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Post subject: Super Champ XD tube change: will I have to do bias adjust?
Posted: Tue Dec 20, 2011 6:42 pm
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I picked up a SCXD tonight from Guitar Center-Mart; I discovered the amp pretty much covers all the bases I need to cover, and for only two bills. Haven't had too much time to try it out but I'm really digging what I've heard so far.

I'd like to go ahead and replace the tubes, hence my stupid question. I was told this is an adjustable bias amp. Does this mean it'll have to be rebiased when I replace them, or will I be able to plug in a matched set of power tubes without any tweaking?

Thanks in advance!

Steven


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Post subject: Re: Super Champ XD tube change: will I have to do bias adjus
Posted: Tue Dec 20, 2011 8:40 pm
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The only tubes you have to worry about biasing are the power tubes.

Yes, your amp has adjustable bias. See the attached schematic and chassis layout. Sorry, I cannot read the "R" number of the pot.

http://support.fender.com/schematics/gu ... ematic.pdf

Yes, you should always check and adjust the bias if necessary when you replace tubes. You need to measure the plate voltage also to get a meaningfull number. Do a search on biasing in the forums. There are a lot of threads about how to do it properly.

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Post subject: Re: Super Champ XD tube change: will I have to do bias adjus
Posted: Wed Dec 21, 2011 3:25 am
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I guess I should have been clearer in my original message. What I was curious about is do I NEED to bias them? Will the amp throw up on me if I install new tubes or can I simply plug and play and continue to pretend I'm Hendrix?

I did a lot of digging last night and found conflicting reports. A lot of folks say you "should" bias, but I saw some other stuff that came from a Fender rep and it said that you can drop a matched set in with no problems- just plug and play- and you should not adjust anything, as this voids your warranty. I am not a big fan of monkeying around inside amps that can give a nasty bite, I don't want to give a tech a hundred bucks to tweak an amp I just spent $200 on, nor do I want to void my warranty.

What to do, what to do?


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Post subject: Re: Super Champ XD tube change: will I have to do bias adjus
Posted: Wed Dec 21, 2011 6:46 am
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SMW17 wrote:
I guess I should have been clearer in my original message. What I was curious about is do I NEED to bias them? Will the amp throw up on me if I install new tubes or can I simply plug and play and continue to pretend I'm Hendrix?

I did a lot of digging last night and found conflicting reports. A lot of folks say you "should" bias, but I saw some other stuff that came from a Fender rep and it said that you can drop a matched set in with no problems- just plug and play- and you should not adjust anything, as this voids your warranty. I am not a big fan of monkeying around inside amps that can give a nasty bite, I don't want to give a tech a hundred bucks to tweak an amp I just spent $200 on, nor do I want to void my warranty.

What to do, what to do?


And I don't know how to be any clearer in my response. You don't "need" to do anything as long as you are willing to take the chance that the bias was adjusted properly in the first place and the parameters of the replacement tubes exactly match the old ones. And no, adjusting the bias will NOT void the warranty. Whoever told you that was either a liar or just plain ignorant. There is no question what I would do and do in fact do whenever I change power tubes.

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Post subject: Re: Super Champ XD tube change: will I have to do bias adjus
Posted: Wed Dec 21, 2011 7:16 am
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+1 bluesky636

SMW17,

If this is true, it begs the question, why change something that sounds great already? "Haven't had too much time to try it out but I'm really digging what I've heard so far." So just leave it alone. You don't want to mess with high voltage, and you don't want to pay a tech, there is no other option so just play the damned thing and stop worrying about it. You have no problem, yet want a solution that doesn't exist. :?

I reiterate, if you change output tubes, you should check the bias. If you know that the bias is correct to begin with (which you have no idea), and if you get the exact same tubes as you have now (which is pointless since they are new), then you can go without checking the bias. For all other output tube changes, it would be foolish not to check the bias. Some people don't care and just slap in new tubes, and in that case some melt, some don't. A few amps are plug and play as far as output tubes (like the C600), and some amps have no adjustment (like the BJr) so setting the bias is more difficult, but checking the bias is like checking the oil in your car.....you don't have to, but wise to do so, and checking the bias never causes a problem. Not checking however, can lead to problems. In the case of the BJr, many change output tubes and don't check the bias, and most of the time it's OK....for that particular amp. Since there is no adjustment for the bias in the BJr, it isn't made for users to adjust, and for the most part it's virtually plug and play, but it is best to always check the bias and adjust if necessary.

Edit:
What the Fender rep may have been talking about was another type of amp with no bias adjust, and in that case if you change a resistor to adjust bias, the warranty will be void. And in some amps without the adjustment, they might recommend just changing the tubes without worrying about adjustment, but is for certain amps, likely not referring to any amp with the adjustment (like yours has). If the amp has a bias adjust, it will not void the warranty to adjust it correctly, that is what it was put there for. :idea:

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