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Scratchy tube amp question

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New Member
Joined: 14 years ago
Posts: 1
Topic starter  

Sometimes on my Fender Princeton tube amp i get a scratchy sound. It is audible when a note is sustained for a while. I changed the tubes and tried a new guitar. Could it be a pot or is it a grounding problem?

Noble Member
Joined: 14 years ago
Posts: 1630

Welcome, sorry about your amp problem, but hope you will enjoy further discussion with members! :D

We must assume that you have changed the tubes, all of them now, for absolutely new and fresh store-bought stock. :shock:

Further to that, if all tubes are optimum and balanced, you are thinking of some likely causes, such as a scratchy pot. But it is dangerous to go inside.

It is better not to work on tube amps yourself. Sor starters, there is the "one hand rule". You can not touch the amp (chassis) with both hands at the same time. You should imagine that you must have a hand tied behind your back.

To seriously work inside a tube amp, the technician, or we, ourselves, discharge the capacitors, and perform the service on a grounding mat, from an electronic or service person's supplier.

You could also have a cold solder joint or partially broken/fragmenting resistor at the socket for one of the power tubes.

Lastly - I didn't look it up. Have the amp biased for good measure if it is biasable, and not fixed biased.

Like a bird on the wire,
like a drunk in a midnight choir
I have tried in my way to be free.

Illustrious Member
Joined: 20 years ago
Posts: 7833

That's overly cautious to tell someone not to work on a tube amp, especially for basic stuff like changing tubes. (Not much different from changing light bulbs, which should only be done with the power to the light turned off.) Read up on tube amp safety, like the page on that in the tube amp tech links at: Follow the rules. A grounding mat will not make you safer. That's for discharging static electricity that can damage solid state components. I think learning to properly work on and adjust (like bias settings, where applicable) your gear is a good thing.

Other common sources of scratchy sounds would be the speaker, or perhaps something loose vibrating in the cabinet or chassis.

"A cheerful heart is good medicine."