Low Frequency feedback on initial power up

[jaster55]

I just finished my Princeton reverb build. Triple checked every component, wire, connection. Followed all the first time power up directions, adjusted the trim pot for maximum negative voltage. Plugged a speaker in. Plugged in the reverb tank. Powered up to check DC Millivolts across the 1 ohm resistors. Within 4 seconds the amp starts to generate a low frequency feedback with all control pots turned all the way down. I am getting between 21 and 22 millivolts across both resistors. I took my time and triple checked everything using the Hoffman method.  Any ideas on something I might have missed.

[HotBluePlates]

I just finished my Princeton reverb build. ... Powered up ... Within 4 seconds the amp starts to generate a low frequency feedback ...

Disconnect the feedback wire running from the speaker jack to the board.  Does your feedback stop?

If no, look first to grounding of the filter caps vs the grounding point of the power transformer.  Verify good soldering for all power filters.

[jaster55]

Thanks for your response...I disconnected the feedback wire. Problem went away. Amp sounds Awesome!!!
What does this mean?

[PRR]

Swap the plate leads on the OT. Test again. OK, but still brash? Connect feedback. It "should" now be stable and feedback-controlled to limit the rudeness of naked tubes.

Ideally the feedback is "NFB", Negative feedback, to reduce error and distortion. If the phase through the amp and back through the feedback is reversed, it is Positive feedback, and tends to amplify its own errors, commonly to a HOWL (or THUB-THUB). In theory we can work this out in advance. In practice it is 50:50 crapshoot. Swapping the OT plate leads is the fix. Disconnecting feedback is a quick/easy diagnostic.

[jaster55]

That did the trick.
This is my first Hoffman build. I learned a lot between the website and the forum.
I'm already thinking about my next amp...
THANKS FOR YOUR HELP

[HotBluePlates]