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Post subject: Feedback on Blues Jr. from higher volumes
Posted: Tue May 28, 2013 6:49 am
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I have noticed that if I turn the Volume past 6, I start to get major feedback from the amp. The amp is about 7 years old. The only time it goes away is when I lower the volume (i.e. nothing in the signal chain is causing this, it even happens when nothing is plugged into the amp). Could the pre-amp tubes be going bad? Any other suggestions?
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Post subject: Re: Feedback on Blues Jr. from higher volumes
Posted: Tue May 28, 2013 8:34 am
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Sure sounds like a microphonic tube alright. :idea:

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Post subject: Re: Feedback on Blues Jr. from higher volumes
Posted: Tue May 28, 2013 9:32 am
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More than likely a preamp tube that's gone microphonic.
Gently tap each one with a pencil and see if you can find the culprit.

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Post subject: Re: Feedback on Blues Jr. from higher volumes
Posted: Wed May 29, 2013 4:18 pm
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At 7 years old, might be worth replacing all of them. Test the old ones out, get rid of the bad one, keep the rest for spares.

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Post subject: Re: Feedback on Blues Jr. from higher volumes
Posted: Thu May 30, 2013 5:08 am
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I have to agree with everyone else,...sounds like a tube has become microphonic,...at seven years old,I'd replace all the tubes just in case!

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Post subject: Re: Feedback on Blues Jr. from higher volumes
Posted: Thu May 30, 2013 6:32 am
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Knock gently on each tube , if you hear a sound = microphonic . Amp must be power ON.

Use piece of plastic like screwdriver handle .


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Post subject: Re: Feedback on Blues Jr. from higher volumes
Posted: Fri May 31, 2013 6:39 pm
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stratele52 wrote:
Knock gently on each tube , if you hear a sound = microphonic . Amp must be power ON.

Use piece of plastic like screwdriver handle .


Ok, so I did this and the only one that made a noise was the V1 tube. Also played this evening and heard it cutting in and out. Ill order new tubes over the weekend, but for Sunday can i pull the V2 tube and use it in the V1 slot leaving V2 open? Wouldn't that just leave it at half power?

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Post subject: Re: Feedback on Blues Jr. from higher volumes
Posted: Fri May 31, 2013 8:04 pm
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The amp won't work if any of the three smaller tubes are pulled. You could swap V1 with V2 and see if that makes a difference, V1 usually being the most sensitive position. The V1 position in most any amp will generally produce some sound if tapped, so that doesn't necessarily mean anything. You might only need one new 12AX7, trying it in each position, checking for the feedback each time. In case you were unaware, V1 is the right-most tube, looking at the back of the amp, counting upwards from right to left. The smaller tubes, V1 and V2 preamp, and V3 PI, while the larger tubes, V4 and V5 are output tubes. If you got sound tapping on one of the larger output tubes, then that could be the microphonic tube, and both should be replaced as a matched pair.

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Post subject: Re: Feedback on Blues Jr. from higher volumes
Posted: Sat Jun 01, 2013 2:00 am
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shimmilou wrote:
The amp won't work if any of the three smaller tubes are pulled. You could swap V1 with V2 and see if that makes a difference, V1 usually being the most sensitive position. The V1 position in most any amp will generally produce some sound if tapped, so that doesn't necessarily mean anything. You might only need one new 12AX7, trying it in each position, checking for the feedback each time. In case you were unaware, V1 is the right-most tube, looking at the back of the amp, counting upwards from right to left. The smaller tubes, V1 and V2 preamp, and V3 PI, while the larger tubes, V4 and V5 are output tubes. If you got sound tapping on one of the larger output tubes, then that could be the microphonic tube, and both should be replaced as a matched pair.



+1

Tapping to ear microphonic tube need some experience too.


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Post subject: Re: Feedback on Blues Jr. from higher volumes
Posted: Mon Jun 03, 2013 10:09 pm
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I'm not sure if this test would work without a guitar plugged in, but it might.

Sometimes the reverb tanks, or should I say the springs inside, will become very sensitive and will resonate and cause serious squealing when the volume is turned up past a certain point.

Try touching the reverb tank with your hand, pressing down slightly and see if the sound goes away or lessens. Also turn the reverb all the way down with the knob and see if the squealing goes away. It could be a bad reverb tank causing the squealing and you might be able to put some sticky sided door insulation foam directly onto the metal of the reverb tank (possibly inside a black bag) and stop the resonating that could be causing the squealing. It is also possible that you might need the reverb tank repaired or replaced. I think some people put the bigger tanks in when they replace them, like the ones in the Hot Rod Deluxe.

It's just a thought. It might not be reverb tank related at all, but I would definitely check it by pressing down on it when the amp is squealing to see if it is resonating to something and causing the problem. This is just my idea, that I personally would check into. It could well be a tube. But obviously something inside the amp is causing the feedback. I think it is possible that reverb tank springs that were somehow out of whack or broken could cause that type of feedback.

I had an amp squeal terribly when turned up because of a reverb tank resonating and then causing really loud feedback. That was a Classic 30 and I sent it back and got a Vox AC15c1 that has been problem free.

I like Blues Juniors though and would get it fixed if it was mine.

I hope that gives you another idea to check out. I guess we eventually have to be able to check out our own problems with our tube amps, at least to some degree, as regular maintenance; and we can do a lot of simple things ourselves before we have to bring the shop technicians into the picture.

Good luck.

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