Strange tube issue w/ Ampeg SB-12 amp

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Strange tube issue w/ Ampeg SB-12 amp

Post by centurymantra » Mon Apr 18, 2011 6:30 am

I sold an Ampeg SB-12 amplifier to a friend of mine. It's a great sounding amp and my friend really likes it, but we have run into a strange tube issue. I had suggested that he might want to re-tube the amplifier, which they probably will be doing. Outside of that, when looking at the amplifier when it is on, it almost looks like the 12ax7 tubes in the preamp section are not lit up. I'm no tech expert, but it would seem that the amp simply would not work if the preamp section tubes were out. Even so, they decided to start the re-tube process by getting new 12ax7 tubes - a pair of Ruby tubes 12ax7tubes (12ax7ac4 or ac5 to be exact). When they put the new tubes in, the amp stopped working. They re-installed the old tubes and everything is fine. They are going to check to see if one of the new tubes is bad. In the event they are both fine, would anyone have some ideas what the problem might be?

Thanks in advance for any input!

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Post by Snarl 12/8 » Mon Apr 18, 2011 1:06 pm

What's ac4 or ac5? Maybe those tubes aren't actually compatible. You might want to check with Bruce at Also, that amp is OLD. There could be some other problems. I just got my sb-12 back from the tech and it still had a very intermittent problem, so I took it back again and it took him 2 hours to track down a bad resistor. There could be a borderline cap or resistor or (god forbid) transformer winding, that can't take a more solid tube with bigger requirements for, say, plate voltage or what have you. Like I said, I took mine to a tech, so I'm just spitballin' here.

Why did you swap out the preamp tubes? Just 'cause they don't "light up"? I just flipped mine on and they glow really bright for a second and then settle down to just barely visible. I'll let it warm up, and then play it for a while, and see if that changes. Like, from 6 feet away, I can only see a slight orange glow reflected off the chassis when I peer into that "cage". From some angles I see no glow at all.
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