Fresh 6G15 Reverb Unit Build - Noise with Dwell at Zero

[mort]

Typical stock circuit. Sounds perfect, very quite except when cold there's a buzzing/humm with the dwell control under 2. Turn it up past 2 and it vanishes. Also vanishes completely after warming up for 15-20 minutes.

Thoughts?

https://manualmachine.com/html/d2/d2...f74PacBKBD.png

[pdf64]

Maybe the 12AT7 heater to cathode insulation isn’t ideal?
Or your 0V common arrangement isn’t ideal, eg the 0V common dwell track terminal should reference to the cathode of the next stage.

[mort]

Yep, replaced the tube and problem solved.

[Chuck H]

I remember finishing up a custom build and it had a hum. Not a typical hum, it sounded more wooshie somehow. I always load new tubes into a new build with the exception of V1 which is usually U(used)OS salvage from my stash. But those tubes are rigorously checked for noise, tone and microphonics before going into said stash. So I had to suspect the build. I spent the better part of a day hunting and pecking only to hit a wall. It HAD TO be the V2 tube itself because I'd ruled out everything else. Sure enough replacing that tube stopped the noise. So I've been preaching here ever since that new doesn't always mean good when it comes to tubes. And this has proven itself out for me in other experiences since. So...

Since a build for me can last months and tubes aren't installed until the end I have had the unfortunate circumstance of taking tubes out of their boxes for the first time only to learn they're no good AND out of warranty. Always test and burn in new tube within the warranty period.

[Enzo]

So I've been preaching here ever since that new doesn't always mean good when it comes to tubes.

New doesn't always mean good for ANY part.

[mort]

New doesn't always mean good for ANY part.

Very true.

I’m an hvac contractor and in 2021 I had 3 condenser fan motors that were bad in the box. 2 of them weren’t even installed due to feeling bumps in the action while spinning the shaft by hand. The other sounded a little off when installed and failed within 72 hours.