I have a Mark V combo. Recently it will be playing fine and then all of the sudden no sound at all, but then a few seconds later it will be back again. I've replaced all of the tubes thinking I had an intermittent tube and I'm still having the same issue. I've tried tapping on the top of the amp thinking it may be a loose connection but it has no impact. Sometimes the sound fades out rather than simply on or off. Here's the link to a video of the amp having this issue. At around 35 seconds you'll hear it begin to come back to regular volume. I tapped on the top of the amp and rocked it slightly and it made no difference as in a loose tube. Any ideas? https://www.youtube.com/watch?v=WdDxssIW7nk
Mark V Combo Intermittent Output
- Thread starter rockn4jc
- Start date
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I have a few suggestions.... this brings me back to 2012 when I got my Mark V. I still have it but prefer the Roadster or RA100.
Look at your power tubes when this happens. I am willing to bet one of the center tubes may be overloaded. I had this issue early on with my Mark V such that I kept burning up either V8 or V9 when I used the 45W mode. Also the tube rectifier can also cause grief as they tend to shut down at times. It is rare that I ever use the 45W mode anymore since it tends to run one tube on the hot side (so much for extended Class A simul-class operation). Does the problem go away in 90W mode?
Did you change the rectifier tube along with the power tubes? The rectifier does not do much but rectify the AC power to DC. However if half of the rectifier is operating and the other is not, the power will drop dramatically and behave like you turned the amp on with the stand-by switch off. Unfortunately, even with the two CH1 and CH2 set to diode or when using CH3, the Rectifier will still be active and when it shorts out even momentarily, it will take the power supply down with it (not in a damaging way so no worries just change the rectifier tube).
Look at the fuse filament, it may still be good since the amp is operating, but if there are tiny metal balls growing out of the wire loop is an indication of drawing too much current usually when a tube overloads. Note that the fuse is designed to pull apart once the element overheats, but since it is a slow blow, sometimes it does not happen. If it was compromised it will usually fail completely and not intermittently.
Some intermittent issues are related to blown grid resistor. Power tube will not operate properly if the grid resistor is gone. Try the amp with the effects loop active, and then turn off the effects loop. This will change the position of the hot tube on the Class A circuit. I do not recall which one is hot when the effects loop is on, either V9 with the loop on, and V8 with it off. If you get no sound at all or barely any sound while in one mode over the other, you may have a bad grid resistor.
Look at your power tubes when this happens. I am willing to bet one of the center tubes may be overloaded. I had this issue early on with my Mark V such that I kept burning up either V8 or V9 when I used the 45W mode. Also the tube rectifier can also cause grief as they tend to shut down at times. It is rare that I ever use the 45W mode anymore since it tends to run one tube on the hot side (so much for extended Class A simul-class operation). Does the problem go away in 90W mode?
Did you change the rectifier tube along with the power tubes? The rectifier does not do much but rectify the AC power to DC. However if half of the rectifier is operating and the other is not, the power will drop dramatically and behave like you turned the amp on with the stand-by switch off. Unfortunately, even with the two CH1 and CH2 set to diode or when using CH3, the Rectifier will still be active and when it shorts out even momentarily, it will take the power supply down with it (not in a damaging way so no worries just change the rectifier tube).
Look at the fuse filament, it may still be good since the amp is operating, but if there are tiny metal balls growing out of the wire loop is an indication of drawing too much current usually when a tube overloads. Note that the fuse is designed to pull apart once the element overheats, but since it is a slow blow, sometimes it does not happen. If it was compromised it will usually fail completely and not intermittently.
Some intermittent issues are related to blown grid resistor. Power tube will not operate properly if the grid resistor is gone. Try the amp with the effects loop active, and then turn off the effects loop. This will change the position of the hot tube on the Class A circuit. I do not recall which one is hot when the effects loop is on, either V9 with the loop on, and V8 with it off. If you get no sound at all or barely any sound while in one mode over the other, you may have a bad grid resistor.
Bandit2013, Wow, that's some great information. Unfortunately my amp is now at church awaiting tomorrow mornings performance so I can't test all of that tonight. I never use the 45 watt settings, either 10 or 90. What I did find out by chance this afternoon though was that if I switched it between the wattage settings while it was happening, it would go away. I took it to church and practiced this afternoon and had no trouble at all with it. Sometimes I can play it for hours with absolutely no issue, and then sometimes it does what I showed in the video. When I get it home though I'll check some of the items you commented on. Thanks a bunch for your help!!
I can only reflect on the issues I had with mine. May not be the same problem you are having but sounds so close. When I first got the Mark V (head) I played for hours every day and used the 45W setting all the time. Also used the Variac mode more than full power as it was not as bright that way. The 90W was too loud for me at the time. The amp would cut out but in CH3 so the rectifier was automatically bypassed. I also found the Diode to sound better than the tube rectifier in the other channels. Within 2 months I was having issues and blew out the first set of tubes. That continued to happen with new tubes when I used the 45W mode. After the amp went dead with a burning smell, turns out I had damage to one of the screen resistors that literally split open. I doubt that you lost a screen resistor yet as the amp will not operate without it since the fuse will continue to blow out. Prior to loosing the screen resistor (happened in 45W mode with a set of JJ 6CA7 uses the EL34 bias setting) I have had no issues for 8 months of hard use in 90W mode which is a Class A/B mode) The 10W mode is a pure Class A circuit where as the 45W mode operates in extended Class A. You may notice the plate on one tube to begin to glow red at the seams (similar to a 6V6 tube which is normal for a 6V6, not a 6L6).
What I would do, sit next to the amp but have the front facing away from you so you have clear view of the power tubes. Play the amp as you normally would do but watch the tubes. In 45W mode, only the center two tubes will have a blue hue, the others will just have the orange glow of the heater element. The plates should appear dark in color. Best if you do this in a dark room or minimal light. continue to play though the amp unit the symptom shows. If one tube turns red hot (the dark metal plates are actually glowing red, aka "red plating"), or if you see a flash or arc in one of the tubes (sometimes the Rectifier tube will arc and if it does the power will drop and take a while to build up which may be the issue here). Replace the rectifier tube if you see lightning inside the tube (also note, the rectifier tube will not have a blue hue or glow to it which is normal).
If you have a power tube that does not have a heater glow, or it is really dim compared to the others there may be a fracture in the tube, however if it has a blue hue to it when playing it probably is ok).
What I would do, sit next to the amp but have the front facing away from you so you have clear view of the power tubes. Play the amp as you normally would do but watch the tubes. In 45W mode, only the center two tubes will have a blue hue, the others will just have the orange glow of the heater element. The plates should appear dark in color. Best if you do this in a dark room or minimal light. continue to play though the amp unit the symptom shows. If one tube turns red hot (the dark metal plates are actually glowing red, aka "red plating"), or if you see a flash or arc in one of the tubes (sometimes the Rectifier tube will arc and if it does the power will drop and take a while to build up which may be the issue here). Replace the rectifier tube if you see lightning inside the tube (also note, the rectifier tube will not have a blue hue or glow to it which is normal).
If you have a power tube that does not have a heater glow, or it is really dim compared to the others there may be a fracture in the tube, however if it has a blue hue to it when playing it probably is ok).
jb's 52
Well-known member
You might be having the same issue I was having here:
http://forum.grailtone.com/viewtopic.php?f=33&t=70544
Are you getting no sound on all channels or just one particular one?
http://forum.grailtone.com/viewtopic.php?f=33&t=70544
Are you getting no sound on all channels or just one particular one?
No sound on any of the channels, or fading sound. I have noticed though that when I switch between wattage settings it goes away, at least for a while. I played through it for a couple of hours on Sunday and it never cut out on me. I hate intermittent issues, because I guarantee, when it cuts out I'll be right in the middle of my 25 or 6 to 4 solo! Thanks for all the help though. I'm going to play for a few hours in low light and see what I can from the back.
Authorized Boogie
Well-known member
bandit2013 said:
10w is Class A, 45w is Class A/B, and 90w is Simul-Class. Thanks!
I stand corrected: on the 90W mode, copied from manual:
90 WATT (SIMUL-CLASSâ„¢ : CLASS AB + CLASS A, PUSH-PULL):
The 90 W position (toggle up) is our patented Simul-Class™ scheme and it delivers the most power with 90 watts of clean, sweet and yet bold power and headroom. In this position all four output tubes are on-line and two different classes of operation are being run simultaneously to create the most musical “hundred watt†output stage in the business. The outside pair (second from far left and the far right) are wired to Class AB and are running cooler - while at the same time creating the bulk of the horsepower. This modern way of running tubes is more efficient and produces the greatest power with the least heat.
The inside pair of tubes run in “extended Class A†with their reduced bias and are “more on†all the time, whether there is signal present or they are at rest (when you aren’t playing). This wiring and bias style puts out less power and creates more heat, but in trade for the lack of efficiency, it features a much smoother clip characteristic and a smoother, warmer sound… especially at the onset of clip where the harsher transients can become unpleasant in a Class AB output section. Simul-Class™ takes care of all that and smooths out the rough edges and re-voices the highs - leaving you with a sweet musical clip in the transition zone and yet plenty of power and headroom. The 90 W (Simul-Class™) position is the way to go for most live performance situations where you need headroom for clean chording and tight articulate low end for crunch rhythm along with a sweet, vocal voice with authority for soloing.
However, manual claims extended class A for 45W mode. Not exactly class A/B but the push-pull would make you think otherwise.
45 WATT (EXTENDED CLASS A, PUSH-PULL):
In this setting only the middle pair of 6L6s are on, and it’s these that run in the extended Class A with their bias reduced. Thus, they run a little hotter but also smoother, especially across the threshold into clip without sounding harsh.
90 WATT (SIMUL-CLASSâ„¢ : CLASS AB + CLASS A, PUSH-PULL):
The 90 W position (toggle up) is our patented Simul-Class™ scheme and it delivers the most power with 90 watts of clean, sweet and yet bold power and headroom. In this position all four output tubes are on-line and two different classes of operation are being run simultaneously to create the most musical “hundred watt†output stage in the business. The outside pair (second from far left and the far right) are wired to Class AB and are running cooler - while at the same time creating the bulk of the horsepower. This modern way of running tubes is more efficient and produces the greatest power with the least heat.
The inside pair of tubes run in “extended Class A†with their reduced bias and are “more on†all the time, whether there is signal present or they are at rest (when you aren’t playing). This wiring and bias style puts out less power and creates more heat, but in trade for the lack of efficiency, it features a much smoother clip characteristic and a smoother, warmer sound… especially at the onset of clip where the harsher transients can become unpleasant in a Class AB output section. Simul-Class™ takes care of all that and smooths out the rough edges and re-voices the highs - leaving you with a sweet musical clip in the transition zone and yet plenty of power and headroom. The 90 W (Simul-Class™) position is the way to go for most live performance situations where you need headroom for clean chording and tight articulate low end for crunch rhythm along with a sweet, vocal voice with authority for soloing.
However, manual claims extended class A for 45W mode. Not exactly class A/B but the push-pull would make you think otherwise.
45 WATT (EXTENDED CLASS A, PUSH-PULL):
In this setting only the middle pair of 6L6s are on, and it’s these that run in the extended Class A with their bias reduced. Thus, they run a little hotter but also smoother, especially across the threshold into clip without sounding harsh.
