Mark V V1 Tube Keeps Blowing
- Thread starter rkaczano
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electric mayhem
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yes- @bandit2013 --> you have your head very much in the V and likely could help with this one. I can only guess a R is blown on the input.
electric mayhem
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Or maybe the power supply is jacked sending too much DC to the B+ rail on the pcb or a load R (100k ohm?) failing between that rail and one of the plate pins for V1.
What would cause that?
tbonesullivan
Well-known member
What exactly do you mean by "blowing"? Does the tube arc and/or crack or something similar? Does it now not work in other positions? How do you know that V1 is the issue? If it's not glowing, that could be an issue with the socket itself, not the tubes.
First question of the day: Is this amp currently covered under warranty? If yes, stop there and email Mesa customer service for assistance.
If no. then we can take a closer look at what may be the issue.
Blowing out a 12AX7 is difficult to do even with the cathode follower circuits used in the Dual Rectifier. Not sure what would cause the tube to fail based on the applied voltages. I am not too familiar with the limits on the preamp tube as they are cathode biased. Guess I do not know much then. Never tried to find the limits on a 12AX7, However the Roadster or MWDR have done that for me with the DC coupled cathode follower circuit. Don't worry, that is no found in the Mark amps.
What tube is it that you are having issues other than the location it is failing in. Is this a Mesa branded 12AX7, SPAX7, or other? Figured I would ask up front.
Most common issue with the V1 tube is that extra long tube shield. Mesa only used that so you can see it is there. Same issue with the Roadster as well. I got into the habit of removing the chassis from the head shell to change preamp tubes in those two amps. I have broken a few tubes struggling to get the shield back on while it was in the head shell. If you must keep it in the housing, try using one of the other short shields as it can be easier to get onto the tube and secure it. If the spring is not going over the end seal on the tube (the pointy part) it can be cracked if you have issues or big hands like mine. I found the shorter tube shield was easier to install while reaching over or around the transformer that is in the way.
Check the silver coating on the tubes you suspect are bad. Black or silver is good. White means the glass has a crack in it. Once the glass envelope has failed or cracked, the heater will not heat up or its glow will be very weak and the tube will remain cold. If they look ok, try them in another tube location that is easy to see. V4 has an AC heater circuit so if it glows there it may still be good.
If you do not have a tube tester, it is difficult to determine if the tube is bad just by looking at it. If the silver getter flash is white, that would indicate cracked glass or leaking tube and would be obvious it is a dead tube.
If you can still change channels and all of the LEDs on the amp are working, even if V1 is removed from the amp, I would assume all is good with the 12V supply. Note that V1, V2 and V5 are sourced from this 12VDC supply as well as all of the relays and the channel bus for each channel.
You can remove the tube shield from V2 and or V5, if those heaters are working you will be able to see that nice orange glow that peaks out from the cathode sleeve. Not all the time but most of the time. If those two tubes are cold, you may have a dead 12V supply but then again, nothing else would be working properly (assuming it is just one diode in the bridge circuit.)
If it was a plate voltage issue, V2 and V3 would also be suspect for damage. they have the same plate voltage source "E" which is 405VDC max on CH3
I assume the default at start up is CH1 so the plate voltage would be 370VDC. Each triode is cathode biased. V1A has a 1.5k cathode resistor and a 100k plate resistor in default CH1. V1B is the same but fixed at 100k plate and 1.5k cathode. V1A does get reconfigured for the three different channels as well as with some of the various modes. I just do not see how the 12AX7 tube would be getting damaged as there is not much current in the circuit to start with.
You can measure the heaters of both triodes at the same time, measure across pins 4 and 5. If you want to check each individual heater elements, it would be measured from pin 4 to 9 and pin 5 to 9. That is about all you can do with a multi-meter. Without a tube tester to heat the tube and measure the emissions from the cathode, that would be hard to do. the air gap between the parts would be difficult to measure the capacitance as what is usually listed is the small signal capacitance while the tube is in operation.
This is the data sheet for the Mesa 12AX7, it is a JJ ECC83S tube.
That is all I got. Sorry if this is no help. Blowing out a preamp tube other than in a Cathode follower circuit of the Rectifer amps, is not something I have come across. the failure is usually gradual and not instant kill. If that is what is happening here, what are you plugging into the front end? I assume that -50V is the limit on the control grid before something craps out. That would be for V1A though, V1B is more internal so it is not likely to be damaged, there are several DC blocking caps in the three tone stacks that I find difficult to believe V1B is exceeding its ratings. Got me good with this one.
If no. then we can take a closer look at what may be the issue.
Blowing out a 12AX7 is difficult to do even with the cathode follower circuits used in the Dual Rectifier. Not sure what would cause the tube to fail based on the applied voltages. I am not too familiar with the limits on the preamp tube as they are cathode biased. Guess I do not know much then. Never tried to find the limits on a 12AX7, However the Roadster or MWDR have done that for me with the DC coupled cathode follower circuit. Don't worry, that is no found in the Mark amps.
What tube is it that you are having issues other than the location it is failing in. Is this a Mesa branded 12AX7, SPAX7, or other? Figured I would ask up front.
Most common issue with the V1 tube is that extra long tube shield. Mesa only used that so you can see it is there. Same issue with the Roadster as well. I got into the habit of removing the chassis from the head shell to change preamp tubes in those two amps. I have broken a few tubes struggling to get the shield back on while it was in the head shell. If you must keep it in the housing, try using one of the other short shields as it can be easier to get onto the tube and secure it. If the spring is not going over the end seal on the tube (the pointy part) it can be cracked if you have issues or big hands like mine. I found the shorter tube shield was easier to install while reaching over or around the transformer that is in the way.
Check the silver coating on the tubes you suspect are bad. Black or silver is good. White means the glass has a crack in it. Once the glass envelope has failed or cracked, the heater will not heat up or its glow will be very weak and the tube will remain cold. If they look ok, try them in another tube location that is easy to see. V4 has an AC heater circuit so if it glows there it may still be good.
If you do not have a tube tester, it is difficult to determine if the tube is bad just by looking at it. If the silver getter flash is white, that would indicate cracked glass or leaking tube and would be obvious it is a dead tube.
If you can still change channels and all of the LEDs on the amp are working, even if V1 is removed from the amp, I would assume all is good with the 12V supply. Note that V1, V2 and V5 are sourced from this 12VDC supply as well as all of the relays and the channel bus for each channel.
You can remove the tube shield from V2 and or V5, if those heaters are working you will be able to see that nice orange glow that peaks out from the cathode sleeve. Not all the time but most of the time. If those two tubes are cold, you may have a dead 12V supply but then again, nothing else would be working properly (assuming it is just one diode in the bridge circuit.)
If it was a plate voltage issue, V2 and V3 would also be suspect for damage. they have the same plate voltage source "E" which is 405VDC max on CH3
I assume the default at start up is CH1 so the plate voltage would be 370VDC. Each triode is cathode biased. V1A has a 1.5k cathode resistor and a 100k plate resistor in default CH1. V1B is the same but fixed at 100k plate and 1.5k cathode. V1A does get reconfigured for the three different channels as well as with some of the various modes. I just do not see how the 12AX7 tube would be getting damaged as there is not much current in the circuit to start with.
You can measure the heaters of both triodes at the same time, measure across pins 4 and 5. If you want to check each individual heater elements, it would be measured from pin 4 to 9 and pin 5 to 9. That is about all you can do with a multi-meter. Without a tube tester to heat the tube and measure the emissions from the cathode, that would be hard to do. the air gap between the parts would be difficult to measure the capacitance as what is usually listed is the small signal capacitance while the tube is in operation.
This is the data sheet for the Mesa 12AX7, it is a JJ ECC83S tube.
That is all I got. Sorry if this is no help. Blowing out a preamp tube other than in a Cathode follower circuit of the Rectifer amps, is not something I have come across. the failure is usually gradual and not instant kill. If that is what is happening here, what are you plugging into the front end? I assume that -50V is the limit on the control grid before something craps out. That would be for V1A though, V1B is more internal so it is not likely to be damaged, there are several DC blocking caps in the three tone stacks that I find difficult to believe V1B is exceeding its ratings. Got me good with this one.
