later...

Search the web for examples of the tone you want, then you have something to show someone when words fail. Send me the links to your clips at Design Your Next Amp on my site.
If nothing else, just leave the link in the link place, so I can get an idea of what people like.


Be safe. Observe rules about measuring high voltages, which can absolutely kill you or at least ruin your day.
Call me at my name and number, but after 9AM, please.


Video: Tone Capacitor Comparison
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Speakers and Power - techish article from Antique Electronics and tubesandmore.com!
Speakers and Power Explained
Which is the best order to hook up your pedals? See Guitar Effects - Order & Tips
Good Video... No narration - watch the screen for tips.

Picture of Modified Fab Echo with knob coming out the sideThe Modified Danelectro D-4 Fab Echo
Sounds a lot like an old-style tape echo - adjustable delay time
Controls -
Mix Knob: Stock is kinda weak, Modified to give much more effect to mix.
Repeats Knob: Stock repeats is about 6 max. Modified to have no limit, can feed back echos like a tape echo!
Time Knob: Part of Mod, shortest time is stock, just a slap-back effect. Max delay time is about 4 seconds.
$85 plus shipping. Battery included!
Click on image to hear a sound sample.

I have to point you to Michael Hoffman, BSEE, at Amptone.com. I saw his site years ago but couldn't find it again until recently. His site may have been my inspiration for starting this one. Be sure and give him a visit. His site is really complete and full of good info, suggestions and tips.
Commercial time...
Vintage Tube Amp Repair! Experienced vintage amp repair. www.fratus-amplification.com


my name and number.
Servicing:
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Would like to see:
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Books:

You can go crazy with books, just like anything else. The thing that I have learned is that first you need to understand electronics, that is, learn how electrons behave and how to tame them. Then you have to learn the skills and practices that make it safe and effective to work on stuff in general. Then you need to learn about the particular type of product that you are interested in.
ePanamorama.net Vast array of references, resources, nearly everything! Electronics Hobbyist Web Site GREAT Links! Don't pass this one by! Electronic references, circuits, data, repair info.

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