Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: marshallguy on August 03, 2023, 08:15:50 pm

Title: Ringing / rattling sound super imposed over guitar notes
Post by: marshallguy on August 03, 2023, 08:15:50 pm: Plugged the guitar also into FX return to isolate, ringing / rattling persists. Changed tubes, speaker cabinets, added dampers, problem persists.

Narrowed it down between FX return and output. Everything else works, sounds fine, just this annoying buzzy, rattling ringing on the chords, notes.

Thank you. :BangHead:
Title: Re: Injected audio is oscillating on the output tube plates only
Post by: sluckey on August 03, 2023, 08:30:45 pm: Maybe just a scope trigger problem. Exactly how are you triggering the scope?
Title: Re: Injected audio is oscillating on the output tube plates only
Post by: AlNewman on August 03, 2023, 09:38:36 pm: You could measure voltages at the power tubes and phase inverter and share.
Title: Re: Injected audio is oscillating on the output tube plates only
Post by: PRR on August 03, 2023, 10:42:08 pm: Putting any sort of probe on power tube plates is begging for trouble. Exploded probes, 'scopes, meters. And oscillations galore when the 400V AC audio radiates from the probe leads back to the sensitive preamp.

With the amp properly shielded--- are you really getting 1000Hz tones behind the music? That's an awful odd freq to happen accidentally.
Title: Re: Injected audio is oscillating on the output tube plates only
Post by: marshallguy on August 04, 2023, 02:22:30 am: :BangHead:
Title: Re: Injected audio is oscillating on the output tube plates only
Post by: marshallguy on August 04, 2023, 02:27:37 am: Quote from: AlNewman on August 03, 2023, 09:38:36 pm
You could measure voltages at the power tubes and phase inverter and share.

Hi,

Output tubes Plate V. 456v /456v

P.I. B+. 335v
82k plate. 178v
91k plate. 175v
Title: Re: Injected audio is oscillating on the output tube plates only
Post by: marshallguy on August 04, 2023, 02:51:36 am: With the amp properly shielded--- are you really getting 1000Hz tones behind the music? That's an awful odd freq to happen accidentally.
[/quote]

I can’t say for sure what freq tones I’m getting behind the music except to say it is present anywhere on the guitar neck which is strummed, picked or plucked.
Title: Re: Injected audio is oscillating on the output tube plates only
Post by: sluckey on August 04, 2023, 07:25:08 am: Quote from: marshallguy on August 04, 2023, 02:22:30 am
Quote from: sluckey on August 03, 2023, 08:30:45 pm
Maybe just a scope trigger problem. Exactly how are you triggering the scope?
Source - Ch. 1
Use a BNC "Tee" to run another cable directly from the signal source to the scope EXT TRIG input and set the scope to trigger on that rather than CH1. That will resolve any triggering issues and makes the trigger level independent of the signal at the probe.
Title: Re: Injected audio is oscillating on the output tube plates only
Post by: marshallguy on August 04, 2023, 10:14:03 am: Quote from: sluckey on August 04, 2023, 07:25:08 am
Quote from: marshallguy on August 04, 2023, 02:22:30 am
Quote from: sluckey on August 03, 2023, 08:30:45 pm
Maybe just a scope trigger problem. Exactly how are you triggering the scope?
Source - Ch. 1
Use a BNC "Tee" to run another cable directly from the signal source to the scope EXT TRIG input and set the scope to trigger on that rather than CH1. That will resolve any triggering issues and makes the trigger level independent of the signal at the probe.

Thanks.

By the way,

Will this work with Global NFB?

https://cdnimages.opentip.com/Docs/CED/snubber_article_k-snub.pdf
Title: Re: Injected audio is oscillating on the output tube plates only
Post by: marshallguy on August 04, 2023, 01:14:50 pm: Quote from: sluckey on August 03, 2023, 08:30:45 pm
Maybe just a scope trigger problem. Exactly how are you triggering the scope?

You nailed it.