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Excessive hiss from tube amp - need to replace tubes?

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New Member
Joined: 16 years ago
Posts: 2
Topic starter  

Peavey 5150 2x12 combo

# Five 12AX7 preamp tubes
# Two 6L6GC power tubes

Hiss is emitted with no guitar plugged in. Hiss volume increases as pre and/or post volumes are increased. My father who studied EE 25 years ago (heh) visually inspected the pre tubes and saw no damage. How would I determine the source of the hiss and eliminate it? It's really, really annoying!

Illustrious Member
Joined: 21 years ago
Posts: 7833

Go to: Click on "Tube Amp Debugging Pages." Read. :D

"A cheerful heart is good medicine."

New Member
Joined: 16 years ago
Posts: 2
Topic starter  

The procedure of locating by removing one tube at at time working from the phase inverter/driver back towards the input until removing a tube no longer stops the hiss should then localize the problem to one tube's worth of circuitry.

When I remove any of the five tubes, there is no output whatsoever.

Reputable Member
Joined: 18 years ago
Posts: 199

First up is to distinguish hiss from hum.

Hiss is often due to bad tubes, usually from the first preamp tube, especially in high gain amps, such as the 5150. Begin by replacing that one with a good tube and see if goes away. Hiss can also come from bad or failing resistors or capacitors, especially those used for cathode bypass.

Hum is 60/120 Hz noise often caused by failing electrolytics in the power supply. They typically have a 10-15 year lifetime.

How old is your amp?

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Paper-in-oil caps rule!