Mark V Problems - 3rd Channel Hum + Volume Drop

[RocknRory]

Hey guys, it's been a while since I posted here. I've had my Mark V for 4 years now and I still love it. I replaced the tubes about 6 months with some tubes selected on dougstubes.com like I have before, with the help of Doug. I haven't replaced my pre-amp tubes in about 2-3 years. Anyway, here's the problem. A few weeks ago my fuse broke out of nowhere. I checked the tubes and looked to see what was wrong, but it wasn't apparent. I noticed the fuse didn't fully break either, like every other time the fuse has broken since I've owned the amp. I put a new fuse in to see what would happen, and the amp worked fine.

Fast forward a couple of weeks. Recently I've noticed my 3rd Channel humming quite loudly, way louder than normal. It hums in 90w and 45w, but not 10w. The clean and rhythm channel are fine in terms of humming. I'm not sure if I'm wrong, but I also think the output has weakened slightly on all channels because I remember it being louder. I could be wrong on that part though.

What do you guys recommend doing? Do you think it's my power tubes which are only half a year old, the pre-amp tubes, or something else?

 

[Authorized Boogie]

There's a reason your fuse blew - most often a shorting power or rectifier tube. Some shorts are mild and temporary, due to over-excited electrons jumping plates in the tube. When you go to standby/off or the fuse blows, the electrons settle and the short disappears - however, its a sign a power tube is headed for failure. This is the most common reason for the fuse blowing - aside from voltage spikes. The fuse acts as a protection device, and instead of simply replacing it and moving on - its a good idea to figure out why it blew and correct the problem before it potentially happens again, and causes further damage.

Most power tube problems will show up across all channels, but not necessarily in all wattage modes (only in the modes that use the "bad" tube). So in this case, its not totally clear whats causing your issue. I recommend running the amp in the 45 watt mode and swapping the inner and outer pair of power tubes. 45 watt mode uses only the inner pair - so you can swap pairs and isolate any potential failures to one pair or the other.

Preamp tube related issues are quite often channel specific - the best way to find a failed power tube is to swap a new 12AX7 into each position, one position at a time. This will pinpoint which tube is troublesome, and allow you to keep any good tubes while getting rid of the suspect tube.

Finally, you can run the amp in 45 or 90 watt modes (set to diode in 45 watt mode) and pull the rectifier tube - which can isolate any noise related to the rectifier tube.

I hope this helps.

 

[RocknRory]

There's a reason your fuse blew - most often a shorting power or rectifier tube. Some shorts are mild and temporary, due to over-excited electrons jumping plates in the tube. When you go to standby/off or the fuse blows, the electrons settle and the short disappears - however, its a sign a power tube is headed for failure. This is the most common reason for the fuse blowing - aside from voltage spikes. The fuse acts as a protection device, and instead of simply replacing it and moving on - its a good idea to figure out why it blew and correct the problem before it potentially happens again, and causes further damage.

Most power tube problems will show up across all channels, but not necessarily in all wattage modes (only in the modes that use the "bad" tube). So in this case, its not totally clear whats causing your issue. I recommend running the amp in the 45 watt mode and swapping the inner and outer pair of power tubes. 45 watt mode uses only the inner pair - so you can swap pairs and isolate any potential failures to one pair or the other.

Preamp tube related issues are quite often channel specific - the best way to find a failed power tube is to swap a new 12AX7 into each position, one position at a time. This will pinpoint which tube is troublesome, and allow you to keep any good tubes while getting rid of the suspect tube.

Finally, you can run the amp in 45 or 90 watt modes (set to diode in 45 watt mode) and pull the rectifier tube - which can isolate any noise related to the rectifier tube.

I hope this helps.

I tried swapping the power tubes and everything was going fine for a while, until I heard a loud humming sound. I checked the tubes and one was lit up, very bright orange. I decided to order a new Quad of Ruby 6L6GCMSTR's from Dougstubes.com. Don't know why a tube is dying after only 6 months, but I'm pretty sure it's a power tube. Thanks for the advice, I'll update after I swap the tubes out.

 

[xyrium]

I'm curious. What were the tubes you originally purchased from Doug, and did the new tubes resolve the problem?