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Post subject: Fender Blues Junior buzzing
Posted: Mon Nov 28, 2011 6:10 am
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Hi. I recently bought a second-hand Fender Blues Junior (2008 model I believe). I demoed it and it sounded great, no apparent issues. I then used it at a practice and since then it seems to buzzing a lot. This happens without a guitar or lead connected and with both volumes at zero.

Can anyone suggest possible reasons for this? I know that valve amps display some noise, but this buzzing seems quite excessive.


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Post subject: Re: Fender Blues Junior buzzing
Posted: Mon Nov 28, 2011 7:02 am
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Hi mrmatt,

It is likely a bad tube, my first suspect would be the PI position V3. It could be a preamp tube, V1 or V2, or even an output tube. The easiest thing to do would be to substitute a known good 12AX7 into each position one at a time (V1, V2, V3). Example, remove V1 tube and put in the "new" tube. If noise stops then the tube that was in V1 is bad. If noise still, put the original back in V1, then proceed to substitute the next tube position, etc. :idea:

Looking at the back of the amp, V1 is to the far right (farthest from output tubes), count right to left V1, V2 (preamp), V3 (PI), and V4, V5 (output).

Before exchanging tubes, switch the amp off and allow tubes to cool for a few minutes. :!:

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---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Fender Blues Junior buzzing
Posted: Mon Nov 28, 2011 7:35 am
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shimmilou wrote:
Hi mrmatt,

It is likely a bad tube, my first suspect would be the PI position V3. It could be a preamp tube, V1 or V2, or even an output tube. The easiest thing to do would be to substitute a known good 12AX7 into each position one at a time (V1, V2, V3). Example, remove V1 tube and put in the "new" tube. If noise stops then the tube that was in V1 is bad. If noise still, put the original back in V1, then proceed to substitute the next tube position, etc. :idea:

Looking at the back of the amp, V1 is to the far right (farthest from output tubes), count right to left V1, V2 (preamp), V3 (PI), and V4, V5 (output).

Before exchanging tubes, switch the amp off and allow tubes to cool for a few minutes. :!:


Many thanks for this, I will give it a go. Someone had mentioned on a different forum that the buzzing with the volumes down could mean it is a power supply smoothing capacitor failure. Is this something you are familiar with?


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Post subject: Re: Fender Blues Junior buzzing
Posted: Mon Nov 28, 2011 10:31 am
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It could be a filter or coupling cap issue, but easier and cheaper to try the tubes first. For an amp this new, I wouldn't suspect the caps first, but certainly the next consideration in this case. Wouldn't hurt to visually inspect the caps, being aware that they may still be charged, even with the amp unplugged.

Make sure amp is switched off before exchanging tubes. For preamp tube testing, remove the the tube on the far right, V1. Power up the amp and check for noise. If no noise V1 suspect, if still noisy, switch amp off, replace V1, then try the same for V2, then V3. If you still have noise after trying with each preamp tube, try the power tubes, replace with a known good set, check for noise.

If the problem is not the tubes, the amp still has some warranty left, if you can get the signed original receipt from the seller.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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