Hi All,
I've been reading the musictoyz forums ALOT and have learned a ton. I have a question maybe someone can help me with...

I just picked up a Fender 600 Champ reissue. I really like it. Small, light, great for a cafe gig and practicing at home.

It's 5 watts with 1 12ax7 pre and 1 6v6 power tube. I've been told I can switch out preamp tubes without adjusting the bias on an amp. Can I put another power amp tube into it without having to adjust the bias? I don't know if you CAN adjust the bias on the amp. I'd have to get a tech to do that...

Not sure if the bias thing is for amps with two power tubes...

Thanks for any answers!!!


i'm also going to replace the speaker with a weber.

To the best of my knowledge... If you are replacing it with the same type of tube (6v6 in your case), I believe it does NOT need to be rebiased. Rebiasing is just for older amps that may have lost their biasing over the years or if the amp was never bias correctly in the first place. Don't quote me on this however, this is just to my knowledge.

Thanks for the info.

biasing must be done in any tube amp ( new or old) when changing tubes, to get the best proformance ...unless they are of a fixed bias design...some ( all? check this one...) class a/cathode bias amps do not need biasing...as to your champ...I don't know...they may be a fixed/cathode bias, I believe the old ones were...

Champs are cathode biased, which means you do not need to rebias the amp ever. A resistor within the amp sets the bias.

You should not have to re-bias the amp. There are some newer tubes (EL84, EL34, and maybe some 6L6), that differ from the original specs, and run too hot on cathode biased amps, but I've never seen a 6V6 with that issue yet.

Tweedledee:
Just to make things a little clearer you do not have to rebias your amp as long as you replace the power tube with another 6v6. If you wish to go to, say, a 6L6 then you you definitely need to change the cathode resistor and make sure your power tranny can supply the necessary current. This is beyond your current skill level or you would not be asking. There are sites on the web (i.e. sewatt.com and others) that can help you safely learn this type of stuff.

With single ended amps the bias is preset or "fixed" via a resistor, in the case of a champ its will be a 470ohm resistor. This will accommodate any 6v6 tube. If you want to experiment with other tubes such as a 6l6, KT66 or other octals you will need to change the bias resistor. Or even better install a trim pot to set the bias exact but this would also involve the use of a multimeter and is not worth the effort unless you plan on changing the type of tube. All a bias pot does is vary the resistance present to the bias pt of the tube. And this determines current flow or draw of the power tube, which ultimately determines breakup characteristics. If you guys have any other amp questions I would be happy to address them.

Thanks alot everyone!