Fratus Amplification repairs tube amps and other audio gear.


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A NEW edition of my book has just been released!

The perfect gift for that tech-head musician or technician that loves to work on guitar amps.
My New Book!
Click for preview of inside.
This new edition has been updated with the latest problems and now includes the parts used in the repair!
New edition of my book, "Guitar Amps and Their Problems, 2.0" - Updated and Improved!
With over 2500 entries of service records for many popular (and rare) makes and models of (mostly) tube guitar amps and other gear!

Some interesting videos about tubes and tube amps

Mullard - The Blackburn Vacuum Tubes Factory

Tesla Tube Factory Visit in Czech Republic 1998

How It's Made - McIntosh Tube Amp. Published on Jun 9, 2014

Winding the output transformer for vaccum tube audio. Published on Mar 17, 2012

Unique Marshall Factory Tour as you have NEVER seen it before

Amp Tone and Mods

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

Copyright Mike Fratus, 9/2014

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 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.

This Month's Tube Amp Video: Vacuum tube amp theory - This vintage film is from the US Army from when tubes were "the thing."

Can't see it? Here's the Youtube link.
(Have suggestions of videos to put here? Send me a message!)

Solid-State Tubes are here! See My Products Page for details. Get the tube sound without the tube problems!
See how much a new amp might cost with Amp Feature Selector. List the features you want, such as watts, channels, options, and get an estimated price immediately! I can then send you a detailed description and an invoice for a down payment to start work on your amp.
Need parts? I have a parts request page for you. Use my new Amp Parts link.
Hints and links concerning the sound of your amp at
Build it yourself DIY information and links at


Electricity and Electronics

Collection of the Best DIY Maker Projects | DIY Hacking Open-source information for Arduino and Raspberry Pi projects.
Electronics Hobbyist Web Site GREAT Links! Don't pass this one by! It has electronic references, circuits, data, and repair info. has some great general electronics tutorials.
Making Electric Circuits and Other Electronics Projects: Lesson Plans
The ARRL Handbook has set the standard for Ham Radio enthusiasts and communication engineers through the years. You learn enough about electronics and radio from one of these handbooks to build radio receivers, transmitters, amplifiers, and to get your FCC license! Great tube information! This is the 1959 edition. Caution: 59MB - Large PDF File!
How to repair circuit board problems, by billm. I use these same methods as well. Good guidance.
THE ELECTRON AGE - 100 years of progress in electronics - by Carlos Fazano In case you missed the last 100 years. In English and Portuguese. Excellent! - This portal is dedicated to Students, Amateur and Professionals in the electronics field.
I have to point you to Michael Hoffman, BSEE, at 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.


Vacuum Tube Knowledge

Tube Terminology for Dummies is the name, but it really should be your first reading about tubes, especially if you are trying to fast-track the technical stuff to do something specific.
J.B. Calvert's wonderful and scholarly reference on Vacuum Tubes. Lots of theory and practice on amplifiers, both solid-state and tube-based.
Fun With Tubes! Great site! This non-commercial site is dedicated to passing on and preserving knowledge of vacuum tube circuits and electronic techniques.
Tube Amp Debug Page by R.G. Keen. This page is designed to lead you through debugging a problem with a tube-type guitar amp. Mr. Keen knows his stuff. You can learn a lot from him.
Bonavolta's Tubes for Newbies Tutorial should not be skipped. Lots of good drawings and simple illustrations.
Amplified Parts presents 12ax7's - a comparison of current-production tubes.
The Tone Lizard Lounge site unfortunately is no longer reachable. I even wrote and asked him if I could mirror his pages, and he said the site was not down... well, it is. If I ever get permission to post his pages, I will. Lots of great stuff there. Excellent for theory and practical information. Visit all his pages. Caution: Strong Opinions (usually right!)
TubeCAD Journal is an offshoot of Glass Audio, and is associated with the TubeCAD series of software packages. Loads of articles about tube amp design if you are interested.


Vacuum Tube Amp Building and Repairing in Particular - Building your own Tube Guitar Amp - Troubleshooting Tube Guitar Amps Amp Stuff presents DIY Amp Kits
DIY Guitarist - Amp Stuff


Tinkering Guide ...STEM, Tinkering and the Maker Movement are quickly rising to the top of activities which involve learning by doing.
Makezine... the major magazine of the Maker Movement
Little Bins For Little Hands STEM activities for kids.
Hackaday, site and blog about what other people are doing with stuff that is surprising!, the DIY headquarters!


Stomp Box Electronics

General Guitar Gadgets sells lots of guitar effects kits and has information.
Sparkfun Proto Guitar Pedal provides a board with the essentials that lets you build the working part of the pedal without worrying about the details of interfacing to the guitar and power supply.
Build Your Own Clone has kits and parts for work-alike versions of famous pedals.
Mammoth Electronics, one of my favorites for parts and kits!
SmallBear Electronics/, a surprisingly well-stocked resource for pedals and builders.


Tube Amp Output Biasing

Off-page video...
Mojotone's "How to bias a tube amp". Good, solid instructions!


Guitar Amp History and Evolution

This extremely informative article by Keenan is about how amps have developed over the years.
Here is an excellent article on how the design of guitar amps came to be where it is today: Guitar Amp Tone History The Amp Guide - Historical Guitar Amplifier Information

Guitar Technology

How to test a guitar

Speaker Technology

Speakers and Power - techy article from Antique Electronics and!
Speakers and Power Explained
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Science and Electronics Vast array of references, resources, nearly everything!



Tucows shareware, freeware, free programs


General Knowledge

ResearchBuzz - The Ins and Outs of Searching. Learn to search like a pro!
SoYouWanna - tells how things are done. Simple to complex explanations.
Reference Desk - whatever you want to know: Dictionaries, encyclopedias, etc.
BabelFish - translate to/from any language! (now part of Google Translate)
Internet Archive has lots of media files, movies, art, books, and the famous Wayback Machine!
Make your own paper! Instructions from the State of Wisconsin Department of Natural Resources, Make your own Paper! EEK!
How to Make Your Own Recycled Paper - Video on Youtube Make Paper!


Listen to my friend's Industrial Shred style. Visit his website at
My friend Dennis Odocharty on lead guitar with videos of the Houston band Lady and the Tramps.
Videos of some Local Bands


Other Artists and Craftspeople of Interest


Web-Only Stuff



Amp Schematics

All Brands

Prowess Amplifiers has a large collection of guitar amp schematics. - Tube Amp Schematics Parts Lists and Schematics for Korg, Marshall and VOX - Schematics
TDSL Schematic Links
The Free Information Society - Audio Electronic Circuit Schematics
Tube Data Schematics & Links Site. It's mostly about tube amplifiers & particularly guitar amps with a little hi-fi stuff thrown in.
Schematic Heaven Where All Good Amp & Effect Designs Await Resurrection...

Single Other Brands

The Carvin Museum - Technical Documents

Fender - Fender Schematics

Marshall - Amp Info and Schematics


Click on "Schematics" on left for Mesa Boogie Schematics



Traynor Amplifier Schematic Archive

Professional Audio - Free Pro Audio Schematics and Manuals - Pro Audio Equipment Schematics - Pro Audio Equipment

Tube Amps (but not particularly guitar amps)

AmpsLab Vintage Tube Amplifier Schematics -

Electronic Product Schematics - all sorts of products - Manual free download

How to Repair Electronic Equipment and Products

RepairFAQS How to Troubleshoot from Sam Goldwasser

Guitars - Guitar_Schematics

Pedals and Boxes and Other Music-related Electronics Designs - Audio - Selected Pedal Schematics
General Guitar Gadgets - Stompbox Kits
DiscoFreq's Effects Database Site
Electroconducive DIY Audio - Schematics
Vintage German modules (Neumann, Siemens, TAB, IRT, Lawo, ANT, Eckmiller, Adis, Monitora and Telefunken modules and Braunbuch scans)

Other Electronic Circuits - Electronic circuits - Electronic Circuits - Various Schematics and Circuits
Technician's Filing Cabinet from -- fx projects
Analogguru Schematics


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Call Mike at my name and number, but please don't call until after 9AM.

See this great tone capacitor video!
If you ever believed that the type of cap made no difference, watch this! Need parts? I'm trying to make an outlet for parts. Don't call for parts, please. Use my new Amp Parts link.


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.
Alibris is a great place to buy books at severely reduced prices. 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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