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Post subject: Blues Junior valve heating problem
Posted: Tue Jul 20, 2010 8:28 am
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Hello!

I have a Blues Jr, that I've bought early this year, and I'm having a strange problem with the amp. What happens is that after I play for a while, one of the EL84 tubes gets too hot and the volume and quality of sound is degraded. If I just power off and power on again, it doesn't help, I have to switch the power tubes position and then the problem won't happen again, only after a few days. I took the amp to a friend who works with this and he tested the amp during almost 1h but the problem didn't happen (yeah, of course). So he tested the valves using a kind of machine where he can see if the valve is OK, and both were fine, according to him. So, I'm wondering whether replacing the valves would fix the issue or not.
Has anyone seen something like this? What else could I check?

Thanks!
Eduardo
Porto Alegre / Brazil


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Posted: Tue Jul 20, 2010 8:40 am
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Since I believe it is a fixed bias you would have to change both power tubes for a matched pair.

If it is biased too hot and cooking tubes, in the long run you may want to make your amp bias adjustable by adding trim pots. Or you can also replace a resistor on the output to keep the bias fixed but restrict the juice going to the power tubes.


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Post subject: Blue Jr
Posted: Tue Jul 20, 2010 9:28 am
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The Blues Junior amps are biased too hot from the factory, heard of this problem on some of these amps. In my opinion, the bias needs to be set closer to the tube specs for cooler operation. I have a trim pot ordered for mine so that I can set the bias correctly for each particular set of tubes.

I agree with the poster above, output tubes should be a matched pair. If one goes bad, change them both.


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Posted: Tue Jul 20, 2010 11:18 am
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escricco said : "So he tested the valves using a kind of machine where he can see if the valve is OK, and both were fine, according to him. "

Tubes tester can't mesure if tube is realy good . Working condition in guitar amp is not the same than in tube tester.
If tube tester said "good" this only mean that your tube is glow and emetting small current and is not short.
Tube need more of that to working properly in amp.

But in your case is improper bias ( too hot)is your problem, like atoletter and shimmilou said


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Posted: Thu Jul 22, 2010 5:24 am
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Thanks guys. I'll try this options.


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Posted: Thu Jul 22, 2010 5:54 am
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If you have a warranty on the amp, take it to a tech. Sounds like a bias problem. Let them handle it. If you mess with it or mod it, you'll void the warranty.

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Posted: Thu Aug 26, 2010 7:52 pm
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I have just purchased a new pair of JJ El84s and 3 EH 12ax7s to install in my new BJr. So i can just replace all 5 stock tubes and not be concerned about having to readjust bias in the new power tubes?

thanx for your time and responses, cheers.


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Posted: Thu Aug 26, 2010 8:19 pm
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I was taught to always check the bias with new power tubes. The gain stage, phase inverter, & effects stage tubes (usually 12AX7 or 12AT7 in Fender amps) do not need their bias to be checked.

As for the runaway EL84 of the OP, try cleaning the EL84 pins and sockets. Sometimes, if the pins are dirty or bent and/or the sockets are dirty you lose bias as the tubes warm up. I use electric contact cleaner (like CAIG D5 Deoxit).

Good luck with your new amp.


Last edited by BMW2002Ti on Fri Aug 27, 2010 1:04 pm, edited 1 time in total.

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Post subject: New tubes
Posted: Thu Aug 26, 2010 8:24 pm
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Hi canucklehead,

You should always check the bias when installing new output tubes. But if you are putting them in a stock BJr, the bias isn't adjustable. For the BJr you can install an adjustable bias, or change the bias supply resistor. I did, and I am very happy with the results. I am sure that it voided my warranty, but I do my own work anyway. ;)

http://www.fender.com/community/forums/viewtopic.php?t=42982

:D

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Posted: Thu Aug 26, 2010 8:57 pm
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Many thanx for the direction, gents. As i do not want to void my warranty, i will take amp and new tubes to a tech who can install and adjust bias.

Any recommendations for a 12" speaker in a single 12 external cabinet (closed back)?

cheers


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Posted: Fri Aug 27, 2010 2:58 am
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Yes a bias adjustment is a must whith new tubes.
IMO, I don't think a bias adjustment will void your waranty because it's a must for saving your new tubes first and have a better sound .

I just do this job ( bias) on a Blues Junior. My custumer had put a new set of EL84 without a bias check : Couple weeks after tubes are scrap.
He bring me the amp and I check bias ; was too hot , over tubes specs. I do a new bias with new tubes now the amp is better than a knew one.

Very often we should also replace de Phase Inverter tube too. 12AX7 just before the EL84


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Posted: Fri Aug 27, 2010 5:07 am
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canucklehead wrote:
Any recommendations for a 12" speaker in a single 12 external cabinet (closed back)?

cheers


Here's the one that I'm getting. I wanted an 8-ohm speaker that I can use with any of my amps. This one is made for the HRDlx, but will work for any amp. I will even use it with my C600.

http://www.guitarcenter.com/Fender-Hot-Rod-Deluxe-112-80W-1x12-Guitar-Extension-Cabinet-105168330-i1398875.gc
:D

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Post subject: Re: Blues Junior valve heating problem
Posted: Fri Dec 31, 2010 4:33 pm
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I have the sound degrading problem on my Hot Rod Deville. The amp tech who looked at it forst thought it was something to do with the relay sticking when the amp gets hot. He tested it for two hours and said it was ok but problem now seems to be reoccuring.


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Post subject: Re: Blues Junior valve heating problem
Posted: Fri Dec 31, 2010 5:24 pm
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Hi chrishulme,

Could you give more detail about your amp problem? Maybe you are talking about an intermittent problem that comes and goes. Is it channel switching on it's own, or is it something else? A common problem with some of these amps is poor solder joints, and unless your tech pulled the main circuit board out, he didn't give it a thorough inspection. When he referred to a relay "sticking", that sounds like a symptom that could indicate a poor connection when the amp heats up, which suggests bad solder joints. :idea: Typically, if a relay "sticks", it stays stuck. :lol:

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