What is this power tube issue?

  • Thread starter Thread starter Mr. Willy
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Mr. Willy

Mr. Willy

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I’m going to do my best to describe what was happening. I’m far from a tech, so bear with me in my description. My SLO was having an issue where the sound would crack and pop and become very loud and then soft. Like it’s cutting in and out. It never cut out all the way, but it was kinda like a radio station that’s not quite zeroed in on the dial.
First thought it was a preamp issue, so I pulled them all and tried multiple tubes in their place. Issue still there. In that process, I noticed the outside power tube on the right (if you’re facing the amp) had a blue glow. Obviously it’s a power tube problem. What is causing this? Is it simply a bad power tube?
 
That’s a very common symptom of a bad power tube. They will crackle and pop when going bad.
 
Could be a problem outside of the tube, but start there - coz it usually ends there.
 
Samhain said:
Blue glow is normal but crack n pop is not.
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Crack and pop might not be the best way to describe the problem. The sound was cutting in and out, like it was popping in and out.
 
Thanks for the responses guys. I’m going to pick up a matched pair of 5881 and go from there. Hopefully that’s all it is.
 
Absolutely worth it. These are the REAL DEAL. New production Sovtek/Mesa is a lower value version of the OG pre-Gorbachev Soviet military spec. Observe, different color plates and tig weld vs tab in slot. The one on the left is current production, the one on the right is a relabeled NOS military version.
 

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Nigel said:
Absolutely worth it. These are the REAL DEAL. New production Sovtek/Mesa is a lower value version of the OG pre-Gorbachev Soviet military spec. Observe, different color plates and tig weld vs tab in slot. The one on the left is current production, the one on the right is a relabeled NOS military version.
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Ok, I don’t understand some of what you’re telling me, but you’re selling me on these. Pardon my ignorance, what is “tig weld vs. tab in slot”?
 
You very likely have 1 power tube cutting in and out. This was common on my SLO. I would just keep a few extra 5881 button bottoms on hand, and swap it out.

Since it's the only amp I ever used those tubes in, it probably was just something common those tubes
 
Sovtek tubes are pretty cheap [affordable] and sound great.
Ive gotten pulls that test new in quads for as little as $40 or $50 bucks.
 
Mr. Willy said:
Ok, I don’t understand some of what you’re telling me, but you’re selling me on these. Pardon my ignorance, what is “tig weld vs. tab in slot”?
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The newer Sovtek "5881" tubes are not made the same way the Soviet military made them. They are cheaper and do not sound as good as the old production. The tig weld is the newer, the tab in slot is the older (left vs right). The older tubes sound much better and can be had for a song, as well as lasting a long, long time.
 
Nigel said:
Absolutely worth it. These are the REAL DEAL. New production Sovtek/Mesa is a lower value version of the OG pre-Gorbachev Soviet military spec. Observe, different color plates and tig weld vs tab in slot. The one on the left is current production, the one on the right is a relabeled NOS military version.
Click to expand...
I’ve tried those tubes(similar with the welds) and in the SLO it’s hands down the Wafer bottoms and it’s not close. Believe me, I tried Mesa 415s, other branded Sylvania single/dual getter 6L6GC, Mesa 430/440, KT66 etc etc and those cheaper wafer bottom Sovtek 5881s won out every time. Put them in, bias 28-32 ma and have fun.
Naylors also were designed with that tube, and same result applies to those amps too.
Wafer bottoms for the win.
 
Donnie B. said:
When was the last time your sockets got cleaned?

View attachment 81511
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Word of caution... there are some high voltages in tube sockets, so solvents should NOT be used when the amp is turned on... the amp should not be turned on while the solvent is still wet either. I use this stuff sparingly and after using, I put in and pull out a few different tubes to clean the sockets. I use different tubes since the pins may be slightly different and cleaning a contact point the other tube may not touch.
 
psychodave said:
Word of caution... there are some high voltages in tube sockets, so solvents should NOT be used when the amp is turned on
Click to expand...

Thanks. I should have included that.

Also, be sure to get the correct solvent for the job.
You can't go wrong with DeoxIT.
D5 for tube sockets and other static metal connections and F5 for moving parts like pots and Mesa slider EQs.
 
Donnie B. said:
Thanks. I should have included that.

Also, be sure to get the correct solvent for the job.
You can't go wrong with DeoxIT.
D5 for tube sockets and other static metal connections and F5 for moving parts like pots and Mesa slider EQs.
Click to expand...
I’m not sure when or if the sockets have been cleaned. It’s a ‘92. Also, for the sake of clarity, the tube that failed is a wafer bottom Sovtek 5881.
 
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