2009 Fender 65' Twin Reverb reissue problems???

[twinmaster]

I noticed today that my inner-most power tube is cold to the touch and unlit. I purchased the amp two months ago and at that time all tubes were lit and warm/hot. Now one of the Blue GT-6L6b tubes is probably blown. There has been absolutely no change in sound dynamics though. I called two different amp techs today and was told by one " to use the amp as is and wait and listen for abnormal sound dynamics or just abnormal sounds until I can afford a new matched quartet and then bring it in for re-biasing"; the second amp tech said "try to purchase a used working Blue GT-6L6b tube and replace the cold tube...no need to re-bias at this time" (money is really tight for me right now). So now, I'm really confused and also concerned that using the amp with a bad tube could cause further damage to the amp itself????
I'm new to your forum and I would most certainly appreciate any helpful input. If someone has a used Blue Groove Tube 6l6b, please email me for purchase - if that is a viable option.
MY usual amp settings are:
Volume: just below 3
Bass:8
Mid.:6
Treble:7
I cranked the volume to 5 today and could not hear any abnormal sound dynamics at that level.
Thank you everyone for your time and cooperation.

[Chuck H]

If you don't hear any difference in the tone with a bad output tube then I suspect that tube has been bad for awhile. Since safety and amp circuit longevity are my primary goal I can't endorse using the amp before it's been repaired. Any number of things could have caused that tube to fail. It can't be discounted that it wasn't a circuit failure that damaged the tube. More likely the tube just got old and died, but we can't know that, can we? So...

If the amp is playing and sounding fine that is probably because you never push it hard enough to exploit those tone anomalies that become apparent with a mismatched push pull stage. Against my better judgment I would suggest that you pull the bad tube and one tube from the other side of the push pull circuit. So, pull the bad tube. If the bad tube was #1 or #2 then you need to also pull #3 or #4. If the bad tube was #3 or #4 then you need to also pull #1 or #2. Each pair of tubes, as placed side by side, work on each side of the push/pull output circuit. So the point here is to balance the circuit. Let's put it another way...

If the failed tube was one of the inner two tubes, pull the other inner tube. If the failed tube was one of the outer two tubes, pull the other outer tube.

With two tubes removed the amp will be playing into a mismatched impedance. This is safe within reason and no worse than what you are doing now. At least the output stage will be balanced like this so you may hear an improvement in the tone.

Lot's of guys actually pull two tube intentionally with these amps to reduce volume. PROVIDED NOTHING ELSE IS WRONG WITH THE AMP it is safe to do.

Perhaps the bias circuit is bad and took out that tube. That would mean the other tubes are also in danger. Tubes don't always go gracefully and the risk of damage to the amp still exists for as long as you play it without service.

Perhaps the tube just got old and died. That would probably mean the other tubes are also old and could die at any time. Not to mention the tonal benefits of good functioning tubes. In this case the other tubes could die at any time. Tubes don't always go gracefully and the risk of damage to the amp exists for as long as you play it without service.

So... If you're going to play the amp no matter what, do pull the two inner or the two outer tubes. Whichever pair has the bad tube. That will at least balance the output stage and the amount of impedance mismatch shouldn't be dangerous to the amp.

[Enzo]

Who says the tube is bad? Your guys talked to you on the phone, right? No one actually tested the tube or tried swapping places with one of the others?

All we know is the tube doesn't light up. It could just as easily be the socket is bad, or more likely, the solder on the socket pins.Swap the cold tube with the one next to it. Now does the cold tube stay cold in its new position? or does the formerly working tube from next door, now go cold in the same socket as the first tube? That is the VERY simple first test, even a tech does that first.

Even if it winds up the tube itself is bad, it is probably just a failed solder joint in pin 2 or 7, which can almost always be repaired with a soldering iron. Actual burned out heaters are pretty darn rare.

But the first step is to determine whether the problem follows the tube or stays with the socket.


If you have cracked solder - my prime suspect - just pulling the tube in and out often will start it up again. But that isn't fixed, it will do it again. But if wigling the tube makes it light up, even for a momebt, that means we have a loose connection.

[pdf64]

Tube amps will inevitably require occasional $ for tubes and tech time.
Also it is very beneficial for users to invest in a spare 'known good' tube of each type that the amp uses, ideally a known good matched pair in regard of the power tubes as appropriate.
That will allow them to swap in the good tubes as required (eg to each slot in turn) in order to identify whether issues are due to a failing tube.

In the first instance, I suggest that you swap the power tubes around, to see if it's a bad tube or something associated with that socket.

If the fault of the tube staying cold follows the tube then it's likely just that tube's heater has failed.

I doubt that running the amp with just 3 of the 4 tubes work, ie imbalanced, will cause a problem.

As long as the amp carries on sounding good, then I think it would be fine to use it until you can afford a matched pair of GT Blue 6L6; then replace the cold tube with one of those.
Swap the other one into each of the power tube sockets in turn and you may find that one of them results in lower background hum and a stronger sound.
If they're all the same then perhaps keep it as your spare.

The advantage of the GT grading system is that the user can fit replacement tubes of the same type and grade without re-biasing; so no necessity to change all 4 together, though the best performance and reliability will likely result from that.

Note that the usual practice is to reduce the bass setting as the volume is increased.

[twinmaster]

Great information from all of you. Thank you for your input. I will move the faulty tube (inner-most tube on the end ) to the outer position and see if it lights up etc, and then I will move the outer tube to the position where the faulty tube resides-to see if it lights up etc. If the socket is bad, then, for the time being, I'll pull the outmost (good tube ) and just use the amp with the inner two power valves. If the socket is fine, I would like to try to resolder the pins on the faulty tube; I am proficient with a soldering iron. Enzo, how would I determine where pins #2 and #7 are located? If no luck with that, I will buy a matching duet of Groove Tubes of the same type and grade as I presently have now and replace one into the faulty site OR remove #1(faulty site) and #4 ( outer end) and install the new replacement duet?

Sounds like a good plan. Guys, I'm 67 years old and have been playing for 35 years; somehow I've never encountered faulty tubes in any of my amps. Maybe the tube-gods had been looking after me or I've been trading off amps frequently enough to just luck out. Thank you for all of the great information; if my above plan is in error, please let me know.

[Chuck H]

You can't re solder the tube itself. Enzo smartly pointed out that the problem could be with the circuit instead of the tube. So, if you move the tube to another socket and it lights we know the problem is with the socket that tube was in and NOT the tube. You can examine the socket connections and re solder them. If you move the tube to another socket and it doesn't light then you have a bad tube and it must be replaced. pdf64 is right that Groove Tubes can be bought with matched characteristics. And he's been pretty good to me so I sure don't like to be contrary, but I probably wouldn't buy just one tube unless you know that the other tubes are fairly young. Fender acquired Groove tubes around the time that that amp was made. So just because they don't say "Fender" doesn't mean they may not be the original, and therefor possibly very well used up tubes. I'm not a tube matching nut job cork sniffer, but buying one new tube to install with three very old ones must cross some kind of line. I mean, you wouldn't buy one new tire for your car to replace a flat and leave three bald ones. I've never even tried to buy just one Groove Tube with a specific rating. I'm sure it can be done though. So, if the tubes are young-ish you'rel probably fine just replacing the tube. If the tubes are old then replacing them all is eminent anyway. pdf64 mentioned this too.

[pdf64]

Chuck, I think Enzo did mean to actually re-flow the solder in the tube pins associated with the heater, see http://www.mif.pg.gda.pl/homepages/f...63/6/6L6GC.pdf
Sometime they're just not soldered very well at manufacture.
You should be able to check the continuity of the heater with a meter, it should be near a short.
Make sure that the solder doesn't 'blob' and make the tube pins too fat for the socket; a craft knife can be used to trim things back a bit

[Chuck H]

Well the link just takes me to a data sheet.?. But I'll be a potato head then. I've never considered resoldering a tubes pins to it's electrode connections. Certainly nothing to lose by trying.

[pdf64]

The link was to show twinmaster how to identify pins 2 & 7
But I seem to remember seeing forum threads on the topic.

[twinmaster]

Well guys, I tried a well lit tube in the "faulty" socket and it lit up and warmed up just fine. So I tried the faulty tube in another socket and it stayed cold and dark. Next, I re-soldered all the pins on the "dead" tube and lo and behold the tube is now lit up and warm/hot as are the others in comparison. The amp also has more brilliance to its sound and is definitely louder on the previously mentioned volume setting. Upon further research, I traced the amp's serial number and found that the amp was manufactured in 2000! So much for my due diligence when purchasing a used amp. Lesson learned. The tubes that are in this amp are actually Sovtek 5881wxt's....with the Fender and Groove Tube label. I replaced the now fixed tube back in its old socket and also checked to be sure all these tubes' labeling is the same and they all match.
Thanks to everyone for your very helpful input...especially Enzo!

[Chuck H]

I'll be damned then! I sure don't go through tubes like the repair professionals here, but I've been through a lot of tubes and I've never suspected one of needing resoldering the pins to the electrode ends.

[Enzo]

In fairness, I have been wielding tubes for 60 years or so, and it has come up, but not very often. In your experience, how many open heaters have you encountered in your career? I bet not a lot. Of those only octal or older four or five pin bases can be resoldered by their nature. Nothing to solder on a 12AX7. COnsidering that 6L6 heaters are just so unlikely to burn out, I think it worth checking that the solder has failed in the base when their heaters go down. But it won't be common. COnductive eyelet boards, rare enough really, will be far more common failures.

[Chuck H]

Only one heater failure (that I could blame on the tube ) Intermittent. I probably could have resoldered it!

[Enzo]

If it is any comfort, I'll be damned, too.

But I'll be too busy shaking hands to notice the heat...

[Leo_Gnardo]

In your experience, how many open heaters have you encountered in your career? I bet not a lot.

Not a lot, right, but often enough to make it a worthwhile check. Nice to make a cheap fast repair whether soldering an octal pin or swapping in a new AX or whatever. In Sherlock Holmes style problem solving, I call this kind of fix "the dog that didn't bark."