6505 low level squealing

gybe!

gybe!

Active member
Have a new to my son 6505 that has a very low level squeal across both channels and doesn’t seem to increase with gain or volume. Takes a minute or so after initial power up and occurs regardless of if anything is plugged in. Definitely a squeal, but low level in the background (can definitely play around it) just a bit above the natural hiss of the amp.

Knee jerk is to suspect a preamp tube, but the lack of change with volume or gain (on either channel) has me questioning. Thoughts?
 
Clean the input jacks and effects loop jacks. Spray some cleaner on a 1/4 jack and insert/remove it a few times. Wipe it clean, re-spray it again and repeat. Do that 5 times or so for each jack.

The fact it stays constant with gain and volume tells me it’s not a failing electrolytic in the signal path.

If after the jacks you still have a problem, time to start assessing the tubes. Start with pre - replace one at a time with a known good tube to find the culprit.

For the power tubes, pull the inner tubes and half the impedance and see if that fixes it. If not replace the two inner tubes and pull the two outer tubes. Impedance on the amp stays half the correct match anytime two tubes are pulled.

If you find it’s a bad power tube pair, order a new matched quad pre-selected for the amp and always select burned in feature so that they’re settled.

If after all of this you get no recourse, it could be a bad or failing electrolytic capacitor in the mains or screen rail supply. Take it to a tech and have them recap the whole amp both power and pre (there are electrolytic bypass caps in the preamp)
 
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I had the 5150 2x12 combo and one day it started doing that. No change in volume or gain. It was a preamp tube. I'd try that first.
 
glpg80 said:
Clean the input jacks and effects loop jacks. Spray some cleaner on a 1/4 jack and insert/remove it a few times. Wipe it clean, re-spray it again and repeat. Do that 5 times or so for each jack.

The fact it stays constant with gain and volume tells me it’s not a failing electrolytic in the signal path.

If after the jacks you still have a problem, time to start assessing the tubes. Start with pre - replace one at a time with a known good tube to find the culprit.

For the power tubes, pull the inner tubes and half the impedance and see if that fixes it. If not replace the two inner tubes and pull the two outer tubes. Impedance on the amp stays half the correct match anytime two tubes are pulled.

If you find it’s a bad power tube pair, order a new matched quad pre-selected for the amp and always select burned in feature so that they’re settled.

If after all of this you get no recourse, it could be a bad or failing electrolytic capacitor in the mains or screen rail supply. Take it to a tech and have them recap the whole amp both power and pre (there are electrolytic bypass caps in the preamp)
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Awesome info
 
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