#1
Alright guys, I have a late 80's Fender Super 60 rack head. All tube, nice amp. But...

Whenever I have the amp at gigging/practice volumes (around 2-3) and turn the gain up to halfway or better, I get this weird "OOOOOOOOOOOOOO" feedback. It's not like normal feedback, like it doesn't... develop, if you get what I mean. It's just there. Hitting it with an OD makes it worse. The same thing happens if I run the gain really low and try to crank it, or run the gain low and boost it really hard with an OD.

I don't think it's a tube problem, but hey, you never know. It might be just what it is; feedback that an ISP or some other noise gate will take care of.

But I don't know, and figured I would ask you guys. I have tried different guitars, and it still does it.

Something else I might should mention is that the amp does have a clipping meter, but the highest that it's ever hit is 2 lights in.

Anything I should look for, or am I just an idiot?
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#2
Don't plug a cable into the input and see what happens when you start cranking. If you get something it is in the output section or speaker cable.

If not, it's probably the guitar's cable, preamp tube, phase inverter, or other preamp problem.
#3
I've tried different cables as well, and still happened.

And alright, I'll have to try that tomorrow. Don't have it here, left it at the practice space.

Kinda random, but if I threw up a schematic, could anyone tell me if it's cathode biased or not? Or, what would tell me in the schem if it is?

EDIT: Almost forgot. Thanks
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#6
It is not cathode biased. It is fixed bias. If you look at the cathodes of the power tubes, you'll notice they are grounded.

In a cathode biased amp, there is a large power resistor and capacitor between the cathodes and ground.


EDITED: Just FYI, fixed bias has a somewhat misleading name if you're not familiar with how it works. Fixed bias amps MUST (actually should) be biased with tube changes.

Cathode bias is also referred to as "Auto-Biasing" which is somewhat of an appropriate name.
Last edited by end_citizen at Dec 6, 2009,
#7
Quote by end_citizen
It is not cathode biased. It is fixed bias. If you look at the cathodes of the power tubes, you'll notice they are grounded.

In a cathode biased amp, there is a large power resistor and capacitor between the cathodes and ground.


EDITED: Just FYI, fixed bias has a somewhat misleading name if you're not familiar with how it works. Fixed bias amps MUST (actually should) be biased with tube changes.

Cathode bias is also referred to as "Auto-Biasing" which is somewhat of an appropriate name.

Yeah, I knew that much . Alright, so I'll need to get it biased. That sucks.. Old amp was cathode.

At least now I can make it hot or cold.

Alright, thanks a lot, end_citizen, you are awesome. I'll see what I can see tomorrow, and report back.

Thanks so much again.
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#8
Not a problem.

If you need any help, there are lots of intellegent people on here. I listed some people with better amp knowledge in the thread linked to in my sig labled "Building an amp? Maybe I can help."
#9
Alright, big update time.

end_citizen, I tried it without a cable plugged in, and nothing happened. So, now I know that the problem is in the preamp.

Now, what should I check first? Switch out the pres?
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#10
I have this same problem on my Musicman 112RD50, it happens when I turn it to about 8... but I never get a chance to turn it above 1 anyway so I've ignored it. I want to see what answers this thread yields though. I've been told however that my amp is cathode biased/autobiasing.
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