PRS Sweet 16 - How to tell if a tube is dead?

[Themady]

Hi all,

I bought a Sweet 16 off a BAMmer for a steal a few months back, but have only just had a chance to put it through its paces. While it sounds fantastic, one thing I noticed is that if I look through the back grill with the amp on in a darkened room, the 2nd small tube (from left to right) doesn't glow at all. I've never had a tube go out on me on my other tube amp (16 year old Fender HRD), so I'm not sure how else to diagnose this other than by visually checking. What effect would a dead tube have on sound (having never heard a normally operating Sweet 16 in person?)

Thanks in advance!

 

[Egads]

Not all tubes glow. If you are getting sound (and it sounds good) and the reverb works, you are fine. Since it's a single channel amp with no effects loops, you would know if a tube was out.

BTW, I absolutely love the Sweet 16. It hits the sweet spot for being able to actually crank the amp into natural compression without having band mates and neighbors yelling, and if you mic it for gigs, you are set! It seems like so many guitarists think they need 50 or 100 watt amps, when most of us small bar/club players can get amazing tone from lower watt amps. Don't get me wrong. There is definitely a different low end from a low watt amp with it's associated transformers. Playing in front of a cranked 50 watt amp and a 4x12 is awesome. I just think it is totally overkill for most of us. I'm also sick of hauling gear. We usually bring the PA, as well as all of our personal gear. I'm done hauling heavy cabs and big heads. I'm between 16, 18, 22, and 36 watt amps now and only occasionally bring more than one 1x12 cab.

 

[László]

If by "small tube" you mean a 12AX7 preamp tube, there are a couple of possibilities: One, lots of tubes hardly seem to glow when working, and the glow can vary from brand to brand. Or you could have a bad tube.

If a preamp tube isn't working, generally an amp won't sound right at all. But here's what I'd do:

A preamp tube is a $12.00 item, and it's always a good idea to have spare tubes just in case. So I'd spend the twelve bucks, get a spare, and try it in the amp. If the amp's tone changes significantly, you had a bad tube. If not, the tube was probably just fine, and you could either leave the new tube in place, or put the old one back in.

Changing tubes is about the same difficulty level as changing a light bulb, so you won't have to work very hard to make this happen.

Edit: I see Egads posted just about the same info while I was replying, sorry about that!

 

[Egads]

Les is right. It's always good to have spare tubes, especially if you gig. I keep matched power tubes and enough preamp tubes to cover every spot. I also keep spare fuses in my kit.

 

[Boogie]

Unless you have a lot of odd noise or completely dead silence off standby, the tube is probably fine. A dead tube would result in silence.

Just because it's a 16w amp, don't be fooled, it's stinkin' loud! And unless you play very clean and need volume with headroom, this little beast delivers the goods. And takes pedals like a champ, too. It's one of my favorite PRS amps.

 

[jdbreshears]

I agree, unless the amp sounds bad the tube is probably fine. I think V2 in my Sweet 16 came with a Tung-Sol and those do not typically glow much (as well as EHX 12ax7 tubes). Your amp should have tube shields on the preamp tubes, but perhaps he previous owner removed them. The V1 tube is probably a JJ (if stock) and the other pre-amp tubes should be NOS Phillips 12at7's, all of which glow.

I love my Sweet 16. It has a nice compression, is dead quiet at idle, and get LOUD. It also is a very responsive circuit design, which plays well with guitar volume adjustment. The only change I made is to replace the V1 JJ tube with an EHX 12ax7 to give a little smoother compression on high gain and a little more "swirl" in the harmonics.

 

[DHW]

I agree, unless the amp sounds bad the tube is probably fine. I think V2 in my Sweet 16 came with a Tung-Sol and those do not typically glow much (as well as EHX 12ax7 tubes). Your amp should have tube shields on the preamp tubes, but perhaps he previous owner removed them. The V1 tube is probably a JJ (if stock) and the other pre-amp tubes should be NOS Phillips 12at7's, all of which glow.

I love my Sweet 16. It has a nice compression, is dead quiet at idle, and get LOUD. It also is a very responsive circuit design, which plays well with guitar volume adjustment. The only change I made is to replace the V1 JJ tube with an EHX 12ax7 to give a little smoother compression on high gain and a little more "swirl" in the harmonics.

I haven't seen a schematic but if the preamp section is linear a truely blown tube would result in a dead amp. A damaged tube would result in excessive noise, pops and crackles, muffled sound, or really anything you wouldn't expect from an amp of this quality. I certainly wouldn't be putting the cheapest tubes I could buy, just because they are cheap, into any socket but especially the preamp section. Using high quality tubes with the correct attributes in each particular socket gives it life so it doesn't sound sterile.

Is there a schematic for this amps power and preamp section?

 

[vchizzle]

Agree with everyone that if the amps sounds good, the tube is likely fine. They don't all glow a lot, even those that are the same brand/make. If you have sound and there's no excessive or obnoxious noise issue, it's probably good. Another thing you can try, with the amp on(not super loud), take a pencil and gently tap the tube with eraser. If you get any big ring, hiss or crash type noise that takes off when you tap it - get a new tube.

 

[jdbreshears]

Is there a schematic for this amps power and preamp section?

Unfortunately, none that I know of... I've even asked PRS customer service and was politely refused.

 

[DHW]

Unfortunately, none that I know of... I've even asked PRS customer service and was politely refused.

That amp has what? 2 power tubes and 2 preamp? One of the preamp tubes probably splits duty with the power stack which means it would have to be linear so if a tube was dead there would be no sound at all. If a tube was damaged you'd hear it for sure. Look at the tube in question and if there is discoloration on the plates or white flecks all over the glass replace it.