How to spot a bad tube...

[modsquad]

I know this is not pedal related but figured with all the tube amp and pedal guys we have someone could point me in the right direction....

I have a Fender Blues Jr and its just not sounding right to me.   I am wondering if I might have a bad tube in the thing.   Looking for input on how to check without blowing myself up or electrocuting myself.  It just doesn't seem to have that warm sound.   Also the dang reverb seems to have gone out.

[Kearns892]

What specifically seems to sound wrong? I know I had tube "rattle" and loss of some tone on my tube amp, and one of my tubes was microphonic.

Testing if a tube is microphonic is simple enough, take a pen pencil or something and tap the tube while the amp is on and turned up fairly high. If the tap is amplified, it may be time to change the tube.

Power tubes (the larger ones) are pushed harder and go bad faster, one of these is likely the culprit, however, because of the way power tubes work if one has gone bad the entire set must be replaced with another matched set. Preamp tubes are smaller and are not pushed as hard as power tubes, so they fail less often, and preamp tubes do not need to come from a matched set. If a Preamp (small) tube has gone microphonic, only the bad tube needs to be replaced.

There are plenty of other things that could go wrong with tubes aside from being microphonic; I'm not a "tube guy" only someone who owns a tube amp. So if I have made a blunder, someone please jump in and correct me or expand on what I said. However, I can assure you checking if your tubes are microphonic is one place that is safe to start.

[wavley]

Ok, look up tube safety, I don't feel like explaining all the things to be safe.  Just remember that even a Blues Jr can KILL you.

I have my suspicions of what is wrong, but first... swap the 12ax7s around and see what happens, if it's a tube the problems should move, if you have an extra tube that's a known good one it makes your job even easier.

The Blues Jr has a TERRIBLE problem with solder cracks on those PC mounted tube sockets, check for those.  This is where I think your problem actually is.

The wire inside the reverb tank going to the little transformer has most likely sheared off.  Pull the nub of wire out of that connector and push the new one in, it will cut the insulation and connect itself... or better yet, do it right and strip the wire and carefully solder it to the connector.

[igerup]

If the reverb has gone bad, it could be the wires from the in/out jacks of the spring reverb can (inside the can) needs to be replaced. They're really thin and very hard to strip insulation off without breaking them. As for the power tubes; it isn't an absolute must to use matched pairs. But it helps.

[modsquad]

Thanks to all, the amp seems to have lost tone and sounds more like my princeton chorus SS amp.

1.   Will check the tubes to see if microphonic with the tap method.
2.   Check to socket solder joints.

Yeah, I read the BillM stuff on the reverb tank and thought I would check the conectors.

[petemoore]

  NO glow or redplating...ways to 'spot' problem tube.
  replacing  with a good tube is probably the easiest way to tell if the last one was up to par, I figured the Jr.'d be pretty forgiving about mathching and biasing output tubes [haven't really studied that one, figured probably no need in this design], usually matching close enough makes working good amp.
  Chopstick test...gently tap or nudge on the tube and listen, might make slight tinking noise but should be small, short lived, try this with a good tube to know what kind of sound to expect out of a tap when [amp is under the conditions it's under such as volume setting.
  how old is it answers probably how old are the capacitors in it...
 

[R.G.]

You spot a bad tube by replacing it with a new, known good one. Full stop.

There is a reason those things are in sockets.

[Paul Marossy]

Yeah, what RG said. It's hard to determine the condition of a tube without a tube tester. You usually can't just look at a tube and see if it's bad. That's like trying to look at a battery and seeing if it's fully charged or not.

[Arn C.]

Yeah this stuff can drive you crazy!  I have a Guild Thunder 1 (no reverb) that with or without a guitar hooked up after a few minutes, I hear a litlle thump/pop then it squeals like a pig.   I recapped it (got rid of the hum), and still same thing happens.  Had the tubes checked, they all checked good (but does that really mean anything...)  Then I swapped out some known good preamp tubes (2 12ax7's)  still same problem.  I didn't have any power tubes(6GW8's) to swap.  I cleaned all the sockets and tightened some loose ones, still have the problem, cleaned all pots... I am lost on this.   Maybe the power supply transformer or output transformer or  ?   Not much left.  Could a resistor cause this?  It does not happen at the smae time each time and when I can play through it , it sounds fine...  3 weeks and still trying!


Peace!
Arn C.

[gmoon]

@Arn C.

Not much left.

That sounds like an arcing power tube to me. I would definitely swap them out for some good ones--both, even if only one tube is arcing. If one is bad, there's a good chance the other has been stressed. Probably a good idea to replace the tubes soon, to prevent more serious problems.

Sometimes you can see the arcing when it pop/thumps. I had an amp that would arc when I turned up the tone knob. It was easy to reproduce, and I could watch the tube change state as I turned the knob. It would still operate (tubes are amazingly robust), although poorly.

'Course you can also have arcing in other areas in the amp, too. But tubes are always the first things to replace. Maybe the tube tester you used was an emissions tester only (like the Mercury 1000 I've got in the basement).

Is the Thunder fixed bias? Replace any caps in the bias supply (if you didn't when you recapped the PS).

[modsquad]

So checked it out and as I expected found out...absolutely nothing.    So I am going to swap out the tubes, I think they might be original.   I am contemplating upgrading the power tube sockets to accept ECC5881s and upgrade the transformer.

As far as the reverb not working, I have the green circuit board so there is no plug, the wires are directly soldered to the board.   

[Arn C.]

Thanks GMOON for your advice.  Yes the tube tester only checks if they are good or bad  and also I changed every cap including the power supply filter cap.
The amp is fixed bias.  Looks like new tubes !   I did notice the power tubes got extremely hot . I only had the amp on for a few minutes and turned it off and had to use gloves to remove the tubes , even after 5 minutes of the amp being off.

Thanks again!
Arn C.

[wavley]

So checked it out and as I expected found out...absolutely nothing.    So I am going to swap out the tubes, I think they might be original.   I am contemplating upgrading the power tube sockets to accept ECC5881s and upgrade the transformer.

As far as the reverb not working, I have the green circuit board so there is no plug, the wires are directly soldered to the board.   

I was speaking of the wires on the transformer in the actual reverb tank, the Jr. is notorious for these breaking, I've probably fixed fifty of these with the same problem.  The Reverb is driven by a TL072 on this amp, not a transformer or tube and capacitor like older amps.

Even out here in the country I can go to a store a buy a tube or two, they're overpriced and chinese, but at least they are known good tubes.

The Blues Jr has a terrible solder problem, I would check to see if you lost a cathode bypass cap to a bad solder, maybe C3 or C4... if the resistor is still in the circuit everything will work, but the gain structure will be all wrong.  I was messing with these values last night on my Bass Mate, which is also a 15 watt amp, and it makes a BIG difference.

Here is the schematic with all of the voltages you should see http://www.blueguitar.org/new/schem/fender/blues_jr.gif

And I BEG you to read this before working on that thing http://www.aikenamps.com/SafetyTips.html

[modsquad]

Oh yeah trust me, I am really paranoid about electrical current.  I got knocked off a ladder in my younger years...long story.   Thanks for the info on the reverb tank.   Will check it out, very carefully of course.   

[Arn C.]

GMOON,
  Thanks again for the advice, I just happened to have another power tube in a tube caddy someone gave me awhile back and it cured the problem!
I always thought that it was the preamp tubes, but not in this case.
YEEEEEEHAAAAAAWWWWWWWW!!!!!!!!!

Peace!
Arn C.

[gmoon]

GMOON,
  Thanks again for the advice, I just happened to have another power tube in a tube caddy someone gave me awhile back and it cured the problem!
I always thought that it was the preamp tubes, but not in this case.
YEEEEEEHAAAAAAWWWWWWWW!!!!!!!!!

Cool! Glad that fixed the problem, and your output transformer is OK.

I've done the same thing--replaced components rather than pull the trigger on new tubes first (because $25 of new caps is cheaper than $70 of NOS tubes...) I think I've finally learned that lesson. At least you don't have to worry about recapping anytime soon.

Next time you have it open you might check the screen resistor(s), especially if both output tubes share a single resistor. If one tube was wonky and the power stage running hot, they sometimes fail.

[modsquad]

I would like to thank everyone who HIJACKED my original thread...

Okay I'm grumpy today.

[R.G.]

I would like to thank everyone who HIJACKED my original thread...

Okay I'm grumpy today.

S'OK. We all wake up with a coffee deficit sometimes. 

But the thread got through the correct advice quickly. There is really very little that you can do to a tube amp that does not involve putting yourself at risk. You can replace tubes, fuses, and cords which do not involve opening it up, and  you can readjust the controls, period. Everything else has the potential (sorry, pun not intended) to kill you if you don't already know how to do it safely.

There are exactly two ways to spot a defective tube that do not involve exposing yourself to hazardous conditions. The first is to visually notice that one tube does not have the heaters glowing. The other is to replace the suspected tube with a known good tube. That's it, full stop as I said. Replacing with a known good tube was the preferred method for all professionals in the tube Golden Age. They got out their meters only when forced.

Like so many things, this is covered in some depth at GEO. In this case, http://geofex.com/ampdbug/ampdebug.htm, the Tube Amp Debugging Page. Read "Safety Warning", "Order of Suspicion" and "Step Zero" especially.

Step Zero is a biggie. Anybody not having a set of fresh, known good tubes ready for their tube amp is going to (as I note) make some amp techs very happy over the amp's life. And the clever guy who has collected not only a set of known good tubes, but a set of known BAD tubes and swapped the bad ones around to know what their amp sounds like with a bad one will be able to do magic with a failing amp. The entire TV repair industry back during the era of tube TV sets depended mostly on a guy being able to swap a tube out of his tube caddy.

[Paul Marossy]

The entire TV repair industry back during the era of tube TV sets depended mostly on a guy being able to swap a tube out of his tube caddy.

I was a kid at the tail end of that era. I remember having our old stereo/TV console fixed several times!

[modsquad]

Addendum...I swapped the tubes around and now I am getting splatty sounding low volume.

Will be replacing the tubes this week.