what can quickly kill 12 ax7 preamp tubes?

Bob Carlucci · Post by **Bob Carlucci** » 27 Feb 2023 9:25 am

Got a nice shape all tube [2x6973] Univox 2 x12 combo amp, around 20 watts RMS.. It was giving all kinds of trouble at times, losing gain, reverb going in and out.. One of the 2 12 ax7 tubes was barely glowing..I went to an amp shop, tested the preamp tubes.. Both 12 AX7 tubes tested bad, and one was shorted.. The 12AT7 was good.. Bought 2 JJ 12AX7 tubes[$50+!!!!], went home, put them in the amp,started her up, all was good.. for a minute or two..Both 12AX7 glowing as they should. Then I heard an audible pop, lost half the volume, bunch of crackling, and once again one of the tubes was not glowing... I'll bet that if I test those brand new tubes,with maybe 2 minutes run time, one will be shorted... any ideas what may be wrong before I spend $350 getting a $300 amp repaired?.... bob

J Fletcher · Post by **J Fletcher** » 27 Feb 2023 10:08 am

Could be the grid resistor is open , or no longer has a return to ground .

Bob Carlucci · Post by **Bob Carlucci** » 27 Feb 2023 11:16 am

Just pulled the tube, and it is blown out, cracked open around the entire circumference of the tube where the pins go into the base... Lasted all of about 90 seconds,, Never in 55 years of using tube amps have I seen this happen to ANY tube on any amp......

Michael Butler · Post by **Michael Butler** » 27 Feb 2023 11:17 am

if you have the schematic, pull all tubes and check the voltages. if ok, plug in the rectifier tube and check. then the preamp, check and then the power tubes, check. most likely would have found the problem by then. you could also check all the resistors but particularly the ones around the faulty tube.

if unable to do the above, may have to take it to a tech.

good luck.

Bob Carlucci · Post by **Bob Carlucci** » 27 Feb 2023 12:15 pm

SS rectifier... I am going to have to pull the chassis.. Whatever the problem is, it seems to be on one tube, not both.. One glows normally and the amp does work.. One 12ax7 socket seems problematic, and I'll check there first.. I have just never in my life seen a problem so severe on a preamp tube circuit that it would cause a brand new tube to actually explode which is what happened here.

I did find the schematic online.

Michael Brebes · Post by **Michael Brebes** » 27 Feb 2023 1:43 pm

It could also be related to bad pin to socket connections on the suspect socket.

Bob Carlucci · Post by **Bob Carlucci** » 27 Feb 2023 2:07 pm

Michael Brebes wrote:It could also be related to bad pin to socket connections on the suspect socket.

Agreed, and that socket is a bit loose... How though could it cause the new tube to explode, and a dead short in the one it replaced?... I have dealt with bad female pins in tube sockets, but usually they just caused noise or intermittent operation. Never saw a bad socket cause a surge that blew a tube open..

Jeff Highland · Post by **Jeff Highland** » 27 Feb 2023 2:22 pm

First things would be checking are the cathode resistors and bypass caps. A short or low value in these would cause excessive current draw and damage.

Bill Hatcher · Post by **Bill Hatcher** » 27 Feb 2023 5:49 pm

bob. it doesnt say fender on it......thats part of the problem right there.

Donny Hinson · Post by **Donny Hinson** » 27 Feb 2023 6:19 pm

If the bias is off, the tube will draw too much current, heat excessively, and then fail due to an inter-electrode short. I’ve seen it happen dozens of times. If the filament voltage isn’t near perfect, that can also cause premature failure. Filament voltage is critical, as are bias voltages. So make sure the tube sockets are making good contact with the pins. Re-tensioning socket pins every few years is required maintenance for tube amps.

And on any old tube amp, resistor and capacitor values should all be checked during any routine service.

Bob Hoffnar · Post by **Bob Hoffnar** » 28 Feb 2023 5:32 am

This is a handy tool for dummies that I use to check and adjust tubes

https://www.stewmac.com/luthier-tools-a ... ias-meter/

Brandon Montgomery · Post by **Brandon Montgomery** » 28 Feb 2023 6:26 am

Sounds like the cathode bypass capacitor ( or cathode resistor) on that stage blew up, left a carbon trace, and the new tube is seeing that carbon trace as a clear path to ground, I.e., that tube isn’t getting control voltage.

If you don’t service your tube amp regularly it’s somewhat like driving a car with an oil leak, it’ll run for a while but eventually seize.

Before I get the peanut gallery comments about never having to fix _______ amp, I must assure you that said amplifier is indeed in dire need of service.

J Fletcher · Post by **J Fletcher** » 28 Feb 2023 7:39 am

So the tube is drawing too much dc current , it heats up excessively and explodes . First of all , you are sure that a 12ax7 belongs in that socket , not some other 9 pin tube ?
If a coupling capacitor from a previous stage is leaky , the grid will go too positive and cause excessive current to flow in the tube . If the grid return resistor is open , or the circuit from the grid is not going to ground , the grid will accumulate electrons from the cathode to plate current flow , and these electrons will have no path to ground . That causes the grid to go more positive , which results in more current flow , which causes more electrons to accumulate on the grid , causing more current flow , and eventually the tube gets so hot it explodes .

Bob Carlucci · Post by **Bob Carlucci** » 28 Feb 2023 9:48 am

J Fletcher wrote:So the tube is drawing too much dc current , it heats up excessively and explodes . First of all , you are sure that a 12ax7 belongs in that socket , not some other 9 pin tube ?
If a coupling capacitor from a previous stage is leaky , the grid will go too positive and cause excessive current to flow in the tube . If the grid return resistor is open , or the circuit from the grid is not going to ground , the grid will accumulate electrons from the cathode to plate current flow , and these electrons will have no path to ground . That causes the grid to go more positive , which results in more current flow , which causes more electrons to accumulate on the grid , causing more current flow , and eventually the tube gets so hot it explodes .

Yes its a 12 ax7.. all tube positions are factory marked on the chassis. I pulled the chassis, found nothing noteworthy yet, have not started digging...

Bob Carlucci · Post by **Bob Carlucci** » 28 Feb 2023 7:07 pm

replaced the tube at the repair shop where I bought the tubes.. I brought the chassis down there, as the guy had used known good tubes, and was willing to sacrifice one.

Amp ran perfectly.. Quiet, clean, good power, no crackles.. Perfect... Brought it home, same thing.. Never plugged the reverb in.. I thought 1/2 of the 12ax7 drove the reverb, but I was wrong its the AT7, which was good. So I suppose,I might have just HAPPENED to have 2 bad 12 AX7 tubes at the same time, and then one of the replacements blew open instantly, or there may be some insidious issue that will rear its head again.. Everything in the chassis looked fine, I found no open resistors, no burning no "hot" smell, checked the PC board traces for carbon tracks..amp is church mouse quiet and runs perfectly...Pulled the reverb tank, checked transformers for shorts/opens, wires for shorts, continuity etc.. Very nice hammond tank, no cheapie... I dunno, I am not going to even try the reverb, as I still think that might be the issue as everything is great without it plugged in..

Can't try it until I get a few sacrificial 12 AX7 tubes cheap somewhere.. I can't use $25 each tubes as a diagnostic tool, after seeing one blow up, and finding 2 bad ones before that.. So still not sure whats going on, but the amp works great, and I am real gunshy as the only variable I can think of when the amp was eating and then exploding tubes, is the reverb circuit, 3 dead tubes with it plugged in, and smooth quiet powerful operation with it out of the circuit... bob

Dave Grafe · Post by **Dave Grafe** » 1 Mar 2023 2:15 am

Running a tube-driven reverb circuit without the pan plugged in can cook the transformer due to the open, i.e. infinite load.

If the 12AT7 reverb driver was not an issue before it can't really have anything to do with your other tubes failing. Anyway, if you smell the magic smoke escaping you'll know why.

Bob Carlucci · Post by **Bob Carlucci** » 1 Mar 2023 5:29 am

Dave Grafe wrote:Running a tube-driven reverb circuit without the pan plugged in can cook the transformer due to the open, i.e. infinite load.

If the 12AT7 reverb driver was not an issue before it can't really have anything to do with your other tubes failing. Anyway, if you smell the magic smoke escaping you'll know why.

Thanks, I never knew that. You are speaking of the reverb transformer correct?...

Dave Grafe · Post by **Dave Grafe** » 1 Mar 2023 7:18 am

Yes, it's a simple output transformer and needs proper termination. Why Fender tube amp reverb tanks all have 8 ohms input impedance when solid state circuits call for much higher Z.

Patrick Huey · Post by **Patrick Huey** » 7 Mar 2023 9:04 pm

Brandon Montgomery wrote:Sounds like the cathode bypass capacitor ( or cathode resistor) on that stage blew up, left a carbon trace, and the new tube is seeing that carbon trace as a clear path to ground, I.e., that tube isn’t getting control voltage.

If you don’t service your tube amp regularly it’s somewhat like driving a car with an oil leak, it’ll run for a while but eventually seize.

Before I get the peanut gallery comments about never having to fix _______ amp, I must assure you that said amplifier is indeed in dire need of service.

Brandon,
EXACTLY. And tube amps and solid state as well all need filter caps, electrolytic caps, etc replaced, tube amps need current and voltages checked, bias checked, etc. regularly. You don’t blow tubes like that unless that tube is red plating big time and to get hot enough to pop suddenly it’s drawing waaaay too much current. Something else is amis. Just like you don’t pop fuses on tube amps because a fuse “goes bad” suddenly

Patrick Huey · Post by **Patrick Huey** » 7 Mar 2023 9:10 pm

Brandon Montgomery wrote:Sounds like the cathode bypass capacitor ( or cathode resistor) on that stage blew up, left a carbon trace, and the new tube is seeing that carbon trace as a clear path to ground, I.e., that tube isn’t getting control voltage.

If you don’t service your tube amp regularly it’s somewhat like driving a car with an oil leak, it’ll run for a while but eventually seize.

Before I get the peanut gallery comments about never having to fix _______ amp, I must assure you that said amplifier is indeed in dire need of service.

I know only a couple steel players that have tube amps they “never have to fix_______” they both have pre CBS Fender twins they bought new and have used em extensively and toured all over with them….still have the original Sylvania 6L6’s in em i believe they are…..however both those guys have their amps in for maintenance and TLC at least once a year.

Bob Carlucci · Post by **Bob Carlucci** » 8 Mar 2023 3:29 pm

Believe it or not, it was THREE bad tubes, plain and simple... The amp is now whisper quiet, sounds great, and runs as new and has for over a week. Runs so quiet now its creepy. It simply had 2 tested bad preamp tubes, and when I installed 2 new JJ's one blew up instantly... Once that was replaced, end of all problems related to the tubes... Reverb is working but is pretty weak,, Tried replacing the 12ax7 reverb driver/ recovery tube, to no avail.. It functions, but there isn't enough in the mix...

At least one forumite that had a repair service tell me he's seen more than one JJ explode on lightup..Not an encouraging observation concerning their quality control.... bob

David Gertschen · Post by **David Gertschen** » 13 Mar 2023 12:51 pm

Interesting outcome. I have never had bad luck yet with JJ brand tubes. I sure hope their QC isn't going downhill, but one never knows? Corporate changes, cost cutting, etc.

Another trend with vintage amps issues in general could be due to line voltage. I routinely see 122+ VAC measured at the outlet in many different venues and places. The old tube amps were designed to be used with 110 VAC input power. That's more than a 10% difference in design to operating value. It can often stress old components to the brink. But the Utility Companies just keep turning it up

Dave Grafe · Post by **Dave Grafe** » 13 Mar 2023 1:05 pm

David Gertschen wrote:Interesting outcome. I have never had bad luck yet with JJ brand tubes. I sure hope their QC isn't going downhill, but one never knows? Corporate changes, cost cutting, etc.

Another trend with vintage amps issues in general could be due to line voltage. I routinely see 122+ VAC measured at the outlet in many different venues and places. The old tube amps were designed to be used with 110 VAC input power. That's more than a 10% difference in design to operating value. It can often stress old components to the brink. But the Utility Companies just keep turning it up

Factual information:

In 1954, the American National Standards Institute (ANSI) published C84.1 “American National Standard for Electric Power Systems and Equipment – Voltage Ratings (60 Hertz)”. This standard established a national standard 120 volt nominal system.

While there may have been regional variances, 120 VAC 60Hz was the prevailing voltage on the west coast and as such was the design voltage for all of Leo Fender's circuits. Any noise to the contrary is just that, noise. Why people repeat stuff thru could easily look up and debunk continues to mystify me.

Carl Gallagher · Post by **Carl Gallagher** » 13 Mar 2023 3:22 pm

JJ tubes are notorious for early failures. I've been repairing tube amps for decades and JJ's are some of the most unreliable tubes I ever experienced.In my and my sons amps I wont even use any current production tubes. I started hoarding thubes back in the early 80's when I saw the writing on the wall concerning production ending in the US and most of europe. I have enough NOS tubes that my sons will never have to buy a new tube for the 20 something amps we have. When I do repairs for clients I wont even buy the tubes, they buy their choice and when, not if, but when it fails they can deal with returning them. I have a couple of good clients that I will sometimes use my good tubes for, but thats very rare. I would recommend a used old stock preamp tube over any current production. Preamp tubes that were used in radio and hi fi applications are not stressed the way guitar amps stress them and are usually like new after years of use. I have a big supply of used preamp tubes or "pulls" that I used to find at ham fests and I have used them in amps that I repaired and have been in use for years still going strong. We have 3 Ampeg Gemini's from the 60's that still have the original Bugle Boy preamps and they are still working like new.
That VHT "tester" that Bob posted about is not for preamp tubes it is a bias tester for power tubes, and I have to say,its probably the worst bias tester I ever used. It will tell you the plate current draw but not the voltage, without knowing the voltage on the plate, the current draw is useless.I bought one a couple years ago because the one i'm currently using is getting old and is not made anymore.I put the VHT aside til I can get around to modifying it to show voltage but havent gotten around to it yet.My oldone is still my go to.

Tony Prior · Post by **Tony Prior** » 16 Mar 2023 12:19 am

reading about Bobs issue, I am not shocked at all that the issue was the tube itself. If and when I replace a suspected bad tubes, I don't use new tubes ,I pull one from another amp or from the junk box . Like many here, I had been servicing amps for a long time and I've seen more bad NEW tubes than bad older used tubes. Go figure. If I look back I don't think I ever had more than 1 or 2 bad 12AX7's on a gig. When they go bad it's more noisy than anything, they sound like a Kitchen Mixer !

If we suspect a bad preamp tube, swap it with another tube position in the same amp, that would conform the socket and any related components. If the problem moves with the tube it isn't the socket or circuit. IF the problem STAYS with the socket, well then there ya go .

Remember the TV repair guys ? Think about this. They carried GOOD TUBES, not necessarily NEW tubes. They are not the same. "NEW" is an unknown, GOOD is a known. When troubleshooting circuits, we go with "KNOWN" components or we may be chasing our tails endlessly. Been there done that...

Back in the days when I was burning daylight, I would sit at the workbench and measure the MU of the 12AX7's, ( gain factor) I would mark the little box they came in and put them away for later use. Sometimes I would order some new tubes and test them, the MU factor was lower the than some of the old tubes I had sittin' around ! It was at that point that If I desired any new tubes I bought the cheap ones .

In addition, by measuring the MU factor it also confirmed that the USED tube had life and could be used if need be. By the way, the 12AX7 and the family of 12xxx tubes can be used in any slot, but the different gain factor of the tubes may be noticeable in certain circuits. BUT for general testing, you can identify a bad tube in a NY Minute. 12AX, 12AU, 12AT they can confirm if the audio circuit is functioning or certainly confirm if the socket is causing the issue.

Another thing I still do, while I have never needed it, I carry an old tested , used , "GOOD" 12AX7 in the gig bag. I think its a $10 JJ or something like that.

I recommend that anyone using a TUBE amp with 12AX7's, 6V6 , 6L6's , EL84's etc... keep an old GOOD one in your pocket for a rainy day.

Have fun!