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Post subject: changing tubes
Posted: Mon Aug 30, 2010 2:48 pm
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i am wishing to change the tubes in my Blues Jr. Other than unplugging the amp, letting the tubes cool before removing, is there anything else i need to be concerned about when swapping tubes?

thanx


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Posted: Mon Aug 30, 2010 2:54 pm
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Make sure your pins are correctly aligned.

The sub-miniatures (12AX7, 7025, EL84, etc) can be a beetch sometimes.

Arjay

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Posted: Mon Aug 30, 2010 4:02 pm
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Thanx for that, Arjay. Good point about aligning the pins. Is there an easier way to remove the tubes than reaching in and wiggling loose the tubes? Can the chamber that holds the 5 tubes be lowered, and if so, do i do this by removing the four screws?

thanx for your assistance.


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Posted: Mon Aug 30, 2010 6:14 pm
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Yes, dropping the "cage" will definitely make tube changing easier. In fact I'd remove it -- PERMANENTLY -- and hook-shot it into the closest dumpster. Unless your local neighborhood representative from the "Ministry of Amplifier Idiot-Proofing" objects......

:mrgreen:

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Posted: Mon Aug 30, 2010 8:34 pm
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canucklehead wrote:
Thanx for that, Arjay. Good point about aligning the pins. Is there an easier way to remove the tubes than reaching in and wiggling loose the tubes? Can the chamber that holds the 5 tubes be lowered, and if so, do i do this by removing the four screws?

thanx for your assistance.


Don't remove the "four screws", that holds the chassis in place! Sounds like you are talking about the chassis when you referred to the "chamber that holds the 5 tubes". If you want easier access to the tubes, remove the "six screws" that hold the back cover on, and remove the cover. If you are changing output tubes, it is easier to change one output tube at a time with one hand, while holding the metal bracket down and to one side with the other hand. That way the springs won't come loose. :)

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Posted: Mon Aug 30, 2010 8:36 pm
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LoL, Arjay. Final thing - one does not have to worry about the potential forshocks or other electrifying surprises if the amp is unplugged?


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Posted: Mon Aug 30, 2010 8:41 pm
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Excellent advise, shimmilou. Thank you very much.


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Posted: Tue Aug 31, 2010 3:37 am
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wrong item sorry for disturbances

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Last edited by Tissan on Tue Aug 31, 2010 7:56 pm, edited 1 time in total.

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Post subject: Re: changing tubes
Posted: Tue Aug 31, 2010 3:51 pm
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canucklehead wrote:
i am wishing to change the tubes in my Blues Jr. Other than unplugging the amp, letting the tubes cool before removing, is there anything else i need to be concerned about when swapping tubes?

thanx


When I took delivery of my NOS BJ, the first thing I did, literally out of the box, was swap out the stock tubes for a set of JJ's from Eurotubes. When I change out my tubes I take the back cover off for ease of access.

The main thing that I have read that is a "must" when changing tubes, is to clean the sockets with contact cleaner. The method I have seen most reported is to apply the contact cleaner to the new tubes and work it in my inserting and removing the tubes a couple of times. I was not comfortable doing this with new tubes, so I cleaned the original tube's pins with contact cleaner and then used them to "clean" the sockets by inserting and removing them a few times. I thin applied contact cleaner to the new tubes pins and inserted them.

I don't think this was necesary in my case, because I never even turned on the amp with the orginal tubes and played it. I basically swapped the still new GT's for JJ's and have been off in tone heaven since.

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Posted: Tue Aug 31, 2010 4:06 pm
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Whith new EL 84 you must check and adjust bias.

You need a tech for that job.

A correctly biased amp will run efficiently and maximizes the life of your tubes.
UNDER-BIASED :amp will lack of punch and the tubes will run noticibly hotter. .
OVER-BIASED : amp will sound thin and brittle, with the tubes running too cool for proper performance.
A CORRECTLY-BIASED amp will sound clean and thight at moderate volumes,than at higher volumes break up and distort musicaly


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Posted: Tue Aug 31, 2010 7:41 pm
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Thanx for the advice, markara. What would you suggest as a quality "contact cleaner" - rubbing alcohol?

My new GT tubes (white labels) are only about 4-6 weeks old, so i doubt there would be much dirt/residue left in the sockets when installing the new JJ El84s (pair of #38s) and EH 12AX7s. My understanding is that the new JJs and the white labelled GT tubes are fairly close in bias rating. Consensus is just "plug and go"!.


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Posted: Wed Sep 01, 2010 2:01 am
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Hi Canucklehead,try Motomaster electrical contact cleaner spray from Canadian Tire,I used it on my JCM 800's pots the other day and it worked better than anything else I've tried.Just give it a few minutes to completely dry if you spray inside tube sockets or pots.

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Posted: Wed Sep 01, 2010 4:01 am
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My understanding is that the new JJs and the white labelled GT tubes are fairly close in bias rating. Consensus is just "plug and go"!.
--canucklehead

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Consensus by who ? Electronics tech ?

Sorry but "plug and play" for power tube does't work. And most of the time Blues Junior are wrong biased ( too hot) when Fender built it


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Post subject: Tube bias
Posted: Wed Sep 01, 2010 8:42 am
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canucklehead wrote:
...My understanding is that the new JJs and the white labelled GT tubes are fairly close in bias rating. Consensus is just "plug and go"!.


stratele52 is right, output tubes should always be biased properly, when possible. I know that we have covered this before. Again, the BJr comes stock with no bias adjustment. It doesn't matter which output tubes that you use, they will be biased way too hot and will cook very quickly. Biasing hot is one thing, biasing above the tubes rated output is just rediculous (as the BJr is from the factory). You can change a resistor, or install a pot to adjust the bias. In the stock BJr (unmodified amp) ALL output tubes that are installed will have the wrong bias. You'll be lucky to get a couple of years use from your tubes, without correcting the bias. :!:

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Posted: Wed Sep 01, 2010 11:52 am
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The Blues Jr is adjustable, it just doesn't have a bias adjustment pot. You either need to install a pot or change out the resistor. The choice is yours. The power tubes always need to be adjusted properly for best performance. Apparently Fender's designers don't seem to worry about the hot bias. They do own Groove Tubes so maybe that has something to do with it. Make the power tubes bias pretty much non user adjustable and sell the tubes. Sounds about right to me.

Just be aware, doing the bias pot mod will void your warranty. Having a certified Fender tech do the resistor change will not.

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