Need a quick bit of advice re: failed preamp tube

[Sideways8]

Been getting some crap sound out of my new AC15...intermittent volume drops, thin sound, mushy distortion...And after all my reading/YouTube watching I felt comfortable getting inside and having a knock around at the tubes to see if anything jumped out at me.

Power Off = nothing really makes an impression

Lo-and-behold when the amp is warmed up and volume up significantly the first position 12AX7 which is barrel shielded (whatever you call it I took it off for the testing) makes a distinctly different "tink tink" sound when tapped with my little bamboo stick. It's also clearly audible through the frontside speaker and none of the others come through the speaker when tapped. "tink tink".

Is that it? Case closed? Or is that possibly normal for a preamp tube? Sorry, it's been ages since I've had a tube amp, and in my teens I never had any problems with the couple I had so my experience is very limited.

Any help is greatly appreciated

 

[Lake Placid Blues]

Re: Need a quick bit of advice re: failed preamp tube

Yes, that tube is faulty. It is microphonic. Replace it with a known good 12ax7

 

[VinceT]

Re: Need a quick bit of advice re: failed preamp tube

Yes, that tube is faulty. It is microphonic. Replace it with a known good 12ax7

What Lake Placid Blues said.


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[Low_fidelity2100]

Re: Need a quick bit of advice re: failed preamp tube

Well, it may be the problem, but in my experience tons of perfectly good 12AX7's will produce some noise in V1 when tapped. Sorta just depends on the amp and the tube itself.
Generally as long as the noise it produces isn't a ringing sort of sound (you'll know it when you hear it), it's not a problem. If you tap a tube and it rings out, or in a high gain amp rings into a microphonic whistle, then yeah replace the tube. But simply hearing a "tink" through the speaker when tapping V1? That's pretty normal.

That said, by all means swap it out for a known good 12AX7 anyways and see if it cures the other problems you described (thin sound, volume drops, etc...). It might solve the issue, it might not. It's pretty easy to check.


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[Gearjoneser]

Re: Need a quick bit of advice re: failed preamp tube

Amp companies use the cheapest tubes they can get away with.

The smartest thing you could do is replace the entire set with JJ or Tungsol, throw away the microphonic pre, and box up the stock ones as emergency spares.

Your problem will likely be solved, and your Vox will sound better than ever.

With EL-84 amps, you don't have to bias them....just pop the new tubes in.

 

[Sideways8]

Re: Need a quick bit of advice re: failed preamp tube

Thanks folks. I think it's probably going to get a single JJ for now since they're easy to find locally , and then I'll order in some Russian replacements once our potential mail stoppage (Canada) gets sorted out.

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[Kamanda~SD]

Re: Need a quick bit of advice re: failed preamp tube

Agreed with everyone in here. Make sure you toss the bad one!

Just an FYI for you - Hopefully they are adding a 30 day extension so mail doesn't stop monday. Also, http://www.thetubestore.com/ is Canadian and IMO the best (I only buy my tubes from them now since local shops usually only have GT and EHX). Also, they are shipping FedEX only until the mail situation gets sorted out. They also have tube packages (in different levels) sorted by amp, which is rather handy TBH. Ex. http://www.thetubestore.com/Shop-by-Amp/Vox-AC15-Custom-Classic-Amp

 

[Sideways8]

Re: Need a quick bit of advice re: failed preamp tube

Agreed with everyone in here. Make sure you toss the bad one!

Just an FYI for you - Hopefully they are adding a 30 day extension so mail doesn't stop monday. Also, http://www.thetubestore.com/ is Canadian and IMO the best (I only buy my tubes from them now since local shops usually only have GT and EHX). Also, they are shipping FedEX only until the mail situation gets sorted out. They also have tube packages (in different levels) sorted by amp, which is rather handy TBH. Ex. http://www.thetubestore.com/Shop-by-Amp/Vox-AC15-Custom-Classic-Amp

Yes! I've got Tubestore bookmarked already and have been reading their reviews, and picking out candidates. Like I say, a full retubing is probably in the cards sometime soon.

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