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Is it time to change tubes?

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9/8/2000 5:35 AM

Is it time to change tubes?

Hello.

Every time I turn on my Sovtek MIG50 head, there is this background noise that isn't supposed to be there. It is a crackling sound, constant, and quite annoying.

I just got this amp a few months ago, and it still has the original tubes. It just started doing this a week or so ago, and it is constantly getting worse. Every time I turn it on now, it gets louder with more crackling. It is getting annoying.

Should I change the tubes? Thanks for any help.

Neil

9/8/2000 3:42 PM

Power tubes should last more than six months and preamp tubes much longer, so its unlikely the transconductance (a measure of if they have “worn out”) has decreased to the point they need changing. Music stores will tell you otherwise because they want to sell tubes – most music stores don’t have a tube tester (at Guitar Center :-)

:-)

?) to measure the transconductance and /or the knowledge of how to do it. However, tubes can measure fine for transconductance and be noisy or microphonic, and may require replacement because of this.

I don’t know how familiar you are with working on amps, so I am going to assume you have no electronics knowledge (no insult intended). Given that, a simple test anyone can do is to pull all the preamp tubes and then replace them one at a time, starting with the phase inverter tube (it’s the one closest to the power tubes) and working toward the input jack end of the amp. If the problem is a noisy tube, it will appear when that tube is replaced and you should be able to subsitute it in different locations and make the noise reappear. If the noise reappears at the same point every time there may be a circuit problem at that point – bad part, bad ground, many other things. If you have another amp, you could substitute the 12AX7 preamp tubes between the two amps also as a test

If the amp makes the crackling with all the preamp tubes removed, there is a problem in the power section, and you should probably take it to an amp tech. Even if it its only bad power tubes, when you replace the power tubes the bias needs to be checked and the tech will have to do that.

You might also check out these links:

http://www.geofex.com/tubeampfaq/taffram.htm

http://www.geofex.com/ampdbug/ampdebug.htm

9/8/2000 6:02 PM
skankin neil	Scott, thank you for your answer. It was just what I was wanting to know. BTW, I have very little knowledge of tube amps and such, so I was not offended. Thanks again. I will try new preamp tubes, and see if that helps. IF not, then I am going to an amp tech.

9/8/2000 8:20 PM
LOU	Check for any loose wire's in side the amp that are just about to break off.

9/8/2000 8:43 PM
Brian	he has no tech experience though-- I would just recommend that he take it to a tech if it isn't the tubes. It can be dangerous inside a chassis if you don't know what your doing. BT

9/9/2000 3:14 AM
skankin neil	It was the first preamp tube. I bought two replacements(just in case), and tried out all three preamp tubes, and it was the last one I checked. So, it seems to be working fine now. Thanks for all the help. Neil

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