Mark IV blowing tubes?

[giantstepjoe]

I just got this Mark IVb combo and I love it but I think it may have an issue. I traded for it and when I brought it home I noticed one of the 6l6s in the inner sockets was blown. I replace the inner tubes with a set of Mesa 6l6's I had hanging around, fired it up, and both it the inner tubes worked again so I just figured I had a bad tube. I used it several times since for a few hours total since and when I fired it up today it sounded a little off so I looked in the back and lo and behold the 6L6 in the same socket as before was dead. I'm really hoping this is just coincedence and I had two bad tubes in the same place twice. I replaced the tube again and now its working again, I guess I'll find out within the next couple of days whether or not it's the amp. Am i correct in assuming that if it's not the tubes that there is an issue with this particular tube socket and or its connections? I really hope that if there is something wrong that it won't be a timely or costly repair. Thoughts?

Also My Mark IV is hums loudly when you switch from standby to on and then settles into a softer but still obvious hum when running. Is this normal? My Bosses' Mark IV seems to react the same way but not quite as loudly

 

[SonicProvocateur]

Is your hum a ground level hum? Maybe try a different socket somewhere in your house. 60hz hum from lights or singlecoils/cables/effects? Does it do it on the clean channel too, or only on the lead channel when the gain is cranked (a little bit of high gain noise is acceptable, but noticable hum on the clean channel is a giveaway that there is noise either coming from the wall, or coming from the guitar/signal chain, or even possibly the amp.

It could be a coincidence that the same tube blew twice, if your spare set had alot of hours on them, or you pummeled them...if you do a noise test this time around and you decide it's ok to play (no noise on the clean channel), i'd stress test it in simul-class mode to see if it goes. (Also check your fuse). If there is excessive noise on the clean channel, i'd take it to a tech. You certainly don't wanna keep dumping money into 6L6s left and right, but its easier to get something like a ground wire/loose connection repaired, or a resistor/cap replaced than pairs of tubes every other jam session.

 

[giantstepjoe]

Is your hum a ground level hum? Maybe try a different socket somewhere in your house. 60hz hum from lights or singlecoils/cables/effects? Does it do it on the clean channel too, or only on the lead channel when the gain is cranked (a little bit of high gain noise is acceptable, but noticable hum on the clean channel is a giveaway that there is noise either coming from the wall, or coming from the guitar/signal chain, or even possibly the amp.

It could be a coincidence that the same tube blew twice, if your spare set had alot of hours on them, or you pummeled them...if you do a noise test this time around and you decide it's ok to play (no noise on the clean channel), i'd stress test it in simul-class mode to see if it goes. (Also check your fuse). If there is excessive noise on the clean channel, i'd take it to a tech. You certainly don't wanna keep dumping money into 6L6s left and right, but its easier to get something like a ground wire/loose connection repaired, or a resistor/cap replaced than pairs of tubes every other jam session.

Thanks for your resoonse. I don't think it's a ground level hum. I've played it at three different locations in 4 different outlets and 2 places had floruescents but one place didn't and I still got the hum. I'm using a super strat and most of the time using humbuckers so that rules out the single coil hum. I'm using no effects right now and and have tried a couple different cables. It does hum on the clean channel but it's not so loud that it stays noticeable once you start playing for a bit. I can live with a little hum-but if there is something wrong I want to take care of it. What concerns me is the intitial volume of the hum when you switch standby to on- it's almost a pop but not quite and then it settles down.

 

[SonicProvocateur]

Hmmm. Mesa's are a creature with a very very low noise floor, there shouldn't be any noise or popping from your Mark IV, really. I would take it to your nearest Mesa service center and have it checked out. I wouldn't wanna risk anymore blown tubes, personally.

Plus, then you'll have the joys of actually spending money on tube rolling to explore the tone of the amp! :wink:

 

[zodiac272]

is it possible to have a failing screen resistor causing that tube to fail?

in any event, a service call is probably needed.

scott

 

[Blueracer]

Well your one inner tube going out twice could be coincidence but I doubt it. The hum and the tube could be related. Try switching to class A mode and see if this helps the hum. Class A basically disables the inner two power tubes. This will also allow you to use the amp without compromising that one power tube as it and the other inner tube will be effectively out of the circuit.

One thing to check for is red plating of the power tube. Look for an orange glow on the large structure in the tube. If this is occuring turn it off immediately. This would be indicative of a bias problem. Checking the screen resistor and voltage is also a good idea. If you are good with electronics and have a voltmeter I would check all the voltages on that suspect tube. If you aren't it would be prudent to take it to a tech. Post back any and all results of what you find.

 

[Blueracer]

One other thing to check is the fuse particularly since this is a new amp for you. Check to make sure it is the proper current which is 3 amp slow blow for the Mark IV. I have seen people put in large fuses so they can play their amps. This does defeat the purpose of the fuse, i.e. the amp fails to save the fuse. There is more than a bit of irony in these cases.

 

[giantstepjoe]

Well your one inner tube going out twice could be coincidence but I doubt it. The hum and the tube could be related. Try switching to class A mode and see if this helps the hum. Class A basically disables the inner two power tubes. This will also allow you to use the amp without compromising that one power tube as it and the other inner tube will be effectively out of the circuit.

One thing to check for is red plating of the power tube. Look for an orange glow on the large structure in the tube. If this is occuring turn it off immediately. This would be indicative of a bias problem. Checking the screen resistor and voltage is also a good idea. If you are good with electronics and have a voltmeter I would check all the voltages on that suspect tube. If you aren't it would be prudent to take it to a tech. Post back any and all results of what you find.

Thanks- I should have thought to run it in class A to see if the hum was still there but didn't think of it. I used the amp for a few hours today teaching lessons with no issue- the tube is still working fine. I kept checking it every so often to see if the tube was red plating or had blown but nothing nothing looked out of the ordinary.

 

[Blueracer]

Good to hear that things are normal. How is the hum. Did it go away. If it did you may have some sort of intermittent connection. If the hum comes back check again for red plating. Also did the poping when going from standby to on stop. The poping could be a sign of arcing. You can check for this by removing the suspect tube and looking at the socket for a line of burnt material. Hopefully your issue has resolved itself and won't come back.

 

[giantstepjoe]

The hum is still there. Although it's really not very loud- maybe it's just noticeable to me because I'm playing it at low volumes right now- when I turn it up I only notice it when I stop playing and listen for it. It's almost inaudible in clean channel one and gets a little louder in the two higher gain channels-. It's hard to describe the sound when switching from standby to on - it's not really a pop but a hum thats initially a slightly louder volume and then settles to a lower volume for the duration of operation.

 

[giantstepjoe]

Well it seems that maybe it was a coincidence- I haven't blown any more tubes- But I'm still keeping a very watchful eye on them when I'm using the amp. Now I just want to get rid of that hum- It's really not that bad at all you can't even notice it when playing- but the fact that my bosses Mark IVa is dead quiet makes me think mine should be too and that it needs to be checked out.

 

[screamingdaisy]

When I bought an old Mark III several years back the contacts in the sockets had stretched from years of use.... a problem that was exacerbated when I installed a set of JJs power tubes in it (JJs have thinner pins). The vibrations would cause the tube to intermittently break contact which would cause the tube to red plate.

Re-tensioning the tube sockets is a relatively simple fix. Or you can do what I did and stick a tube with fatter pins back in it.

Just tossing that one out there in the event you have problems again in the future.

 

[giantstepjoe]

When I bought an old Mark III several years back the contacts in the sockets had stretched from years of use.... a problem that was exacerbated when I installed a set of JJs power tubes in it (JJs have thinner pins). The vibrations would cause the tube to intermittently break contact which would cause the tube to red plate.

Re-tensioning the tube sockets is a relatively simple fix. Or you can do what I did and stick a tube with fatter pins back in it.

Just tossing that one out there in the event you have problems again in the future.

Good call-I did notice that the tube sockets need retensioning. No more blown tubes as of yet and I've been using the amp alot so that's a good sign methinks.