Tube Amp Troubles...Help With Diagnosis, Please?

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ghetto3jon

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my fender hot rod deluxe 112 is acting strangely. the volume seems to be fluctuating on its own. it's not really drastic, but it's obvious. the level quickly jumps up and down...i've tried many different guitars and many different cables, bypassing all of my pedals...so it's definitely the amp. does anyone have any idea what is going on? i'd really appreciated any advice.
 
I had an Ampeg SVT do that before, it was a long time ago but I think it was overheating. I ended up putting a small clip fan behind it in the flight case to cool it. That probably isn't a long term solution but I was on the road and taking it out of the case to aid in air circulation wasn't an option.

Have you recently done any modification's to it?
 
It may be as simple as a bad tube. If you bought it locally see if they have some tubes they will let you try in it. Start with the preamp tubes, changing one at a time. The power tubes should be changed as a set.
 
i was hoping it was preamp tubes. it still has the stock tubes in it...and the amp is about 5 years old. preamp tubes can just be popped in, right?

thanks so much for the advice.
 
ghetto3jon said:
i was hoping it was preamp tubes. it still has the stock tubes in it...and the amp is about 5 years old. preamp tubes can just be popped in, right?

thanks so much for the advice.
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yeah ... just pop them in (pre-amp) ... it seems to be a nice amp ... do yourself a favor and get some JJ-Tesla from www.eurotubes.com ... probably the best 20 bucks you'll spend ...

other than that ... make a visual inspection for poor soldering, the obvious stuff, you know ...

alfred
 
i was thinking about trying out the mullard re-issues from gt. i think i can get them from gc, which is important to me because i'm in the middle of a session, and i need to fix this issue asap.
 
does anyone know if there is a way to determine that it is indeed the preamp tubes (three 12AX7s) before i spend the money on new ones?
 
Check the tubes with a tube checker. Also, does your channel switching light come on by itself when the volume fluctuates? If so this could be a bad internal switch. It happened to my Blues DeLuxe.
 
ghetto3jon said:
does anyone know if there is a way to determine that it is indeed the preamp tubes (three 12AX7s) before i spend the money on new ones?
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If the tubes are 5 years old and you play a lot, you need to replace them. the 12ax7s are less than $10 each. The last time I had the problem you are describing, it was an effects loop jack.
 
AlfredB said:
... and need to be re-biased...

best of luck
alfred
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I think the devilles come with the GT color coded tubes, in which case you could probably just get the same color (red is stock I believe) and won't need to re-bias. I would change the power tubes before the pre tubes. They tend to wear out over time and five years with regular use is a lot of time. The pre tubes will last a very long time unless there was a latent defect.

EDIT: oh yeah, and if you notice certain "dead" notes, that are quiet and lifeless it's a telltale sign of tubes about to go. A shorted tube can potentially take out a lot of other things, so considering the age of the amp, you should probably replace the power tubes even if it isn't the problem
 
Try tapping each of your preamp tubes lightly with a pencil, while the amp is on, vol. turned up, and no instruments connected.

A microphonic (or failing tube) will sometimes ring loudly when tapped. This may or may not be your problem tube, if you find a noisy one.
 
I'm having that same problem with my amp. I changed the preamp tubes and power tubes but no go. Someone said it is the filter caps and that sounds about right
 
no, the gain knobs are clean. i could try the fx loop thing, that would be great. the amp is at the studio right now, so i won't be able to try any of this stuff until thursday night...but if anything works out, i'll post it here to help anyone else with the same issue.
 
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