Mod help: aa764 champ. converting to fixed bias.

[Jimmybjj]

I search the forums and can't find any material stating how this is done. the only things I can find are adding a switch for both types of bias. is it as simple as a trim pot and a resistor, if so what values are suggested? what about a supply voltage? any help would be greatly appreciated. I have a basic undertanding of electronics, I'm learning everyday. this was my first build. links to anything that would hop me in the biasing adventure would be great to, thanks.

[Enzo]

Bias refers to the voltage difference between the cathode and the grid of a tube. It really doesn;t matter if the grid is at zero volts and the cathode at +20v, or if the cathode is at zero and the grid is at -20v. Either way, that grid is 20v more negative than the cathode.

The Champ uses cathode bias, which uses the current through the tube and its cathode resistor to cause a positive voltage at the cathode. The grid is grounded through the 220k resistor

To make fixed bias, you would then ground the cathode, and connect that 220k grid resistor to a negative power source of about 20v.

Grounding the cathode is simple, just wire it. Making the negative bias is your challenge. There is no negative supply in this amp, you will have to invent one. You can tap off the high voltage winding to make one, or even install an extra small transformer. However you do that, you need to make something around -20vDC for your bias supply. SInce you are going that far, a few simple changes would make that voltage supply adjustable.

[Jimmybjj]

Bias refers to the voltage difference between the cathode and the grid of a tube. It really doesn;t matter if the grid is at zero volts and the cathode at +20v, or if the cathode is at zero and the grid is at -20v. Either way, that grid is 20v more negative than the cathode.

The Champ uses cathode bias, which uses the current through the tube and its cathode resistor to cause a positive voltage at the cathode. The grid is grounded through the 220k resistor

To make fixed bias, you would then ground the cathode, and connect that 220k grid resistor to a negative power source of about 20v.

Grounding the cathode is simple, just wire it. Making the negative bias is your challenge. There is no negative supply in this amp, you will have to invent one. You can tap off the high voltage winding to make one, or even install an extra small transformer. However you do that, you need to make something around -20vDC for your bias supply. SInce you are going that far, a few simple changes would make that voltage supply adjustable.

Thanks for the reply. Very educational and most importantly understandable I understand what your saying about tapping for the high voltage winding, but I have no idea of how to implement what your saying. How do I convert (change) the high voltage to -20v ?

Your last sentence, what changes are you referring to?

Thanks so much for taking the time to help me.

[Enzo]

Let us assume you already have a basic negative voltage supply. From that, the actual bias voltage would be taken from a voltage divider. Fancy term for a couple resistors. The resistors would be set in the amp design by the factory, Fender for example. A voltage divider works on ratios between two resistors, so by replacing one of the resistors with a variable resistor, we can make the voltage variable.

Pick any 10 amps you care to that might get converted to adjustable bias, and read the changes. You will see they all are done the same way. Might be 25k in one amp and 50k in another and 15k in a third, but the overall approach will be the same. Like a 12-bar blues progression. Doesnlt matter what key it is in, it still works the same way.

Do some research into bias supplies. A number of Marshall models tap off the high voltage winding for their bias needs. Find a circuit and copy it. The Marshall might be wanting -50v instead of -20v, but the difference is just a few resistor values.

[Jimmybjj]

again, thanks for all of your help. i keep reading your post over and over and pick somehing up new everytime I read it. I'm pretty confident of my understanding of what I have read. what I don't know how to do, is place in the need values to make the circuit. this is what I think I shoud emulate.

http://i996.photobucket.com/albums/a...BiasSupply.jpg

if it provides any help this is the transformer that I'm using.

Dual Primary 120/240v Champ Princeton Power Transformer

Let us assume you already have a basic negative voltage supply. From that, the actual bias voltage would be taken from a voltage divider. Fancy term for a couple resistors. The resistors would be set in the amp design by the factory, Fender for example. A voltage divider works on ratios between two resistors, so by replacing one of the resistors with a variable resistor, we can make the voltage variable.

Pick any 10 amps you care to that might get converted to adjustable bias, and read the changes. You will see they all are done the same way. Might be 25k in one amp and 50k in another and 15k in a third, but the overall approach will be the same. Like a 12-bar blues progression. Doesnlt matter what key it is in, it still works the same way.

Do some research into bias supplies. A number of Marshall models tap off the high voltage winding for their bias needs. Find a circuit and copy it. The Marshall might be wanting -50v instead of -20v, but the difference is just a few resistor values.

[tubeman]

For what it is worth, I played with a fixed bias champ a couple of years ago... It did not make a big differance compared to a cathode bias version. See pic.

[Jimmybjj]

For what it is worth, I played with a fixed bias champ a couple of years ago... It did not make a big differance compared to a cathode bias version. See pic.

thanks for the reply. if you don't me asking what values did use? if you have time can you give a idea of how you came up with the values or point me to a link. I have looked a lot and can't find anything.

[tubeman]

I hate to say it but I used the version Gerald Weber drew up in his book "Tube Guitar Amplifiers Essentials" I scanned in the drawing. Have fun.

[Jimmybjj]

I hate to say it but I used the version Gerald Weber drew up in his book "Tube Guitar Amplifiers Essentials" I scanned in the drawing. Have fun.

Thanks, that was very cool of you to scan that for me. I know what my next book purchase is :-)