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Replacement Tubes


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I own one of the earlier Fender Pro Juniors. It makes a squealing noise at times. With no reputable service center near, I am wanting to replace the tubes first. (Any comments on if this makes sense are welcomed). The preamp tubes are 12AX7A. I cannot find them anywhere. Contacting the local GuitarCenter, the employee said he was not familiar with the Pro Junior (although they sell them) Any help would be appreciated.

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The "A" in the 12AX7A had a specific meaning in the sixties and seventies as a "improved" version of the 12AX7. Although the designations remains in use today, a 12AX7 without the "A" is the same tube.


Also, every tube type has at least three code designations. The most common are American, Military, and European.

12AX7 (American) = 7025 (Military) = ECC83 (European). These are all the same tube, although may have slight differences, but all do the same thing. GC should have the tube you need labeled as one of the above, I believe my local GC's are labeled ECC83.


Here's a couple of other sources of tubes and information:



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I'm not an amp tech and there are others here who know more about tube amps than I so....


Guys' date=' would a pre-amp tube cause squealing?[/quote']


It can if it goes overly microphonic. Here are some tips for how to diagnose:






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Upon further investigation' date=' it looks like a cap might be my problem. If you don't see any more posts from me, you know I have been electrocuted.[/quote']





Discharge the cap by making a wire with at least a 100k 5watt resister:


alligator clip---------------100k 5watt resistor------------------alligator clip



I made one with a 470k 5watt because the amp tech in my town said it was faster and that's what he had on hand to sell me.


1. Unplug the amp

2. Clip one end to the amp chassis

3. clip the other to the biggest ELECTROLYTIC cap - it will have a + (clip it to the side with the +)

4. let it drain the electricity

5. If you work with the amp plugged in - work with one hand behind your back.

6. Replace the bad cap with a quality cap (Sprague) not some Chinese junk.


I'm not an amp tech, but these simple things I've learned since I started building an amp.

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