Squealing PRRI

**vashondan** · 10-09-2020, 08:15 AM

I've got a 65 PRRI that starts to make this little squealing sound after the amp has been on for about 10 minutes. Stays for the rest of the time the amp is on. Doesn't get louder with volume. I changed the speaker but it's still there. So, I'm guessing it's the tubes (?) but wondering what makes that happen. Never had this problem before. Thoughts? Thanks.

**Sam Sherry** · 10-09-2020, 08:33 AM

It could be anything from a failing tube to a failing component to a dirty contact. (That's not a 'speaker' symptom as you found out.)

If you're not geared up and comfortable with high voltage your smart move is to shut it down and bring it to the tech.

**Paulie2** · 10-09-2020, 09:22 AM

Yep, would be willing to bet its a failing microphonic tube. You can sometimes find which one it is by GENTLY tapping on them with something and seeing if the squealing changes. I could be a failing interior component, but with a newer amp like that I would doubt it.

**KirkP** · 10-09-2020, 12:57 PM

Every tube amp owner should have at least one spare of every tube type in the amp. You could try substituting one at a time to see if that eliminates the squeal.

If you don’t have any spare tubes, you could try removing the smaller tubes one at a time. That can indicate which stage of amplification may be defective. It won’t tell you whether the root cause is the tube or other components, but it will at least give a clue as to where to look.

Don’t remove the larger octal tubes. Power tubes are push-pull so must be replaced in pairs. Without a rectifier tube you’ll hear nothing. Of course when changing tubes you should power down and cool off. To be extra-cautious you can unplug the amp.

If you report your results here you might get more suggestions, but if it’s not a tube you’ll need a good tech to pull the chassis.

**vashondan** · 10-09-2020, 01:04 PM

Thanks to all of you for your responses. I had already ordered some tubes which are on their way after getting similar advice about having a set handy.

I'll follow up with your suggestions.

Thanks once again!