Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cbevilaqua on February 09, 2022, 08:18:36 pm

Title: Squeak sound if speaker jack is grounded
Post by: cbevilaqua on February 09, 2022, 08:18:36 pm
Some time ago, I built my 1st tube amp, the 5f1 model, all from scratch.
I left the speaker jack ungrounded ( insulated from the chassis by a plastic ring ) and it works good.
Some days ago I decided to try to change the RCA insulated jack to a J10 non insulated jack ( the negative side is attached to the chassis ) and in this case once I turn on the amp, I can hear a Squeak sound coming from the speaker. I jus turned off the amp immediately and don't even tested anything else with the ungrounded speaker just afraid of doing some kind of damage.
Actually, my speaker jack is located between the preamp and the power amp section.
Anyone had some similar sympthom? Should I ground the speaker jack or it may be completely insulated from the chassis? May be this is caused by it's location?

You can see an attached image withe the location of the RCA Jack ( with it I have no problems as it is insulated ).
Thanks!
Title: Re: Squeak sound if speaker jack is grounded
Post by: sluckey on February 09, 2022, 09:01:37 pm
The speaker jack should be grounded to chassis because of the NFB circuit. Since your amp "squeaks" when the jack is grounded, the NFB phase is wrong. To correct this, swap the two OT primary leads. Or, if the OT only has one secondary tap, usually green and black wires, you can just swap those at the speaker jack.
Title: Re: Squeak sound if speaker jack is grounded
Post by: cbevilaqua on February 10, 2022, 05:59:56 am
Thank you so much, I will do that!
Title: Re: Squeak sound if speaker jack is grounded
Post by: sluckey on February 10, 2022, 06:26:07 am
While you are working on the amp, consider moving the speaker jack as far away from the input jack as possible. As is, the speaker jack is in the worst possible location. Lot's of bad things can happen with the speaker jack next to the input jack.
Title: Re: Squeak sound if speaker jack is grounded
Post by: cbevilaqua on February 10, 2022, 09:09:31 am
Quote from: sluckey on February 10, 2022, 06:26:07 am
While you are working on the amp, consider moving the speaker jack as far away from the input jack as possible. As is, the speaker jack is in the worst possible location. Lot's of bad things can happen with the speaker jack next to the input jack.

Thanks! I will also change the speaker jack positiion.
Title: Re: Squeak sound if speaker jack is grounded
Post by: cbevilaqua on February 10, 2022, 06:14:12 pm
That worked exactly as you said, thank you so much again!
Here are some pictures of the new location of the output RCA Jack. I also reverted the OT output wires. As it is of insulated type, I grounded it in the input jack ground point.
Now, I have almost no "hum" and everything works 100%.