Smoked one of the power tubes

[bandit2013]

I had a few issues with the mark V. On several occasions while playing through channel 3 the amp shut down. My fist thoughts were either power tube or speaker blowing out. In all occurances I was operating in the 45W mode at full power. I am also using the effects loop so I can have a master volume for all 3 channels.

The first time I was running a Digitect 40000 delay and thought it was the Digitech checking out (still works on the MKIV)
The other two times, no effects at all. The last event of the three I decided to replace the tubes with old STR420's I kept as backups after retubing my mkIV.

Also, the first three failues occured with V30's in the 4x12 cab. Everytime after shutting down and powering back up it would go away. If I played for a short while no issue but longer than an hour it seemed to be hit or miss whein it would occur.

Not completely sure it was a power tube, I discoverd on this board that similar issues may be caused by V6 or V7. In all events I failed to look in the back to see if any power tubes were glowing red hot since the standby switch was always "the go to" cure to prevent any further damage in the event of catastrophic failure.

I had many spare 12ax7's on hand so I replaced both V6 and V7. I assumed the original power tubes were fine so I put them back in. Since then I had changed speakers from V30's to EVM12L. This time, while playing channel 3 in 45W mode at full power, the MKV "sH!T the bed" and it smelled like burnt resistor or circuit board. This time I took the amp out of the shell to inspect for blackend resistors or burnt solder pads, traces, etc (without removing anything). All looked well. Although considering the operating temperature, some of the solder paste from connecting wires is a bit excessive in some areas. All of the solder joints looked good (usually will change color from bluish to dark gray if they get overheated).

V8 got smoked. This time while the sound died out , the amp began to hum like a noisy flourecent ballast. Side of the tube opposite of the MESA stamp has blackened. On the same side, the plate is also dark, the seam is no longer straight. 3 months is a short life span. The original tubes I had in my MKIV lasted 13 years and are still going (now installed in the MKV). I guess I will find out if this corrected the issue.

Just a typical tube fault. However, a bit premature.

 

[farren]

I was going to ask if it got you high, but then I realized it was your amp that smoked the power tube...

I didn't have great luck with my stock tubes either, except in my case I had several pre-amp tubes quickly die. I'm replacing the whole set before another stock 12AX7 bites the dust.

Three months isn't long, so if you're the first owner of your amp, you should be able to get a warranty replacement.

 

[bandit2013]

I was going to ask if it got you high, but then I realized it was your amp that smoked the power tube...

I didn't have great luck with my stock tubes either, except in my case I had several pre-amp tubes quickly die. I'm replacing the whole set before another stock 12AX7 bites the dust.

Three months isn't long, so if you're the first owner of your amp, you should be able to get a warranty replacement.

Yes it did. LOL.
It is becomming a trouble. I replaced all 4 with another set (old ones from Mark IV STR420 6L6GC) and they did not survive very long. It seems V8 begings to run hot after 30 minutes of use. That was the same location the stock STR440 blew out in. This time before hitting the standby switch I was able to see what was happening. The entire tube was cherry red. Of cource V9 was red hot too but not as bright as V8. I wonder if there is a bias issue, or the EL34/6L6 bias switch is defective. I do have three of the stock 440's to use. Any ideas of what could be the issue? I am not too concerned about the STR420 since they are old. I do have new tubes comming but would hate to burn them up if there is a problem with the amp.

 

[bandit2013]

I at least have two good tubes that were stock to last another week. STR440's in the center (V8 and V9) with STR420 in the outer A/B circuit (V11 and V10). I was able to find the one tube that was paired with the one that red plated. It was lacking the UV glow in the 45w mode. Yep, I put it in the V8 socket. Could have tested the 10W mode. However, I discovered that pentode worked fine, triode mode did not. Replaced it with the remainder, so far still okay. That should suffice until the replacments arrive. I almost thought the inverter tube went sour since only one tube was working in the A/B 45W.

I played for at least an hour non stop without incident. In case anybody was wondering, I sat behind the amp in the dark to observe the tubes. Just add a bit of mercury pellets, inert gass, some magnetic ballast and bingo: UV Lamp and lots of Ozone. Skin cancer while playing guitar anyone? The bar is soon to close, keep out the irishmen. At least that analogy was better than the "train conductor" or "traffic cop" cartoon used to describe how bipolar transistors work.

 

[bandit2013]

Yell I figured out the issue. My skill level at playing the guitar is so bad the am would rather die than have me play through it. After 30 years (off and on) I should have at least learned something, no reason to quit the day job.

So far so good, may have been a bad tube causing the other to one to degrade. One other clue to the problem, I still have one V30 in the cabinet. Replacement will arrive this week. I was suspecting a bad speaker hence the change from V30's to EVMs. Cab is 13 years old. Also, I have now been using half the 412 cab with the two EVM and no issue what so ever (still feel it was bad tubes from the start). Hopefully I can rule out a grid resistor or bypass cap for now (or a bad solder joint) since I have not burned out the remaining tubes. As for the Cathode bias resistors, they appear to be of the carbon comp variety, metal film resistors typically do not have that shape. Carbon resistors tend to have thermal related issues (on some occasions they can form micro fractures in the helix element due to temperature cycling). It is rare to have failure with new parts, but I have experienced similar issues with carbon power resistors operating at 250Vdc and less than 6mA to drive a base on an NPN transistor. At least it was not one of my designs, but I did help with the Root Cause Analysis.

 

[domct203]

Glad it looks like you are back up and rocking!

FYI- If a screen grid resistor did fail, that powertube position is essentially off and not drawing current, & a replacement tube in that position would never fail.

Dom

 

[bandit2013]

I would agree if the resistor opens, however, a drop in resistance will occur at elevated temperatures due to poor thermal drift characteristics of carbon type resistors. In that case, the cathode bias resistors may drop in resistance considerably if they overheat. Same would apply to grid resistors if made of the same material and have similar temp coefficients. I am only assuming they are carbon comp based on appearance. Even metal oxide or film resistors will be affected by temperature. I hope it is not a thermal issue, just stating it could be. If it was widespread defects of parts, everybody else would be having similar issues. With my amp, it is time dependent. I have left the amp soak for a bit in standby after playing for almost 2 hours non stop. Loss of 4 tubes has me a bit concerned, However, two of them were worn out to stat with.

Would a bad speaker cause power tubes to red plate?

 

[bandit2013]

Let me see if I understand this... Grid resistor is used to bias the grid to be more negative than the cathode. It is primarily used to prevent paracitic oscillations, prevents rectification of radio frequencies. However is the resistor value is to low, paracitic oscillations will occur that may be out of the audiable range, symptoms of such would include squeal, and redplate at safe bias conditions, harsh treble response and undesireable overtones. Yep, that fits my issues exactly. The center tubes are supposed to be biased to run cooler than the outer two, I have located the series grid resistors, but not the bias feed resistors. Typical connection should be a voltage divider network. Perhaps they are mounted on the component side of the PCB. ( the exposed area of the PCB that connects the power tube sockets is the solder side facing up, term used for wave soldering. I could measure resistance from ground to grid to get an idea what the pull down (or bias feed ) resistance is. I believe the thevinin equivalent resistance should be around 300k ohms. (I am still thinking silicon amplifiers, ie. BJT and FET's, there are some similarities to Valves but the circuits are different in some respects). It is possible that one of the tubes was out of spec. First time I noticed the redplate, only one tube V8 was running cherry red, V9 was beggining to glow two. No squealing, just harsh tones and sounded like crap. I did not have this issue when I replaced the 6L6 with the 7581's. They never redplated, but they did go flat within 120 hours of use. I now have a quad of matched GT6L6-R2 (early 80's vintage, Groove Tubes relabeled SED Winged =C=) Great tones so far. I just do not want to redplate them or strip the cathode too soon. As for the STR420's they were used and already spent. I had just retubed the Mark IV and wanted to preserve at least one functional amp if the Mark V was having issues.

 

[67mike]

I was going to ask if it got you high, but then I realized it was your amp that smoked the power tube...

I didn't have great luck with my stock tubes either, except in my case I had several pre-amp tubes quickly die. I'm replacing the whole set before another stock 12AX7 bites the dust.

Three months isn't long, so if you're the first owner of your amp, you should be able to get a warranty replacement.

Yes it did. LOL.
It is becomming a trouble. I replaced all 4 with another set (old ones from Mark IV STR420 6L6GC) and they did not survive very long. It seems V8 begings to run hot after 30 minutes of use. That was the same location the stock STR440 blew out in. This time before hitting the standby switch I was able to see what was happening. The entire tube was cherry red. Of cource V9 was red hot too but not as bright as V8. I wonder if there is a bias issue, or the EL34/6L6 bias switch is defective. I do have three of the stock 440's to use. Any ideas of what could be the issue? I am not too concerned about the STR420 since they are old. I do have new tubes comming but would hate to burn them up if there is a problem with the amp.

My Mark V is REDPLATING the tube in V8 as well.....Hope this isn't a design issue!!!!!

 

[bandit2013]

I lost two of the originals, Unfortunately what was occuring I thought was something else. The first time it took a nose dive on me was while I was using the effects loop connected to a Digitech RDS4000. Sounded like a surge and everything faded away. Opps, the Digitech got fried, turns out it is okay. Suggestions were to replace V6 since I thought it was associated to the loop. After I saw the red tubes, It has been such a long time, say 20 years, since I had experienced redplate tubes after burning up tubes in the MKIII. The MKIII ate EL34 for lunch, replacements were not so great and never lasted long. Ended that problem when I switched over to all 6L6. I do not have specifics on plate current, grid current or voltage. I doubt the cathode bias has anything to do with it. It could be a bias issue or uncontrolled oscillation. Tubes can be unpredictable.

I have not redplated any tubes yet after replacing my speakers, that just may be coincedential at best. The only original survivors were the tubes in V10 and V11. I have used them in V8 and V9 along with STR420's I found as reserves for the mark IV. I did run a quad of matched 7581 but they went flat after 120 hours. Without measuring the associated currents and voltages to determine if the bias is correct or not, it is more speculative at this point.

When is Orange going to release the VT1000. I know that will not resolve the issues if there are any with the MKV, but at least I could test all the tubes I now have. I have been searching for a bias meter, Eurotubes has one that may work out.

 

[ricorocks]

Roll the other one up qnd smoke it man LOL...... :lol:

 

[bandit2013]

Roll the other one up qnd smoke it man LOL...... :lol:

When I burn up one of the tube I have now, I can say it was Groovy.