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Post by dickieg on Feb 11, 2006 0:41:36 GMT -7
This just started - after the amp has been on for about 10 minutes it gets a hum. Not loud enough to hear over playing, but too loud to be normal Is this tubes? PreAmp or Power amp? Also, I've been tilting it back to play - noticed another noise that it started to make- but that noise went away when not tilted. What can that be??
Thanks in advance.
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Post by guitargeorge on Feb 11, 2006 4:53:51 GMT -7
Is your MAZ 18 a newer one or is it from the late 90's?
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Post by dickieg on Feb 11, 2006 9:56:52 GMT -7
I bought it used, but I'm pretty sure it's a newer one.
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gstan
Full Member
What, me worry?
Posts: 180
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Post by gstan on Feb 11, 2006 11:38:57 GMT -7
Make sure the amp and/or you and your guitar aren't near anything that might induce noise.
Since it's used let's give it a quick physical. Run the amp again after doing each of these steps, let it warm up and play thru it for 15-20 minutes to see if the problem is fixed, tilt it back etc., if not fixed then go on to the next step, rather than doing it all at once, could make for less work.
First, check the tightness of the screws that attach the chassis to the cabinet. The fact that it hums when tilted and the hum stops when upright makes me think something is moving.
Second, remove and reseat every tube.
Next, remove the top back panel and take a look at the big transformer, examine the chassis where the tranny is attached. Since this is a used amp you'll want to make sure it hasn't been dropped. If it has taken a bad fall there may be evidence around the transformer where the g forces caused the chassis to bend under the stress Does the amp have any major dings or dents in the cabinet? Is the tranny snug to the chassis?
Next, and this is a bit of a hassle depending on which knobs you have, remove the knobs and make sure the nuts on the pots are snugged down, my amp was making all sorts of racket that I chased in vain until I read one of the Doc's posts that mentioned this.
Once you've done all of this, if the problem perisists, it's time to start going thru the tubes. Since it was used, a new set of power tubes and a new phase inverter would be a good idea anyway and you do need to keep spares on hand.
Lastly, this is not a quiet amp compared to say a Fender or some other boutique models, there is an inherent hiss and it will never be dead quiet, probably related to how hot the Doc runs the EL84's to get that great tone.
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Post by dickieg on Feb 11, 2006 18:09:07 GMT -7
The amp is physically perfect. And the hum is now there when the amp is level. Everything is snug and pretty. Would hum be more likely from the power amp tubes or the preamps? The guy the had the amp before me just put in the following tubes
"brand new NOS Tesla (not JJ) EL84's and a NOS Mullard CV4024/ECC81/12AT7 reverb tube"
so my guess would be to check preamp tubes. I have a pair of Fender 12AX7s- but don't know if those would be the right tubes??? (How are they different from ECC83?)
GStan - I sure do appreciate the time you've spent helping - needed to say that
The amp worked beautifully for the first week - then I tilted it back (slightly) for a few days and that's when (maybe coincidence) the hum started - but it's now there when upright. BTW, when I turn on the amp, it's not there - it takes about 6-10 minutes for the noise to start.
Any thoughts???
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Post by dickieg on Feb 11, 2006 21:39:05 GMT -7
Well - I've certainly narrowed it down! The noise stops when I turn the reverb all the way down! (Usually only have it between 9 and 10:30) So, I've checked the leads and they're fine - could it still be a tube?? What else?? Thanks
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gstan
Full Member
What, me worry?
Posts: 180
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Post by gstan on Feb 11, 2006 22:47:34 GMT -7
It probably is the 12AX in the V4 position, though I wouldn't rule out the NOS Mullard as a suspect as well. Doc usually puts a long plate in V4 (LPS).
The V3 has been a problem for me, I think it was because of the problem I had with loose pots, but for a while I was frying a 12AT in V3 every couple of weeks. Do some swapping in these positions with some different breeds of 12AT in V3 and 12AX in V4.
While we're talking tubes I'll let you in on my tube source: ESTATE SALES. Look for them in the paper or better yet contact an Estate Sale company and get on their email list. Every now and then you'll find a sale where the decedent was a Ham radio buff or there may be old TVs, radios, or stereos with tubes in them. I have acquired literally dozens of new, in the box, as well as used, 12AX, 12AT, 12AU, 5751, 6V6, 6L6, EL84, EL34, you name it, from GE, WE, Mullard, Sylvania, Zenith, Phillips, Bugle Boy, Telefunken.
For instance, I bought a tube driven home intercom system, early '60s vintage, for $5.00. I got two Telefunken ECC83s out of it, one of which sits in V1 of my Junior. If you check tube prices on the web you'll see these going for from $65 to $95.
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