Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Nizzle on April 21, 2020, 04:22:00 pm

Title: Rising Hi Pitched Squeal
Post by: Nizzle on April 21, 2020, 04:22:00 pm

Hi all,

I've got Plexi style amp that has just started making a Rising Hi Pitched Squeal(like a tea kettle) when I power up and sometimes out of nowhere, while playing. It, eventually, goes away after 15-30 seconds of rising in pitch. Sounds like a bomb is about to detonate.I've also noticed slight to moderate increased level of the noise floor of the amp. PreAmp tubes seem to be fine. One was slightly microphonic so I switched positions and it's operating fine in the new configuration, BUT - I still get the tea-kettle whine. Not sure if it's the Power Tubes, but they are not noisy to the light tap. Any thoughts?

best,

-t

Title: Re: Rising Hi Pitched Squeal
Post by: shooter on April 21, 2020, 04:39:29 pm

new build?

Quote

rising in pitch

typically positive feedback from somewhere.
had similar a couple days back when I put a hi-gain tube in a low gain spot squealed like a pig  :think1:

Title: Re: Rising Hi Pitched Squeal
Post by: Nizzle on April 27, 2020, 02:48:12 pm

No. Startup up out of nowhere. Rearranged PreAmp Tube placement - no change. Replaced Power Tubes - no change. Replaced Preamp tubes - Problem gone! Went back to old Power Tubes and problem was still gone. One or more of the Preamp tubes went bad. Sadly - I have no way of testing the preamp tubes and no visual indication or rattling to indicate a bad tube...Oh well. At least it's back to a healthy state of operation.

Thnx for the advice.

-t

Title: Re: Rising Hi Pitched Squeal
Post by: tubeswell on April 27, 2020, 03:07:49 pm

Send me your bad old tubes. I know what to do with them LoL

Title: Re: Rising Hi Pitched Squeal
Post by: shooter on April 27, 2020, 03:19:35 pm

Quote

Sadly - I have no way of testing the preamp tubes

sure you do, you have a working amp, better than any tube tester  :icon_biggrin: