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Post subject: SCXD tube change rebias?
Posted: Mon Mar 02, 2009 8:42 pm
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Hi,
Sorry if this has been asked before. I searched but could not find out if one must rebias a Fender Super Champ XD after a 6V6 tube change. I am looking into putting in a pair of JJ 6V6s'. Anyone?

Kind thanks,
Will


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Posted: Tue Mar 03, 2009 11:30 am
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You should definitely rebias your amp when changing output tubes. It's easy to do on a SCXD, but if you don't have any experience working on high voltage electronics, take it to a tech. It shouldn't cost much to have it done.
Otherwise, if you want to do it yourself, you need a basic multimeter and here's how it's done:

Remove chassis. Set chassis on something so tubes are clear of bench. Plug speaker in. Use clip or jumper to ground negative probe of multimeter. Turn on amp and check reading at pin 8 of power tubes. Pin 1 is marked. Pin 8 is just to the left with chassis up-side-down. Keep one hand completely away from chassis, that's why you want to connect the negative probe to ground. The bias pot is the large blue knob on the circuit board. You want a reading of around 40 mV. Adjust to desired setting. Total time to pull chassis, set bias and reinstall takes about 10 minutes.


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Posted: Tue Mar 03, 2009 4:31 pm
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gtrman100 wrote:
You should definitely rebias your amp when changing output tubes. It's easy to do on a SCXD, but if you don't have any experience working on high voltage electronics, take it to a tech. It shouldn't cost much to have it done.
Otherwise, if you want to do it yourself, you need a basic multimeter and here's how it's done:

Remove chassis. Set chassis on something so tubes are clear of bench. Plug speaker in. Use clip or jumper to ground negative probe of multimeter. Turn on amp and check reading at pin 8 of power tubes. Pin 1 is marked. Pin 8 is just to the left with chassis up-side-down. Keep one hand completely away from chassis, that's why you want to connect the negative probe to ground. The bias pot is the large blue knob on the circuit board. You want a reading of around 40 mV. Adjust to desired setting. Total time to pull chassis, set bias and reinstall takes about 10 minutes.


Thanks guitar man. That is very helpful info. I think I am a little intimidated by this. lol. Any idea how much it should cost to have a local amp tech person do this?
Cheers,
Will


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Posted: Tue Mar 03, 2009 5:05 pm
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Since I do it myself I'm not sure how much it would cost, but I imagine it would be the minimum bench charge-$25-40. If you're not familiar with high voltage electronics, DON'T MESS WITH IT! There are voltages that can kill you. Take it to a tech.


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Posted: Fri Apr 10, 2009 4:20 pm
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I just took mine to my Tube guru yesterday in order to change the 6V6.
Yes he will rebias it but only because I asked to do so.
He's been clear about it : with a pair of 6V6 in this kind of circuitery, you can drop your 6v6 in it and rock!! No bias at all.
That's what's Fender tell us anyway. Why shouldn't we trust them ?


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Posted: Fri Apr 10, 2009 7:59 pm
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You might not need to re bias your amp if you replace it with the same color code Groove Tubes, but using a different brand you definitely should re bias. I'd re bias even if I was using the same brand of tube, because I like to keep my amps running as good as possible.


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Posted: Sat Apr 11, 2009 1:48 am
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My tech is an 60 years old man who learned on tubes and works on tubes.
He does only that.
He made my 72 super reverb alive again, after a few "tech" had only messed with it.
I trust him.
So again, we insist saying that such a man is wrong and that Fender is wrong also saying that you don't have to bias ??


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Posted: Sat Apr 11, 2009 8:26 am
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Avirex wrote:
My tech is an 60 years old man who learned on tubes and works on tubes.
He does only that.
He made my 72 super reverb alive again, after a few "tech" had only messed with it.
I trust him.
So again, we insist saying that such a man is wrong and that Fender is wrong also saying that you don't have to bias ??

First, I don't know where Fender says you're not supposed to re bias an amp when you change tubes. They do say you don't have to if you replace them with the same color Groove Tubes, but a SCXD dpesn't come with Groove Tubes. Also, since it's so easy, why not do it anyway? Since you've replaced the original EH tubes with JJ's, they are most likely not the same spec, and should be checked.
If your tech doesn't understand that, then you've been lucky with how he's performed repairs for you in the past. If you don't believe me, call Fender and ask them if you should re bias an amp when changing tube brands.

Anyway, that's my $.02, good luck with your amp.


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Posted: Sat Apr 11, 2009 9:24 am
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As you can see, somebody call them before me:
http://www.fender.com/community/forums/viewtopic.php?t=7134&highlight=bias

But you are free to think what you want about that.:P :P
I've always made my HRdx and '72 super reverb rebias when changing power tubes, I asked to do so for my SCXD too ( I'm very old fashioned somewhere, and don't like risk anyway)...

Now I suppose that guys at Fender, and my tech arn't totaly irresponsable.
If they say that, there must be a reason somewhere :P :P

I'm not saying that you don't have to rebias, I report what specialists tell about it.


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Posted: Sat Apr 11, 2009 10:06 am
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Avirex wrote:
As you can see, somebody call them before me:
http://www.fender.com/community/forums/viewtopic.php?t=7134&highlight=bias

But you are free to think what you want about that.:P :P
I've always made my HRdx and '72 super reverb rebias when changing power tubes, I asked to do so for my SCXD too ( I'm very old fashioned somewhere, and don't like risk anyway)...

Now I suppose that guys at Fender, and my tech arn't totaly irresponsable.
If they say that, there must be a reason somewhere :P :P

I'm not saying that you don't have to rebias, I report what specialists tell about it.

If you read the thread, it's one persons version of what a Fender tech told him, not a Fender tech saying it. Here's the quote from Shane Nicolas, the Fender marketing guy:
"While we realize there are many enthusiasts on the Forum who do their own repairs and modifications, we must state "no user servicable parts inside." A situation that requires getting inside your amp (high voltage!) is best left to a qualified technician; if it's a Warranty situation, one of our authorized service centers. "
In other words he's saying, take it to a 'qualified' technician, don't do it yourself. It's understandabe, you can get electrocuted if you don't know what you're doing.
But he never said that a SCXD shouldn't be re biased when changing tubes. As a matter of fact, Billm, a very well qualified Fender tech, says the opposite in the last post of the thread. Anyway, I'm done with this dicussion, good luck again.


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Posted: Wed Dec 09, 2009 1:54 pm
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gtrman100 wrote:
You should definitely rebias your amp when changing output tubes. It's easy to do on a SCXD, but if you don't have any experience working on high voltage electronics, take it to a tech. It shouldn't cost much to have it done.
Otherwise, if you want to do it yourself, you need a basic multimeter and here's how it's done:

Remove chassis. Set chassis on something so tubes are clear of bench. Plug speaker in. Use clip or jumper to ground negative probe of multimeter. Turn on amp and check reading at pin 8 of power tubes. Pin 1 is marked. Pin 8 is just to the left with chassis up-side-down. Keep one hand completely away from chassis, that's why you want to connect the negative probe to ground. The bias pot is the large blue knob on the circuit board. You want a reading of around 40 mV. Adjust to desired setting. Total time to pull chassis, set bias and reinstall takes about 10 minutes.


I just got the Super Champ XD and was wondering about this. How does one remove the chasis? Do you remove the screws on top of the amp? Would you need to take the knobs off the front of the amp?


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Posted: Wed Dec 09, 2009 2:42 pm
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If they don't come with GT's then what tubes come in them?

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Posted: Wed Dec 09, 2009 3:07 pm
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Celtic Cyclonus wrote:
If they don't come with GT's then what tubes come in them?

CC


Not sure, I think I read something about EHX tubes before.....


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Posted: Wed Dec 09, 2009 3:12 pm
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I just got the Super Champ XD and was wondering about this. How does one remove the chasis? Do you remove the screws on top of the amp? Would you need to take the knobs off the front of the amp?[/quote]

It's been said a million times. If you're not sure of what you're doing, take it to a tech. I pulled my first amp apart when I was 12 or 13 and figured it out for myself. Put it back together properly too. I also got into radios. There were lots of tube driven radios around when I was a kid and every drugstore in my neighborhood had a tube tester in it. I come from a very mechanically inclined family. I also work on my own cars too, timing belts, brakes yadda, yadda, yadda.

Not to sound like an A Hole, If you can't get that far yourself, take it to a tech and see if he'll let you watch him. You'll learn that way and not get yourself electrocuted. I worry about tube newbies wandering around in unfamiliar territory.

As for biasing, yes if you care about your tone and the lifespan of your power tubes, check the bias. Believe it or not, lots of techs are not musicians. They are electronic technicians. You tell them what you want, and hopefully they can do it for you. The more your stuff breaks, the more money they make. I wouldn't trust the color bands on GT's grading system anyway. It's just more BS and overpriced hype :wink:

www.eurotubes.com has instructional videos on how to bias and other information. The thing that worries me is Bob at Eurotubes biases an amp while wearing jewelry. NEVER wear rings or other jewelry while working in high voltage situations. :wink:


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Posted: Wed Dec 09, 2009 3:31 pm
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Good advice Supro... if you don't feel comfortable opening up an amp- don't do it. I agree with everything Supro said, BUT, I've checked the bias on several GT tubes of the same color code, and they're close enough for the average home user. If you're a working musician or a person that really cares about their equipment, I'd definitely get my amp biased when changing output tubes. YMMV


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