Tube amp biasing-reference book?

[Tom Wolverton]

Can someone recommend a good book that explains how to adjust the bias on Fender tube amps?

[Ken Fox]

http://www.duncanamps.com/technical/lvbias.html

Good read at that website, Tom.

Tweed era amps had no bias control adjustment. Blackface amps had bias adjust, for the most part. The Princeton reverb for example did not have a bias adjustment. Most of the Silverface era amps had bias balance, not bias adjustment.

A good bias calculator here:

http://www.webervst.com/tubes1/calcbias.htm

The Weber uses 70 percent idle current, the max setting that should be used. I prefer 60 percent or less for extended tube life and a lot less strain on the output transformer primary windings.


I use a Bias King Pro (retails at $259.00) for my bias settings.

http://biasking.com/two_kings.htm

My backup is a Weber Bias Rite with dual heads, retailing at $140.00. I also use a sine wave gennerator and scope to check the amps for cross-over distortion (a result of cold biasing) or any other unusual output problems.

http://taweber.powweb.com/biasrite/br_page.htm

[Tom Wolverton]

Thanks Ken. I'll check these sources out.

[Larry Robinson]

Instead of buying a high price piece of equipment, I have used the following method to check bias setting for Fender Twin Reverb.
Buy precision one ohm one watt resistors and solder them between the cathode and ground in each of the 6l6GC tube circuits. The reading you get when measuring the voltage drop across the one ohm resistor will be in milli-volts which, when using Ohm's law, will equate to total current through the tube. I have found there is approximately 2ma through the screen resistor in TR amps. I also have found that setting the bias resistor for approximately 36-38 ma works well. However, there are a few out there that won't agree with my method, but it does work and it doesn't cost $250. Now if you are unfamiliar with soldering and electronic theory, then I don't recommend this method except to a qualified technician.

[Charlie Tryon]

Here is another great product. It show 2 current measuremets at once.
http://www.compu-bias.com/
Charlie

[Dave Grafe]

A note of caution before you start sticking your fingers in there or trying to solder anything to the tube sockets: The plate voltage stored in the capacitors does not go away even after being turned off for many months. If you fail to drain it and you touch one of those contacts the shock will send you across the room and leave a nasty taste in your mouth to boot. If you have a weak heart it can do much worse than that, so be certain that you act accordingly. Really!

[Tom Wolverton]

In the words of Tim Allen: "Son, you're grounded" : )

Yes, I suppose learning to drain caps would be the first lesson to learn before messing with any amps.

[Stephen Cowell]

Soldering resistors to the Fender chassis is not for the faint-of-heart... takes a large iron or 200w gun.

Also, since the bias will have to be adjusted with the resistors available, the case will be open... more opportunity for shock-therapy.

A bias probe is cheap insurance against this... although many Fender amps don't have true bias adjust, they just adjust one side ('balance'). Best is to have both bias and balance available... a modification.

'Biasing a Fender' is not as simple a proposition as it seems. Obtaining a book like Tom Mitchell's 'How To Service Your Own Tube Amp' will go a long way to getting you started... if you're the learning type.
The Hoffman Forum will be of help too.

http://www.el34world.com/forum/index.php